Learn how to implement For Loops in Python for iterating a sequence, or the rows and columns of a pandas dataframe.
Like other programming languages, for
loops in Python are a little different in the sense that they work more like an iterator and less like a for
keyword. In Python, there is not C
like syntax for(i=0; i<n; i++)
but you use for in n
.
They can be used to iterate over a sequence of a list
, string
, tuple
, set
, array
, data frame
.
Given a list of elements, for
loop can be used to iterate over each item in that list and execute it.
To iterate over a series of items For loops use the range
function. The range
function returns a new list with numbers of that specified range based on the length of the sequence.
While iterating over a sequence you can also use the index of elements in the sequence to iterate, but the key is first to calculate the length of the list and then iterate over the series within the range of this length.
The for
loops in Python are zero-indexed.
Let’s quickly jump onto the implementation part of it.
To start with, let’s print numbers ranging from 1-10. Since the for
loops in Python are zero-indexed you will need to add one in each iteration; otherwise, it will output values from 0-9.
for i in range(10):
print (i+1)
1
2
3
4
5
6
7
8
9
10
Let’s iterate over a string of a word Datacamp
using for loop and only print the letter a
.
for i in "Datacamp":
if i == 'a':
print (i)
a
a
a
Let’s say you want to define a list of elements and iterate over those elements one by one.
sequence = [1,2,8,100,200,'datacamp','tutorial']
for i in sequence:
print (i)
1
2
8
100
200
datacamp
tutorial
But what if you want to find the length of the list and then iterate over it? You will use the in-built len
function for it and then on the length output you will apply range
.
Remember, the range
always expects an integer value.
for i in range(len(sequence)):
print (sequence[i])
1
2
8
100
200
datacamp
tutorial
Great! But why do you need to use the len
function when you can directly use for i in numbers
? The answer is simple. What if you would like to modify or work with the indices of the sequence like changing the element of an existing list, then you would need range(len(sequence))
.
for i in range(len(sequence)):
element = sequence[i]
if type(element) == int:
sequence[i] = element + 4
sequence
5, 6, 12, 104, 204, 'datacamp', 'tutorial']
Cool, isn’t it? You were able to modify the elements of the list based on the if
condition.
Let’s now see how you can print odd numbers between 1 - 20. To accomplish this, you will have to define three things in the range
function. The starting point, the ending point and the increment value (or steps) at which the loop will increment over the numbers 1 - 20.
for i in range(1,20,2):
print (i)
1
3
5
7
9
11
13
15
17
19
for i in range(11):
for j in range(i):
print (i, end=' ')
print()
1
2 2
3 3 3
4 4 4 4
5 5 5 5 5
6 6 6 6 6 6
7 7 7 7 7 7 7
8 8 8 8 8 8 8 8
9 9 9 9 9 9 9 9 9
10 10 10 10 10 10 10 10 10 10
import pandas as pd
iris = pd.read_csv('https://raw.githubusercontent.com/mwaskom/seaborn-data/master/iris.csv')
iris.head()
len(iris)
150
for i in range(len(iris)):
Class = iris.iloc[i,4]
if Class == 'versicolor' and i < 70:
print (Class)
versicolor
versicolor
versicolor
versicolor
versicolor
versicolor
versicolor
versicolor
versicolor
versicolor
versicolor
versicolor
versicolor
versicolor
versicolor
versicolor
versicolor
versicolor
versicolor
versicolor
Next, let’s add two to every row of columns sepal_length
,sepal_width
, petal_length
and petal_width
.
columns = ['sepal_length','sepal_width','petal_length','petal_width']
for indices, row in iris.iterrows():
for column in columns:
iris.at[indices,column] = row[column] + 2
iris.head()
Python’s lambda function is fast and powerful as compared to the basic for
loop. It is widely used, especially when dealing with Dataframes. You can process your data with the help of Lambda
function with very little code. Although, it sometimes becomes difficult to understand it.
x = [20, 30, 40, 50, 60]
y = []
for v in x :
y += [v * 5]
y
[100, 150, 200, 250, 300]
Now, let’s try this with a lambda and map function.
Map
takes in a function, for example, a lambda function and a sequence x
and then returns a new sequence.
y = map(lambda x: x * 5,x)
y
<map at 0x11be7cc88>
It returns a generator function and to get the output from the generator; you pass the output as an argument to list
.
list(y)
[100, 150, 200, 250, 300]
Now, let’s take a step back and look at both the for
loop way and lambda/map() combination. You will notice that the difference between the two is adding map
, lambda
, and removal of “for” and “in”. And also, within one line, you were able to code it.
Congratulations on finishing this basic Python For loop tutorial.
For loops are the backbone of every programming language and when it is Python, using For loops are not at all hard to code, and they are similar in spirit to writing an English sentence.
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