Sofia Kelly

Sofia Kelly

1572582002

30 Days of Vue - Methods and Computed Properties

Methods in a Vue instance behave like normal JavaScript functions and are evaluated only when explicitly called. Instead of using methods we could always write our intended functionality change inline in the template.

Let’s see an example of this. We’ll reuse an example seen in the Vue documentation that involves reversing a series of characters from a string. We’ll first create a Vue instance that contains a single message property that has a value of 'Greetings!':

new Vue({
  el: '#app',
  data: {
    message: 'Greetings!',
  },
});

In the template, we’ll look to bind the message directly and also bind the message in its reversed state. We’ll reverse the value of message by splitting the property into an array of characters (.split('')), reversing the elements in the array (.reverse()), and rejoining the reversed array back into a single string (.join('')).

<html>
  <head>
    <link rel="stylesheet" href="./styles.css" />
    <link rel="stylesheet"
      href="https://unpkg.com/bulma/css/bulma.css" />
  </head>
  <body>
    <div id="app">
      <div class="card">
        <header class="card-header card-header-title">
          <span>Original:</span>
          {{ message }}
        </header>
        <header class="card-header card-header-title">
          <span>Reversed:</span>
          {{ message.split('').reverse().join('') }}
        </header>
      </div>
    </div>
    <script src="https://unpkg.com/vue"></script>
    <script src="./main.js"></script>
  </body>
</html>

With the help of the styling given to us by Bulma, our simple app will look like the following:

This is image title

There’s nothing inherently wrong with specifying functionality change, like the above, inline. However, methods are often times more appropriate to use when the intended changes get harder to decipher.

We can change the above example to instead use a method in our Vue instance to help reverse the message string:

new Vue({
  el: '#app',
  data: {
    message: 'Greetings!',
  },
  methods: {
    reverseString(string) {
      return string.split('').reverse().join('');
    },
  }
});

The method is given a name of reverseString and expects a payload. We can declare this method in the template and pass in the message property as the payload:

<html>
  <head>
    <link rel="stylesheet" href="./styles.css" />
    <link rel="stylesheet"
      href="https://unpkg.com/bulma/css/bulma.css" />
  </head>
  <body>
    <div id="app">
      <div class="card">
        <header class="card-header card-header-title">
          <span>Original:</span>
          {{ message }}
        </header>
        <header class="card-header card-header-title">
          <span>Reversed:</span>
          {{ reverseString(message) }}
        </header>
      </div>
    </div>
    <script src="https://unpkg.com/vue"></script>
    <script src="./main.js"></script>
  </body>
</html>

Our UI would behave just the way it had before by displaying the message greeting and the reversed version right below it:

This is image title

Functionality wise — the above two examples achieve the same thing. Methods might be seen to be more appropriate since it keeps the template cleaner and easier to understand.

We’re also able to achieve the same outcome as above with the use of another property — called the computed property.

Computed Properties

Computed properties are used to handle complex calculations of information that need to be displayed in the view. For our third iteration in building the same simple app, we’ll introduce a computed property called reverseMessage that simply reverses the message data property as we’ve done before:

new Vue({
  el: '#app',
  data: {
    message: 'Greetings!',
  },
  computed: {
    reverseMessage() {
      return this.message.split('').reverse().join('');
    },
  }
});

In the template, we can render the value of the reverseMessage computed property just as we would have rendered any other data property:

<html>
  <head>
    <link rel="stylesheet" href="./styles.css" />
    <link rel="stylesheet"
      href="https://unpkg.com/bulma/css/bulma.css" />
  </head>
  <body>
    <div id="app">
      <div class="card">
        <header class="card-header card-header-title">
          <span>Original:</span>
          {{ message }}
        </header>
        <header class="card-header card-header-title">
          <span>Reversed:</span>
          {{ reverseMessage }}
        </header>
      </div>
    </div>
    <script src="https://unpkg.com/vue"></script>
    <script src="./main.js"></script>
  </body>
</html>

With this, our app will behave as desired:

This is image title
This begs the question, what difference is there to using a computed property or having a method instead return a value?

Methods vs. Computed Properties

In the examples above, using a method or a computed property pretty much achieved the exact same outcome. The key difference in using computed properties is that computed properties are cached based on the dependencies they depend on.

If we take a look at the reverseMessage computed property we’ve declared, we can see it has one data dependency - the message property.

computed: {
  reverseMessage() {
    return this.message.split('').reverse().join('');
  },
}

The value of reverseMessage directly depends on the message data property. When the value of message changes, so does reverseMessage. Computed properties are useful because as long as the dependant data property (message) remains constant (i.e. unchanged), calling the computed property (reverseMessage) multiple times will always return the same cached value.

Let’s see a simple example of this visually. We can place a console.log()message in the computed property function to alert us when the function has been run:

computed: {
  reverseMessage() {
    console.log('computed function is run!');
    return this.message.split('').reverse().join('');
  },
}

In the template, we can aim to render the reverseMessage computed property a couple of times:

<div id="app">
  <div class="card">
    <header class="card-header card-header-title">
      <span>Original:</span>
      {{ message }}
    </header>
  </div>

  <div class="card">
    <header class="card-header card-header-title">
      <span>Reversed:</span>
      {{ reverseMessage }}
    </header>
  </div>

  <div class="card">
    <header class="card-header card-header-title">
      <span>Reversed:</span>
      {{ reverseMessage }}
    </header>
  </div>

  <div class="card">
    <header class="card-header card-header-title">
      <span>Reversed:</span>
      {{ reverseMessage }}
    </header>
  </div>
</div>

By running the application and opening our browser console, we’ll see the console.log() message logged only once:

This is image title

The first time the reverseMessage property is computed, its value is cached. With every other call to render the value of reverseMessage, the messageproperty hasn’t changed, so the cached result is simply returned without running the computed function again.

If we repeat a similar example but instead call methods multiple times in the template, the console.log() message will be run every single time the method is declared:

This is image title

In conclusion, though methods can be used in place of computed properties; computed properties should essentially be used if we intend to compute a value from a data property. Caching can help our application with performance once our application starts to have countless properties with each derived functionality potentially being somewhat computationally expensive.

Here’s a table that highlights the main differences between using methods or computed properties:

This is image title

A good rule of thumb to follow:

  • Use methods when responding to changes (e.g. clicking a button, submitting a form, etc.) or to run explicit functionality change within the instance (e.g. have a method be called from a lifecycle hook).
  • Use computed properties for data manipulation (e.g. create a sorted array from an unsorted array in the instance).

If you have any questions whatsoever, feel free to leave a comment and I’ll answer as best as I can. I’m always available on Twitter as well - @djirdehh!

You can find all the code samples of today in the Github Repo - fullstackio/30-days-of-vue.

If you’re brand new to Vue.js and are interested in diving into other topics -feel free to check out https://www.fullstack.io/30-days-of-vue/.

#vue #vuejs #javascript

What is GEEK

Buddha Community

30 Days of Vue - Methods and Computed Properties
Luna  Mosciski

Luna Mosciski

1600583123

8 Popular Websites That Use The Vue.JS Framework

In this article, we are going to list out the most popular websites using Vue JS as their frontend framework.

Vue JS is one of those elite progressive JavaScript frameworks that has huge demand in the web development industry. Many popular websites are developed using Vue in their frontend development because of its imperative features.

This framework was created by Evan You and still it is maintained by his private team members. Vue is of course an open-source framework which is based on MVVM concept (Model-view view-Model) and used extensively in building sublime user-interfaces and also considered a prime choice for developing single-page heavy applications.

Released in February 2014, Vue JS has gained 64,828 stars on Github, making it very popular in recent times.

Evan used Angular JS on many operations while working for Google and integrated many features in Vue to cover the flaws of Angular.

“I figured, what if I could just extract the part that I really liked about Angular and build something really lightweight." - Evan You

#vuejs #vue #vue-with-laravel #vue-top-story #vue-3 #build-vue-frontend #vue-in-laravel #vue.js

Sofia Kelly

Sofia Kelly

1572582002

30 Days of Vue - Methods and Computed Properties

Methods in a Vue instance behave like normal JavaScript functions and are evaluated only when explicitly called. Instead of using methods we could always write our intended functionality change inline in the template.

Let’s see an example of this. We’ll reuse an example seen in the Vue documentation that involves reversing a series of characters from a string. We’ll first create a Vue instance that contains a single message property that has a value of 'Greetings!':

new Vue({
  el: '#app',
  data: {
    message: 'Greetings!',
  },
});

In the template, we’ll look to bind the message directly and also bind the message in its reversed state. We’ll reverse the value of message by splitting the property into an array of characters (.split('')), reversing the elements in the array (.reverse()), and rejoining the reversed array back into a single string (.join('')).

<html>
  <head>
    <link rel="stylesheet" href="./styles.css" />
    <link rel="stylesheet"
      href="https://unpkg.com/bulma/css/bulma.css" />
  </head>
  <body>
    <div id="app">
      <div class="card">
        <header class="card-header card-header-title">
          <span>Original:</span>
          {{ message }}
        </header>
        <header class="card-header card-header-title">
          <span>Reversed:</span>
          {{ message.split('').reverse().join('') }}
        </header>
      </div>
    </div>
    <script src="https://unpkg.com/vue"></script>
    <script src="./main.js"></script>
  </body>
</html>

With the help of the styling given to us by Bulma, our simple app will look like the following:

This is image title

There’s nothing inherently wrong with specifying functionality change, like the above, inline. However, methods are often times more appropriate to use when the intended changes get harder to decipher.

We can change the above example to instead use a method in our Vue instance to help reverse the message string:

new Vue({
  el: '#app',
  data: {
    message: 'Greetings!',
  },
  methods: {
    reverseString(string) {
      return string.split('').reverse().join('');
    },
  }
});

The method is given a name of reverseString and expects a payload. We can declare this method in the template and pass in the message property as the payload:

<html>
  <head>
    <link rel="stylesheet" href="./styles.css" />
    <link rel="stylesheet"
      href="https://unpkg.com/bulma/css/bulma.css" />
  </head>
  <body>
    <div id="app">
      <div class="card">
        <header class="card-header card-header-title">
          <span>Original:</span>
          {{ message }}
        </header>
        <header class="card-header card-header-title">
          <span>Reversed:</span>
          {{ reverseString(message) }}
        </header>
      </div>
    </div>
    <script src="https://unpkg.com/vue"></script>
    <script src="./main.js"></script>
  </body>
</html>

Our UI would behave just the way it had before by displaying the message greeting and the reversed version right below it:

This is image title

Functionality wise — the above two examples achieve the same thing. Methods might be seen to be more appropriate since it keeps the template cleaner and easier to understand.

We’re also able to achieve the same outcome as above with the use of another property — called the computed property.

Computed Properties

Computed properties are used to handle complex calculations of information that need to be displayed in the view. For our third iteration in building the same simple app, we’ll introduce a computed property called reverseMessage that simply reverses the message data property as we’ve done before:

new Vue({
  el: '#app',
  data: {
    message: 'Greetings!',
  },
  computed: {
    reverseMessage() {
      return this.message.split('').reverse().join('');
    },
  }
});

In the template, we can render the value of the reverseMessage computed property just as we would have rendered any other data property:

<html>
  <head>
    <link rel="stylesheet" href="./styles.css" />
    <link rel="stylesheet"
      href="https://unpkg.com/bulma/css/bulma.css" />
  </head>
  <body>
    <div id="app">
      <div class="card">
        <header class="card-header card-header-title">
          <span>Original:</span>
          {{ message }}
        </header>
        <header class="card-header card-header-title">
          <span>Reversed:</span>
          {{ reverseMessage }}
        </header>
      </div>
    </div>
    <script src="https://unpkg.com/vue"></script>
    <script src="./main.js"></script>
  </body>
</html>

With this, our app will behave as desired:

This is image title
This begs the question, what difference is there to using a computed property or having a method instead return a value?

Methods vs. Computed Properties

In the examples above, using a method or a computed property pretty much achieved the exact same outcome. The key difference in using computed properties is that computed properties are cached based on the dependencies they depend on.

If we take a look at the reverseMessage computed property we’ve declared, we can see it has one data dependency - the message property.

computed: {
  reverseMessage() {
    return this.message.split('').reverse().join('');
  },
}

The value of reverseMessage directly depends on the message data property. When the value of message changes, so does reverseMessage. Computed properties are useful because as long as the dependant data property (message) remains constant (i.e. unchanged), calling the computed property (reverseMessage) multiple times will always return the same cached value.

Let’s see a simple example of this visually. We can place a console.log()message in the computed property function to alert us when the function has been run:

computed: {
  reverseMessage() {
    console.log('computed function is run!');
    return this.message.split('').reverse().join('');
  },
}

In the template, we can aim to render the reverseMessage computed property a couple of times:

<div id="app">
  <div class="card">
    <header class="card-header card-header-title">
      <span>Original:</span>
      {{ message }}
    </header>
  </div>

  <div class="card">
    <header class="card-header card-header-title">
      <span>Reversed:</span>
      {{ reverseMessage }}
    </header>
  </div>

  <div class="card">
    <header class="card-header card-header-title">
      <span>Reversed:</span>
      {{ reverseMessage }}
    </header>
  </div>

  <div class="card">
    <header class="card-header card-header-title">
      <span>Reversed:</span>
      {{ reverseMessage }}
    </header>
  </div>
</div>

By running the application and opening our browser console, we’ll see the console.log() message logged only once:

This is image title

The first time the reverseMessage property is computed, its value is cached. With every other call to render the value of reverseMessage, the messageproperty hasn’t changed, so the cached result is simply returned without running the computed function again.

If we repeat a similar example but instead call methods multiple times in the template, the console.log() message will be run every single time the method is declared:

This is image title

In conclusion, though methods can be used in place of computed properties; computed properties should essentially be used if we intend to compute a value from a data property. Caching can help our application with performance once our application starts to have countless properties with each derived functionality potentially being somewhat computationally expensive.

Here’s a table that highlights the main differences between using methods or computed properties:

This is image title

A good rule of thumb to follow:

  • Use methods when responding to changes (e.g. clicking a button, submitting a form, etc.) or to run explicit functionality change within the instance (e.g. have a method be called from a lifecycle hook).
  • Use computed properties for data manipulation (e.g. create a sorted array from an unsorted array in the instance).

If you have any questions whatsoever, feel free to leave a comment and I’ll answer as best as I can. I’m always available on Twitter as well - @djirdehh!

You can find all the code samples of today in the Github Repo - fullstackio/30-days-of-vue.

If you’re brand new to Vue.js and are interested in diving into other topics -feel free to check out https://www.fullstack.io/30-days-of-vue/.

#vue #vuejs #javascript

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 

Teresa  Bosco

Teresa Bosco

1598685221

Vue File Upload Using vue-dropzone Tutorial

In this tutorial, I will show you how to upload a file in Vue using vue-dropzone library. For this example, I am using Vue.js 3.0. First, we will install the Vue.js using Vue CLI, and then we install the vue-dropzone library. Then configure it, and we are ready to accept the file. DropzoneJS is an open source library that provides drag and drops file uploads with image previews. DropzoneJS is lightweight doesn’t depend on any other library (like jQuery) and is  highly customizable. The  vue-dropzone is a vue component implemented on top of Dropzone.js. Let us start Vue File Upload Using vue-dropzone Tutorial.

Vue File Upload Using vue-dropzone

First, install the Vue using Vue CLI.

#vue #vue-dropzone #vue.js #dropzone.js #dropzonejs #vue cli