There are situations in programming when you want to repeat some tasks. In this condition, although we can write the same statements multiple times, this is better managed using loops. Loops are basically a mechanism to repeat certain statements multiple times.

Overview

In Python, we mainly have two types of loops.

1. While loop
2. For loop

Let’s see some examples of these looping structures.

While Loop

The syntax for while loop looks like this.

1# initialize counter
2while condition:
3    # ... ...
4    # statements of while block
5    # ... ...
6    # Modify counter variable

flowchart TD
    A[Start] -->|Initialize variable| B{Condition Check}
    B -->|fa:fa-check True| C[Statements in While]
    C --> |fa:fa-repeat Check Again| B{Condition}
    B -->|fa:fa-xmark False| E[Outside While]
    C --> E[Outside While]
    E --> F[End]

flowchart TD
    A[Start] -->|Initialize variable| B{Condition Check}
    B -->|fa:fa-check True| C[Statements in While]
    C --> |fa:fa-repeat Check Again| B{Condition}
    B -->|fa:fa-xmark False| E[Outside While]
    C --> E[Outside While]
    E --> F[End]

First we initialize a variable which will be used for iterating in while loop. In the while statement, we check for a condition. If this condition holds True, we execute statements inside the while loop. At the end of the while block, we check for the condition again. When this condition check becomes False, the execution moves outside the while block.

1i = 0
2while i < 5:
3    print(i)
4    i += 1

Output

0
1
2
3
4

There is one problem with while loop usually. If we forget to modify the counter variable inside the while block, this will result in Infinite loop. It will basically keep repeating indefinitely.

While loop is particularly useful when we do not know upfront how many times to iterate. For example, when we want to act based on user input, when user enters certain word, we want to quit, until then we want to keep repeating the statements of the while loop.

For Loop

For loop is very convenient way to loop through any sequence. A simple for..in loop looks like this.

1for item in sequence:
2    # for loop block

Let’s take an example, where we iterate through each character of a word.

1str = 'Hello'
2for character in str:
3    print(character)

This will print all characters of a string “Hello”. Output

H
e
l
l
o

We will revisit this loop again when we learn about lists and other collection data types.

Iterating using Range

If we want to iterate certain number of times to perform something, we can use range function. This returns immutable sequence using Range object. Range function takes three arguments range(start, stop, step).

1for number in range(0, 5):
2    print(number)

Output

0
1
2
3
4

The default value for start is 0 and step1 is 1. That’s why we didn’t have to specify step. Just like step, we can ignore start. Below code produces same output.

1for number in range(5):
2    print(number)

We can also iterate through only odd numbers using step value of 2.

1for number in range(0, 10, 2):
2    print(number)

We also have else block for both loops. This gets executed when the condition becomes False first time.

1for i in range(3,0,-1):
2    print(i)
3else:
4    print("Go!")

Output

3
2
1
Go!

With For loop, we don’t have to track the loop variable and hence we do not have to worry about infinite loops.