Nat  Grady

Nat Grady

1670058554

How Update Array Elements in a Document with MQL Positional Operators

MongoDB offers a rich query language that’s great for create, read, update, and delete operations as well as complex multi-stage aggregation pipelines. There are many ways to model your data within MongoDB and regardless of how it looks, the MongoDB Query Language (MQL) has you covered.

One of the lesser recognized but extremely valuable features of MQL is in the positional operators that you’d find in an update operation.

Let’s say that you have a document and inside that document, you have an array of objects. You need to update one or more of those objects in the array, but you don’t want to replace the array or append to it. This is where a positional operator might be valuable.

In this tutorial, we’re going to look at a few examples that would benefit from a positional operator within MongoDB.

Use the $ Operator to Update the First Match in an Array

Let’s use the example that we have an array in each of our documents and we want to update only the first match within that array, even if there’s a potential for numerous matches.

To do this, we’d probably want to use the $ operator which acts as a placeholder to update the first element matched.

For this example, let’s use an old-school Pokemon video game. Take look at the following MongoDB document:

{
    "_id": "red",
    "pokemon": [
        {
            "number": 6,
            "name": "Charizard"
        },
        {
            "number": 25,
            "name": "Pikachu",
        },
        {
            "number": 0,
            "name": "MissingNo"
        }
    ]
}

Let’s assume that the above document represents the Pokemon information for the Pokemon Red video game. The document is not a true reflection and it is very much incomplete. However, if you’re a fan of the game, you’ll probably remember the glitch Pokemon named “MissingNo.” To make up a fictional story, let’s assume the developer, at some point in time, wanted to give that Pokemon an actual name, but forgot.

We can update that particular element in the array by doing something like the following:

db.pokemon_game.update(
    { "pokemon.name": "MissingNo" },
    {
        "$set": {
            "pokemon.$.name": "Agumon"
        }
    }
);

In the above example, we are doing a filter for documents that have an array element with a name field set to MissingNo. With MongoDB, you don’t need to specify the array index in your filter for the update operator. In the manipulation step, we are using the $ positional operator to change the first occurrence of the match in the filter. Yes, in my example, I am renaming the “MissingNo” Pokemon to that of a Digimon, which is an entirely different brand.

The new document would look like this:

{
    "_id": "red",
    "pokemon": [
        {
            "number": 6,
            "name": "Charizard"
        },
        {
            "number": 25,
            "name": "Pikachu",
        },
        {
            "number": 0,
            "name": "Agumon"
        }
    ]
}

Had “MissingNo” appeared numerous times within the array, only the first occurrence would be updated. If “MissingNo” appeared numerous times, but the surrounding fields were different, you could match on multiple fields using the $elemMatch operator to narrow down which particular element should be updated.

More information on the $ positional operator can be found in the documentation.

Use the $[] Operator to Update All Array Elements Within a Document

Let’s say that you have an array in your document and you need to update every element in that array using a single operation. To do this, we might want to take a look at the $[] operator which does exactly that.

Using the same Pokemon video game example, let’s imagine that we have a team of Pokemon and we’ve just finished a battle in the game. The experience points gained from the battle need to be distributed to all the Pokemon on your team.

The document that represents our team might look like the following:

{
    "_id": "red",
    "team": [
        {
            "number": 1,
            "name": "Bulbasaur",
            "xp": 5
        },
        {
            "number": 25,
            "name": "Pikachu",
            "xp": 32
        }
    ]
}

At the end of the battle, we want to make sure every Pokemon on our team receives 10 XP. To do this with the $[] operator, we can construct an update operation that looks like the following:

db.pokemon_game.update(
    { "_id": "red" },
    {
        "$inc": {
            "team.$[].xp": 10
        }
    }
);

In the above example, we use the $inc modifier to increase all xp fields within the team array by a constant number. To learn more about the $inc operator, check out the documentation.

Our new document would look like this:

[
    {
        "_id": "red",
        "team": [
            {
                "number": 1,
                "name": "Bulbasaur",
                "xp": 15
            },
            {
                "number": 25,
                "name": "Pikachu",
                "xp": 42
            }
        ]
    }
]

While useful for this example, we don’t exactly get to provide criteria in case one of your Pokemon shouldn’t receive experience points. If your Pokemon has fainted, maybe they shouldn’t get the increase.

We’ll learn about filters in the next part of the tutorial.

To learn more about the $[] operator, check out the documentation.

Use the $[<identifier>] Operator to Update Elements that Match a Filter Condition

Let’s use the example that we have several array elements that we want to update in a single operation and we don’t want to worry about excessive client-side code paired with a replace operation.

To do this, we’d probably want to use the $[<identifier>] operator which acts as a placeholder to update all elements that match an arrayFilters condition.

To put things into perspective, let’s say that we’re dealing with Pokemon trading cards, instead of video games, and tracking their values. Our documents might look like this:

db.pokemon_collection.insertMany(
    [
        {
            _id: "nraboy",
            cards: [
                {
                    "name": "Charizard",
                    "set": "Base",
                    "variant": "1st Edition",
                    "value": 200000
                },
                {
                    "name": "Pikachu",
                    "set": "Base",
                    "variant": "Red Cheeks",
                    "value": 300
                }
            ]
        },
        {
            _id: "mraboy",
            cards: [
                {
                    "name": "Pikachu",
                    "set": "Base",
                    "variant": "Red Cheeks",
                    "value": 300
                },
                {
                    "name": "Pikachu",
                    "set": "McDonalds 25th Anniversary Promo",
                    "variant": "Holo",
                    "value": 10
                }
            ]
        }
    ]
);

Of course, the above snippet isn’t a document, but an operation to insert two documents into some pokemon_collection collection within MongoDB. In the above scenario, each document represents a collection of cards for an individual. The cards array has information about the card in the collection as well as the current value.

In our example, we need to update prices of cards, but we don’t want to do X number of update operations against the database. We only want to do a single operation to update the values of each of our cards.

Take the following query:

db.pokemon_collection.update(
    {},
    {
        "$set": {
            "cards.$[elemX].value": 350,
            "cards.$[elemY].value": 500000
        }
    },
    {
        "arrayFilters": [
            {
                "elemX.name": "Pikachu",
                "elemX.set": "Base",
                "elemX.variant": "Red Cheeks"
            },
            {
                "elemY.name": "Charizard",
                "elemY.set": "Base",
                "elemY.variant": "1st Edition"
            }
        ],
        "multi": true
    }
);

The above update operation is like any other, but with an extra step for our positional operator. The first parameter, which is an empty object, represents our match criteria. Because it is empty, we’ll be updating all documents within the collection.

The next parameter is the manipulation we want to do to our documents. Let’s skip it for now and look at the arrayFilters in the third parameter.

Imagine that we want to update the price for two particular cards that might exist in any person’s Pokemon collection. In this example, we want to update the price of the Pikachu and Charizard cards. If you’re a Pokemon trading card fan, you’ll know that there are many variations of the Pikachu and Charizard card, so we get specific in our arrayFilters array. For each object in the array, the fields of those objects represent an and condition. So, for elemX, which has no specific naming convention, all three fields must be satisfied.

In the above example, we are using elemX and elemY to represent two different filters.

Let’s go back to the second parameter in the update operation. If the filter for elemX comes back as true because an array item in a document matched, then the value field for that object will be set to a new value. Likewise, the same thing could happen for the elemY filter. If a document has an array and one of the filters does not ever match an element in that array, it will be ignored.

If looking at our example, the documents would now look like the following:

[
    {
        "_id": "nraboy",
        "cards": [
            {
                "name": "Charizard",
                "set": "Base",
                "variant": "1st Edition",
                "value": 500000
            },
            {
                "name": "Pikachu",
                "set": "Base",
                "variant": "Red Cheeks",
                "value": 350
            }
        ]
    },
    {
        "_id": "mraboy",
        "cards": [
            {
                "name": "Pikachu",
                "set": "Base",
                "variant": "Red Cheeks",
                "value": 350
            },
            {
                "name": "Pikachu",
                "set": "McDonalds 25th Anniversary Promo",
                "variant": "Holo",
                "value": 10
            }
        ]
    }
]

If any particular array contained multiple matches for one of the arrayFilter criteria, all matches would have their price updated. This means that if I had, say, 100 matching Pikachu cards in my Pokemon collection, all 100 would now have new prices.

More information on the $[<identifier>] operator can be found in the documentation.

Conclusion

You just saw how to use some of the positional operators within the MongoDB Query Language (MQL). These operators are useful when working with arrays because they prevent you from having to do full replaces on the array or extended client-side manipulation.

To learn more about MQL, check out my previous tutorial titled, Getting Started with Atlas and the MongoDB Query Language (MQL).

If you have any questions, take a moment to stop by the MongoDB Community Forums.

This content first appeared on MongoDB.

Original article source at: https://www.thepolyglotdeveloper.com/

#mongodb #array #elements 

What is GEEK

Buddha Community

How Update Array Elements in a Document with MQL Positional Operators
Connor Mills

Connor Mills

1670560264

Understanding Arrays in Python

Learn how to use Python arrays. Create arrays in Python using the array module. You'll see how to define them and the different methods commonly used for performing operations on them.
 

The artcile covers arrays that you create by importing the array module. We won't cover NumPy arrays here.

Table of Contents

  1. Introduction to Arrays
    1. The differences between Lists and Arrays
    2. When to use arrays
  2. How to use arrays
    1. Define arrays
    2. Find the length of arrays
    3. Array indexing
    4. Search through arrays
    5. Loop through arrays
    6. Slice an array
  3. Array methods for performing operations
    1. Change an existing value
    2. Add a new value
    3. Remove a value
  4. Conclusion

Let's get started!


What are Python Arrays?

Arrays are a fundamental data structure, and an important part of most programming languages. In Python, they are containers which are able to store more than one item at the same time.

Specifically, they are an ordered collection of elements with every value being of the same data type. That is the most important thing to remember about Python arrays - the fact that they can only hold a sequence of multiple items that are of the same type.

What's the Difference between Python Lists and Python Arrays?

Lists are one of the most common data structures in Python, and a core part of the language.

Lists and arrays behave similarly.

Just like arrays, lists are an ordered sequence of elements.

They are also mutable and not fixed in size, which means they can grow and shrink throughout the life of the program. Items can be added and removed, making them very flexible to work with.

However, lists and arrays are not the same thing.

Lists store items that are of various data types. This means that a list can contain integers, floating point numbers, strings, or any other Python data type, at the same time. That is not the case with arrays.

As mentioned in the section above, arrays store only items that are of the same single data type. There are arrays that contain only integers, or only floating point numbers, or only any other Python data type you want to use.

When to Use Python Arrays

Lists are built into the Python programming language, whereas arrays aren't. Arrays are not a built-in data structure, and therefore need to be imported via the array module in order to be used.

Arrays of the array module are a thin wrapper over C arrays, and are useful when you want to work with homogeneous data.

They are also more compact and take up less memory and space which makes them more size efficient compared to lists.

If you want to perform mathematical calculations, then you should use NumPy arrays by importing the NumPy package. Besides that, you should just use Python arrays when you really need to, as lists work in a similar way and are more flexible to work with.

How to Use Arrays in Python

In order to create Python arrays, you'll first have to import the array module which contains all the necassary functions.

There are three ways you can import the array module:

  1. By using import array at the top of the file. This includes the module array. You would then go on to create an array using array.array().
import array

#how you would create an array
array.array()
  1. Instead of having to type array.array() all the time, you could use import array as arr at the top of the file, instead of import array alone. You would then create an array by typing arr.array(). The arr acts as an alias name, with the array constructor then immediately following it.
import array as arr

#how you would create an array
arr.array()
  1. Lastly, you could also use from array import *, with * importing all the functionalities available. You would then create an array by writing the array() constructor alone.
from array import *

#how you would create an array
array()

How to Define Arrays in Python

Once you've imported the array module, you can then go on to define a Python array.

The general syntax for creating an array looks like this:

variable_name = array(typecode,[elements])

Let's break it down:

  • variable_name would be the name of the array.
  • The typecode specifies what kind of elements would be stored in the array. Whether it would be an array of integers, an array of floats or an array of any other Python data type. Remember that all elements should be of the same data type.
  • Inside square brackets you mention the elements that would be stored in the array, with each element being separated by a comma. You can also create an empty array by just writing variable_name = array(typecode) alone, without any elements.

Below is a typecode table, with the different typecodes that can be used with the different data types when defining Python arrays:

TYPECODEC TYPEPYTHON TYPESIZE
'b'signed charint1
'B'unsigned charint1
'u'wchar_tUnicode character2
'h'signed shortint2
'H'unsigned shortint2
'i'signed intint2
'I'unsigned intint2
'l'signed longint4
'L'unsigned longint4
'q'signed long longint8
'Q'unsigned long longint8
'f'floatfloat4
'd'doublefloat8

Tying everything together, here is an example of how you would define an array in Python:

import array as arr 

numbers = arr.array('i',[10,20,30])


print(numbers)

#output

#array('i', [10, 20, 30])

Let's break it down:

  • First we included the array module, in this case with import array as arr .
  • Then, we created a numbers array.
  • We used arr.array() because of import array as arr .
  • Inside the array() constructor, we first included i, for signed integer. Signed integer means that the array can include positive and negative values. Unsigned integer, with H for example, would mean that no negative values are allowed.
  • Lastly, we included the values to be stored in the array in square brackets.

Keep in mind that if you tried to include values that were not of i typecode, meaning they were not integer values, you would get an error:

import array as arr 

numbers = arr.array('i',[10.0,20,30])


print(numbers)

#output

#Traceback (most recent call last):
# File "/Users/dionysialemonaki/python_articles/demo.py", line 14, in <module>
#   numbers = arr.array('i',[10.0,20,30])
#TypeError: 'float' object cannot be interpreted as an integer

In the example above, I tried to include a floating point number in the array. I got an error because this is meant to be an integer array only.

Another way to create an array is the following:

from array import *

#an array of floating point values
numbers = array('d',[10.0,20.0,30.0])

print(numbers)

#output

#array('d', [10.0, 20.0, 30.0])

The example above imported the array module via from array import * and created an array numbers of float data type. This means that it holds only floating point numbers, which is specified with the 'd' typecode.

How to Find the Length of an Array in Python

To find out the exact number of elements contained in an array, use the built-in len() method.

It will return the integer number that is equal to the total number of elements in the array you specify.

import array as arr 

numbers = arr.array('i',[10,20,30])


print(len(numbers))

#output
# 3

In the example above, the array contained three elements – 10, 20, 30 – so the length of numbers is 3.

Array Indexing and How to Access Individual Items in an Array in Python

Each item in an array has a specific address. Individual items are accessed by referencing their index number.

Indexing in Python, and in all programming languages and computing in general, starts at 0. It is important to remember that counting starts at 0 and not at 1.

To access an element, you first write the name of the array followed by square brackets. Inside the square brackets you include the item's index number.

The general syntax would look something like this:

array_name[index_value_of_item]

Here is how you would access each individual element in an array:

import array as arr 

numbers = arr.array('i',[10,20,30])

print(numbers[0]) # gets the 1st element
print(numbers[1]) # gets the 2nd element
print(numbers[2]) # gets the 3rd element

#output

#10
#20
#30

Remember that the index value of the last element of an array is always one less than the length of the array. Where n is the length of the array, n - 1 will be the index value of the last item.

Note that you can also access each individual element using negative indexing.

With negative indexing, the last element would have an index of -1, the second to last element would have an index of -2, and so on.

Here is how you would get each item in an array using that method:

import array as arr 

numbers = arr.array('i',[10,20,30])

print(numbers[-1]) #gets last item
print(numbers[-2]) #gets second to last item
print(numbers[-3]) #gets first item
 
#output

#30
#20
#10

How to Search Through an Array in Python

You can find out an element's index number by using the index() method.

You pass the value of the element being searched as the argument to the method, and the element's index number is returned.

import array as arr 

numbers = arr.array('i',[10,20,30])

#search for the index of the value 10
print(numbers.index(10))

#output

#0

If there is more than one element with the same value, the index of the first instance of the value will be returned:

import array as arr 


numbers = arr.array('i',[10,20,30,10,20,30])

#search for the index of the value 10
#will return the index number of the first instance of the value 10
print(numbers.index(10))

#output

#0

How to Loop through an Array in Python

You've seen how to access each individual element in an array and print it out on its own.

You've also seen how to print the array, using the print() method. That method gives the following result:

import array as arr 

numbers = arr.array('i',[10,20,30])

print(numbers)

#output

#array('i', [10, 20, 30])

What if you want to print each value one by one?

This is where a loop comes in handy. You can loop through the array and print out each value, one-by-one, with each loop iteration.

For this you can use a simple for loop:

import array as arr 

numbers = arr.array('i',[10,20,30])

for number in numbers:
    print(number)
    
#output
#10
#20
#30

You could also use the range() function, and pass the len() method as its parameter. This would give the same result as above:

import array as arr  

values = arr.array('i',[10,20,30])

#prints each individual value in the array
for value in range(len(values)):
    print(values[value])

#output

#10
#20
#30

How to Slice an Array in Python

To access a specific range of values inside the array, use the slicing operator, which is a colon :.

When using the slicing operator and you only include one value, the counting starts from 0 by default. It gets the first item, and goes up to but not including the index number you specify.


import array as arr 

#original array
numbers = arr.array('i',[10,20,30])

#get the values 10 and 20 only
print(numbers[:2])  #first to second position

#output

#array('i', [10, 20])

When you pass two numbers as arguments, you specify a range of numbers. In this case, the counting starts at the position of the first number in the range, and up to but not including the second one:

import array as arr 

#original array
numbers = arr.array('i',[10,20,30])


#get the values 20 and 30 only
print(numbers[1:3]) #second to third position

#output

#rray('i', [20, 30])

Methods For Performing Operations on Arrays in Python

Arrays are mutable, which means they are changeable. You can change the value of the different items, add new ones, or remove any you don't want in your program anymore.

Let's see some of the most commonly used methods which are used for performing operations on arrays.

How to Change the Value of an Item in an Array

You can change the value of a specific element by speficying its position and assigning it a new value:

import array as arr 

#original array
numbers = arr.array('i',[10,20,30])

#change the first element
#change it from having a value of 10 to having a value of 40
numbers[0] = 40

print(numbers)

#output

#array('i', [40, 20, 30])

How to Add a New Value to an Array

To add one single value at the end of an array, use the append() method:

import array as arr 

#original array
numbers = arr.array('i',[10,20,30])

#add the integer 40 to the end of numbers
numbers.append(40)

print(numbers)

#output

#array('i', [10, 20, 30, 40])

Be aware that the new item you add needs to be the same data type as the rest of the items in the array.

Look what happens when I try to add a float to an array of integers:

import array as arr 

#original array
numbers = arr.array('i',[10,20,30])

#add the integer 40 to the end of numbers
numbers.append(40.0)

print(numbers)

#output

#Traceback (most recent call last):
#  File "/Users/dionysialemonaki/python_articles/demo.py", line 19, in <module>
#   numbers.append(40.0)
#TypeError: 'float' object cannot be interpreted as an integer

But what if you want to add more than one value to the end an array?

Use the extend() method, which takes an iterable (such as a list of items) as an argument. Again, make sure that the new items are all the same data type.

import array as arr 

#original array
numbers = arr.array('i',[10,20,30])

#add the integers 40,50,60 to the end of numbers
#The numbers need to be enclosed in square brackets

numbers.extend([40,50,60])

print(numbers)

#output

#array('i', [10, 20, 30, 40, 50, 60])

And what if you don't want to add an item to the end of an array? Use the insert() method, to add an item at a specific position.

The insert() function takes two arguments: the index number of the position the new element will be inserted, and the value of the new element.

import array as arr 

#original array
numbers = arr.array('i',[10,20,30])

#add the integer 40 in the first position
#remember indexing starts at 0

numbers.insert(0,40)

print(numbers)

#output

#array('i', [40, 10, 20, 30])

How to Remove a Value from an Array

To remove an element from an array, use the remove() method and include the value as an argument to the method.

import array as arr 

#original array
numbers = arr.array('i',[10,20,30])

numbers.remove(10)

print(numbers)

#output

#array('i', [20, 30])

With remove(), only the first instance of the value you pass as an argument will be removed.

See what happens when there are more than one identical values:


import array as arr 

#original array
numbers = arr.array('i',[10,20,30,10,20])

numbers.remove(10)

print(numbers)

#output

#array('i', [20, 30, 10, 20])

Only the first occurence of 10 is removed.

You can also use the pop() method, and specify the position of the element to be removed:

import array as arr 

#original array
numbers = arr.array('i',[10,20,30,10,20])

#remove the first instance of 10
numbers.pop(0)

print(numbers)

#output

#array('i', [20, 30, 10, 20])

Conclusion

And there you have it - you now know the basics of how to create arrays in Python using the array module. Hopefully you found this guide helpful.

You'll start from the basics and learn in an interacitve and beginner-friendly way. You'll also build five projects at the end to put into practice and help reinforce what you learned.

Thanks for reading and happy coding!

Original article source at https://www.freecodecamp.org

#python 

How to Create Arrays in Python

In this tutorial, you'll know the basics of how to create arrays in Python using the array module. Learn how to use Python arrays. You'll see how to define them and the different methods commonly used for performing operations on them.

This tutorialvideo on 'Arrays in Python' will help you establish a strong hold on all the fundamentals in python programming language. Below are the topics covered in this video:  
1:15 What is an array?
2:53 Is python list same as an array?
3:48  How to create arrays in python?
7:19 Accessing array elements
9:59 Basic array operations
        - 10:33  Finding the length of an array
        - 11:44  Adding Elements
        - 15:06  Removing elements
        - 18:32  Array concatenation
       - 20:59  Slicing
       - 23:26  Looping  


Python Array Tutorial – Define, Index, Methods

In this article, you'll learn how to use Python arrays. You'll see how to define them and the different methods commonly used for performing operations on them.

The artcile covers arrays that you create by importing the array module. We won't cover NumPy arrays here.

Table of Contents

  1. Introduction to Arrays
    1. The differences between Lists and Arrays
    2. When to use arrays
  2. How to use arrays
    1. Define arrays
    2. Find the length of arrays
    3. Array indexing
    4. Search through arrays
    5. Loop through arrays
    6. Slice an array
  3. Array methods for performing operations
    1. Change an existing value
    2. Add a new value
    3. Remove a value
  4. Conclusion

Let's get started!

What are Python Arrays?

Arrays are a fundamental data structure, and an important part of most programming languages. In Python, they are containers which are able to store more than one item at the same time.

Specifically, they are an ordered collection of elements with every value being of the same data type. That is the most important thing to remember about Python arrays - the fact that they can only hold a sequence of multiple items that are of the same type.

What's the Difference between Python Lists and Python Arrays?

Lists are one of the most common data structures in Python, and a core part of the language.

Lists and arrays behave similarly.

Just like arrays, lists are an ordered sequence of elements.

They are also mutable and not fixed in size, which means they can grow and shrink throughout the life of the program. Items can be added and removed, making them very flexible to work with.

However, lists and arrays are not the same thing.

Lists store items that are of various data types. This means that a list can contain integers, floating point numbers, strings, or any other Python data type, at the same time. That is not the case with arrays.

As mentioned in the section above, arrays store only items that are of the same single data type. There are arrays that contain only integers, or only floating point numbers, or only any other Python data type you want to use.

When to Use Python Arrays

Lists are built into the Python programming language, whereas arrays aren't. Arrays are not a built-in data structure, and therefore need to be imported via the array module in order to be used.

Arrays of the array module are a thin wrapper over C arrays, and are useful when you want to work with homogeneous data.

They are also more compact and take up less memory and space which makes them more size efficient compared to lists.

If you want to perform mathematical calculations, then you should use NumPy arrays by importing the NumPy package. Besides that, you should just use Python arrays when you really need to, as lists work in a similar way and are more flexible to work with.

How to Use Arrays in Python

In order to create Python arrays, you'll first have to import the array module which contains all the necassary functions.

There are three ways you can import the array module:

  • By using import array at the top of the file. This includes the module array. You would then go on to create an array using array.array().
import array

#how you would create an array
array.array()
  • Instead of having to type array.array() all the time, you could use import array as arr at the top of the file, instead of import array alone. You would then create an array by typing arr.array(). The arr acts as an alias name, with the array constructor then immediately following it.
import array as arr

#how you would create an array
arr.array()
  • Lastly, you could also use from array import *, with * importing all the functionalities available. You would then create an array by writing the array() constructor alone.
from array import *

#how you would create an array
array()

How to Define Arrays in Python

Once you've imported the array module, you can then go on to define a Python array.

The general syntax for creating an array looks like this:

variable_name = array(typecode,[elements])

Let's break it down:

  • variable_name would be the name of the array.
  • The typecode specifies what kind of elements would be stored in the array. Whether it would be an array of integers, an array of floats or an array of any other Python data type. Remember that all elements should be of the same data type.
  • Inside square brackets you mention the elements that would be stored in the array, with each element being separated by a comma. You can also create an empty array by just writing variable_name = array(typecode) alone, without any elements.

Below is a typecode table, with the different typecodes that can be used with the different data types when defining Python arrays:

TYPECODEC TYPEPYTHON TYPESIZE
'b'signed charint1
'B'unsigned charint1
'u'wchar_tUnicode character2
'h'signed shortint2
'H'unsigned shortint2
'i'signed intint2
'I'unsigned intint2
'l'signed longint4
'L'unsigned longint4
'q'signed long longint8
'Q'unsigned long longint8
'f'floatfloat4
'd'doublefloat8

Tying everything together, here is an example of how you would define an array in Python:

import array as arr 

numbers = arr.array('i',[10,20,30])


print(numbers)

#output

#array('i', [10, 20, 30])

Let's break it down:

  • First we included the array module, in this case with import array as arr .
  • Then, we created a numbers array.
  • We used arr.array() because of import array as arr .
  • Inside the array() constructor, we first included i, for signed integer. Signed integer means that the array can include positive and negative values. Unsigned integer, with H for example, would mean that no negative values are allowed.
  • Lastly, we included the values to be stored in the array in square brackets.

Keep in mind that if you tried to include values that were not of i typecode, meaning they were not integer values, you would get an error:

import array as arr 

numbers = arr.array('i',[10.0,20,30])


print(numbers)

#output

#Traceback (most recent call last):
# File "/Users/dionysialemonaki/python_articles/demo.py", line 14, in <module>
#   numbers = arr.array('i',[10.0,20,30])
#TypeError: 'float' object cannot be interpreted as an integer

In the example above, I tried to include a floating point number in the array. I got an error because this is meant to be an integer array only.

Another way to create an array is the following:

from array import *

#an array of floating point values
numbers = array('d',[10.0,20.0,30.0])

print(numbers)

#output

#array('d', [10.0, 20.0, 30.0])

The example above imported the array module via from array import * and created an array numbers of float data type. This means that it holds only floating point numbers, which is specified with the 'd' typecode.

How to Find the Length of an Array in Python

To find out the exact number of elements contained in an array, use the built-in len() method.

It will return the integer number that is equal to the total number of elements in the array you specify.

import array as arr 

numbers = arr.array('i',[10,20,30])


print(len(numbers))

#output
# 3

In the example above, the array contained three elements – 10, 20, 30 – so the length of numbers is 3.

Array Indexing and How to Access Individual Items in an Array in Python

Each item in an array has a specific address. Individual items are accessed by referencing their index number.

Indexing in Python, and in all programming languages and computing in general, starts at 0. It is important to remember that counting starts at 0 and not at 1.

To access an element, you first write the name of the array followed by square brackets. Inside the square brackets you include the item's index number.

The general syntax would look something like this:

array_name[index_value_of_item]

Here is how you would access each individual element in an array:

import array as arr 

numbers = arr.array('i',[10,20,30])

print(numbers[0]) # gets the 1st element
print(numbers[1]) # gets the 2nd element
print(numbers[2]) # gets the 3rd element

#output

#10
#20
#30

Remember that the index value of the last element of an array is always one less than the length of the array. Where n is the length of the array, n - 1 will be the index value of the last item.

Note that you can also access each individual element using negative indexing.

With negative indexing, the last element would have an index of -1, the second to last element would have an index of -2, and so on.

Here is how you would get each item in an array using that method:

import array as arr 

numbers = arr.array('i',[10,20,30])

print(numbers[-1]) #gets last item
print(numbers[-2]) #gets second to last item
print(numbers[-3]) #gets first item
 
#output

#30
#20
#10

How to Search Through an Array in Python

You can find out an element's index number by using the index() method.

You pass the value of the element being searched as the argument to the method, and the element's index number is returned.

import array as arr 

numbers = arr.array('i',[10,20,30])

#search for the index of the value 10
print(numbers.index(10))

#output

#0

If there is more than one element with the same value, the index of the first instance of the value will be returned:

import array as arr 


numbers = arr.array('i',[10,20,30,10,20,30])

#search for the index of the value 10
#will return the index number of the first instance of the value 10
print(numbers.index(10))

#output

#0

How to Loop through an Array in Python

You've seen how to access each individual element in an array and print it out on its own.

You've also seen how to print the array, using the print() method. That method gives the following result:

import array as arr 

numbers = arr.array('i',[10,20,30])

print(numbers)

#output

#array('i', [10, 20, 30])

What if you want to print each value one by one?

This is where a loop comes in handy. You can loop through the array and print out each value, one-by-one, with each loop iteration.

For this you can use a simple for loop:

import array as arr 

numbers = arr.array('i',[10,20,30])

for number in numbers:
    print(number)
    
#output
#10
#20
#30

You could also use the range() function, and pass the len() method as its parameter. This would give the same result as above:

import array as arr  

values = arr.array('i',[10,20,30])

#prints each individual value in the array
for value in range(len(values)):
    print(values[value])

#output

#10
#20
#30

How to Slice an Array in Python

To access a specific range of values inside the array, use the slicing operator, which is a colon :.

When using the slicing operator and you only include one value, the counting starts from 0 by default. It gets the first item, and goes up to but not including the index number you specify.

import array as arr 

#original array
numbers = arr.array('i',[10,20,30])

#get the values 10 and 20 only
print(numbers[:2])  #first to second position

#output

#array('i', [10, 20])

When you pass two numbers as arguments, you specify a range of numbers. In this case, the counting starts at the position of the first number in the range, and up to but not including the second one:

import array as arr 

#original array
numbers = arr.array('i',[10,20,30])


#get the values 20 and 30 only
print(numbers[1:3]) #second to third position

#output

#rray('i', [20, 30])

Methods For Performing Operations on Arrays in Python

Arrays are mutable, which means they are changeable. You can change the value of the different items, add new ones, or remove any you don't want in your program anymore.

Let's see some of the most commonly used methods which are used for performing operations on arrays.

How to Change the Value of an Item in an Array

You can change the value of a specific element by speficying its position and assigning it a new value:

import array as arr 

#original array
numbers = arr.array('i',[10,20,30])

#change the first element
#change it from having a value of 10 to having a value of 40
numbers[0] = 40

print(numbers)

#output

#array('i', [40, 20, 30])

How to Add a New Value to an Array

To add one single value at the end of an array, use the append() method:

import array as arr 

#original array
numbers = arr.array('i',[10,20,30])

#add the integer 40 to the end of numbers
numbers.append(40)

print(numbers)

#output

#array('i', [10, 20, 30, 40])

Be aware that the new item you add needs to be the same data type as the rest of the items in the array.

Look what happens when I try to add a float to an array of integers:

import array as arr 

#original array
numbers = arr.array('i',[10,20,30])

#add the integer 40 to the end of numbers
numbers.append(40.0)

print(numbers)

#output

#Traceback (most recent call last):
#  File "/Users/dionysialemonaki/python_articles/demo.py", line 19, in <module>
#   numbers.append(40.0)
#TypeError: 'float' object cannot be interpreted as an integer

But what if you want to add more than one value to the end an array?

Use the extend() method, which takes an iterable (such as a list of items) as an argument. Again, make sure that the new items are all the same data type.

import array as arr 

#original array
numbers = arr.array('i',[10,20,30])

#add the integers 40,50,60 to the end of numbers
#The numbers need to be enclosed in square brackets

numbers.extend([40,50,60])

print(numbers)

#output

#array('i', [10, 20, 30, 40, 50, 60])

And what if you don't want to add an item to the end of an array? Use the insert() method, to add an item at a specific position.

The insert() function takes two arguments: the index number of the position the new element will be inserted, and the value of the new element.

import array as arr 

#original array
numbers = arr.array('i',[10,20,30])

#add the integer 40 in the first position
#remember indexing starts at 0

numbers.insert(0,40)

print(numbers)

#output

#array('i', [40, 10, 20, 30])

How to Remove a Value from an Array

To remove an element from an array, use the remove() method and include the value as an argument to the method.

import array as arr 

#original array
numbers = arr.array('i',[10,20,30])

numbers.remove(10)

print(numbers)

#output

#array('i', [20, 30])

With remove(), only the first instance of the value you pass as an argument will be removed.

See what happens when there are more than one identical values:

import array as arr 

#original array
numbers = arr.array('i',[10,20,30,10,20])

numbers.remove(10)

print(numbers)

#output

#array('i', [20, 30, 10, 20])

Only the first occurence of 10 is removed.

You can also use the pop() method, and specify the position of the element to be removed:

import array as arr 

#original array
numbers = arr.array('i',[10,20,30,10,20])

#remove the first instance of 10
numbers.pop(0)

print(numbers)

#output

#array('i', [20, 30, 10, 20])

Conclusion

And there you have it - you now know the basics of how to create arrays in Python using the array module. Hopefully you found this guide helpful.

Thanks for reading and happy coding!

#python #programming 

Nat  Grady

Nat Grady

1670058554

How Update Array Elements in a Document with MQL Positional Operators

MongoDB offers a rich query language that’s great for create, read, update, and delete operations as well as complex multi-stage aggregation pipelines. There are many ways to model your data within MongoDB and regardless of how it looks, the MongoDB Query Language (MQL) has you covered.

One of the lesser recognized but extremely valuable features of MQL is in the positional operators that you’d find in an update operation.

Let’s say that you have a document and inside that document, you have an array of objects. You need to update one or more of those objects in the array, but you don’t want to replace the array or append to it. This is where a positional operator might be valuable.

In this tutorial, we’re going to look at a few examples that would benefit from a positional operator within MongoDB.

Use the $ Operator to Update the First Match in an Array

Let’s use the example that we have an array in each of our documents and we want to update only the first match within that array, even if there’s a potential for numerous matches.

To do this, we’d probably want to use the $ operator which acts as a placeholder to update the first element matched.

For this example, let’s use an old-school Pokemon video game. Take look at the following MongoDB document:

{
    "_id": "red",
    "pokemon": [
        {
            "number": 6,
            "name": "Charizard"
        },
        {
            "number": 25,
            "name": "Pikachu",
        },
        {
            "number": 0,
            "name": "MissingNo"
        }
    ]
}

Let’s assume that the above document represents the Pokemon information for the Pokemon Red video game. The document is not a true reflection and it is very much incomplete. However, if you’re a fan of the game, you’ll probably remember the glitch Pokemon named “MissingNo.” To make up a fictional story, let’s assume the developer, at some point in time, wanted to give that Pokemon an actual name, but forgot.

We can update that particular element in the array by doing something like the following:

db.pokemon_game.update(
    { "pokemon.name": "MissingNo" },
    {
        "$set": {
            "pokemon.$.name": "Agumon"
        }
    }
);

In the above example, we are doing a filter for documents that have an array element with a name field set to MissingNo. With MongoDB, you don’t need to specify the array index in your filter for the update operator. In the manipulation step, we are using the $ positional operator to change the first occurrence of the match in the filter. Yes, in my example, I am renaming the “MissingNo” Pokemon to that of a Digimon, which is an entirely different brand.

The new document would look like this:

{
    "_id": "red",
    "pokemon": [
        {
            "number": 6,
            "name": "Charizard"
        },
        {
            "number": 25,
            "name": "Pikachu",
        },
        {
            "number": 0,
            "name": "Agumon"
        }
    ]
}

Had “MissingNo” appeared numerous times within the array, only the first occurrence would be updated. If “MissingNo” appeared numerous times, but the surrounding fields were different, you could match on multiple fields using the $elemMatch operator to narrow down which particular element should be updated.

More information on the $ positional operator can be found in the documentation.

Use the $[] Operator to Update All Array Elements Within a Document

Let’s say that you have an array in your document and you need to update every element in that array using a single operation. To do this, we might want to take a look at the $[] operator which does exactly that.

Using the same Pokemon video game example, let’s imagine that we have a team of Pokemon and we’ve just finished a battle in the game. The experience points gained from the battle need to be distributed to all the Pokemon on your team.

The document that represents our team might look like the following:

{
    "_id": "red",
    "team": [
        {
            "number": 1,
            "name": "Bulbasaur",
            "xp": 5
        },
        {
            "number": 25,
            "name": "Pikachu",
            "xp": 32
        }
    ]
}

At the end of the battle, we want to make sure every Pokemon on our team receives 10 XP. To do this with the $[] operator, we can construct an update operation that looks like the following:

db.pokemon_game.update(
    { "_id": "red" },
    {
        "$inc": {
            "team.$[].xp": 10
        }
    }
);

In the above example, we use the $inc modifier to increase all xp fields within the team array by a constant number. To learn more about the $inc operator, check out the documentation.

Our new document would look like this:

[
    {
        "_id": "red",
        "team": [
            {
                "number": 1,
                "name": "Bulbasaur",
                "xp": 15
            },
            {
                "number": 25,
                "name": "Pikachu",
                "xp": 42
            }
        ]
    }
]

While useful for this example, we don’t exactly get to provide criteria in case one of your Pokemon shouldn’t receive experience points. If your Pokemon has fainted, maybe they shouldn’t get the increase.

We’ll learn about filters in the next part of the tutorial.

To learn more about the $[] operator, check out the documentation.

Use the $[<identifier>] Operator to Update Elements that Match a Filter Condition

Let’s use the example that we have several array elements that we want to update in a single operation and we don’t want to worry about excessive client-side code paired with a replace operation.

To do this, we’d probably want to use the $[<identifier>] operator which acts as a placeholder to update all elements that match an arrayFilters condition.

To put things into perspective, let’s say that we’re dealing with Pokemon trading cards, instead of video games, and tracking their values. Our documents might look like this:

db.pokemon_collection.insertMany(
    [
        {
            _id: "nraboy",
            cards: [
                {
                    "name": "Charizard",
                    "set": "Base",
                    "variant": "1st Edition",
                    "value": 200000
                },
                {
                    "name": "Pikachu",
                    "set": "Base",
                    "variant": "Red Cheeks",
                    "value": 300
                }
            ]
        },
        {
            _id: "mraboy",
            cards: [
                {
                    "name": "Pikachu",
                    "set": "Base",
                    "variant": "Red Cheeks",
                    "value": 300
                },
                {
                    "name": "Pikachu",
                    "set": "McDonalds 25th Anniversary Promo",
                    "variant": "Holo",
                    "value": 10
                }
            ]
        }
    ]
);

Of course, the above snippet isn’t a document, but an operation to insert two documents into some pokemon_collection collection within MongoDB. In the above scenario, each document represents a collection of cards for an individual. The cards array has information about the card in the collection as well as the current value.

In our example, we need to update prices of cards, but we don’t want to do X number of update operations against the database. We only want to do a single operation to update the values of each of our cards.

Take the following query:

db.pokemon_collection.update(
    {},
    {
        "$set": {
            "cards.$[elemX].value": 350,
            "cards.$[elemY].value": 500000
        }
    },
    {
        "arrayFilters": [
            {
                "elemX.name": "Pikachu",
                "elemX.set": "Base",
                "elemX.variant": "Red Cheeks"
            },
            {
                "elemY.name": "Charizard",
                "elemY.set": "Base",
                "elemY.variant": "1st Edition"
            }
        ],
        "multi": true
    }
);

The above update operation is like any other, but with an extra step for our positional operator. The first parameter, which is an empty object, represents our match criteria. Because it is empty, we’ll be updating all documents within the collection.

The next parameter is the manipulation we want to do to our documents. Let’s skip it for now and look at the arrayFilters in the third parameter.

Imagine that we want to update the price for two particular cards that might exist in any person’s Pokemon collection. In this example, we want to update the price of the Pikachu and Charizard cards. If you’re a Pokemon trading card fan, you’ll know that there are many variations of the Pikachu and Charizard card, so we get specific in our arrayFilters array. For each object in the array, the fields of those objects represent an and condition. So, for elemX, which has no specific naming convention, all three fields must be satisfied.

In the above example, we are using elemX and elemY to represent two different filters.

Let’s go back to the second parameter in the update operation. If the filter for elemX comes back as true because an array item in a document matched, then the value field for that object will be set to a new value. Likewise, the same thing could happen for the elemY filter. If a document has an array and one of the filters does not ever match an element in that array, it will be ignored.

If looking at our example, the documents would now look like the following:

[
    {
        "_id": "nraboy",
        "cards": [
            {
                "name": "Charizard",
                "set": "Base",
                "variant": "1st Edition",
                "value": 500000
            },
            {
                "name": "Pikachu",
                "set": "Base",
                "variant": "Red Cheeks",
                "value": 350
            }
        ]
    },
    {
        "_id": "mraboy",
        "cards": [
            {
                "name": "Pikachu",
                "set": "Base",
                "variant": "Red Cheeks",
                "value": 350
            },
            {
                "name": "Pikachu",
                "set": "McDonalds 25th Anniversary Promo",
                "variant": "Holo",
                "value": 10
            }
        ]
    }
]

If any particular array contained multiple matches for one of the arrayFilter criteria, all matches would have their price updated. This means that if I had, say, 100 matching Pikachu cards in my Pokemon collection, all 100 would now have new prices.

More information on the $[<identifier>] operator can be found in the documentation.

Conclusion

You just saw how to use some of the positional operators within the MongoDB Query Language (MQL). These operators are useful when working with arrays because they prevent you from having to do full replaces on the array or extended client-side manipulation.

To learn more about MQL, check out my previous tutorial titled, Getting Started with Atlas and the MongoDB Query Language (MQL).

If you have any questions, take a moment to stop by the MongoDB Community Forums.

This content first appeared on MongoDB.

Original article source at: https://www.thepolyglotdeveloper.com/

#mongodb #array #elements 

Ray  Patel

Ray Patel

1619565060

Ternary operator in Python?

  1. Ternary Operator in Python

What is a ternary operator: The ternary operator is a conditional expression that means this is a comparison operator and results come on a true or false condition and it is the shortest way to writing an if-else statement. It is a condition in a single line replacing the multiline if-else code.

syntax : condition ? value_if_true : value_if_false

condition: A boolean expression evaluates true or false

value_if_true: a value to be assigned if the expression is evaluated to true.

value_if_false: A value to be assigned if the expression is evaluated to false.

How to use ternary operator in python here are some examples of Python ternary operator if-else.

Brief description of examples we have to take two variables a and b. The value of a is 10 and b is 20. find the minimum number using a ternary operator with one line of code. ( **min = a if a < b else b ) **. if a less than b then print a otherwise print b and second examples are the same as first and the third example is check number is even or odd.

#python #python ternary operator #ternary operator #ternary operator in if-else #ternary operator in python #ternary operator with dict #ternary operator with lambda

 iOS App Dev

iOS App Dev

1655889360

Element IOS: A Glossy Matrix Collaboration Client for IOS

Element iOS

Element iOS is an iOS Matrix client provided by Element. It is based on MatrixSDK. 

Beta testing

You can try last beta build by accessing our TestFlight Public Link. For questions and feedback about latest TestFlight build, please access the Element iOS Matrix room: #element-ios:matrix.org.

Build instructions

If you have already everything installed, opening the project workspace in Xcode should be as easy as:

$ xcodegen                  # Create the xcodeproj with all project source files
$ pod install               # Create the xcworkspace with all project dependencies
$ open Riot.xcworkspace     # Open Xcode

Else, you can visit our installation guide. This guide also offers more details and advanced usage like using MatrixSDK in its development version.

Contributing

If you want to contribute to Element iOS code or translations, go to the contribution guide.

Support

When you are experiencing an issue on Element iOS, please first search in GitHub issues and then in #element-ios:matrix.org. If after your research you still have a question, ask at #element-ios:matrix.org. Otherwise feel free to create a GitHub issue if you encounter a bug or a crash, by explaining clearly in detail what happened. You can also perform bug reporting (Rageshake) from the Element application by shaking your phone or going to the application settings. This is especially recommended when you encounter a crash.

Download Details:
Author: vector-im
Source Code: https://github.com/vector-im/element-ios
License: Apache-2.0 license

#swift #ios #mobileapp