for Loop in Python Programming

Loops in Python

What is for Loop

A for loop in Python is a repetition control structure which allows us to write a loop that is executed a specific number of times. Python for loop can be used in a list, tuple, set, string, etc.

for x in sequence:
    # body of the for loop

Here x is a variable which takes values from a sequence. A sequence can be a tuple, set, string or range() function.

Let’s see some examples for more understanding.

# List of numbers
num=[10,12,8,25,82,64]

for x in num:
    print(x)

10
12
8
25
82
64

In the above example, we have run a for loop. Each time when the loop runs, it reads a number from the num list in variable x one by one, and then we print those numbers on the screen using the print() function.

fruit='Pineapple'

for x in fruit:
    print(x)

P
i
n
e
a
p
p
l
e

In the above example, we have run a for loop. Each time when the loop runs, it reads a character from the string variable fruit in variable x one by one, and then we print those characters on the screen using the print() function.

Use of range() Function in for Loop

Python range() function is used to generate a sequence of numbers. It is commonly used in Python for loop and Python while loop. The range() function accept only integer value as its parameters.

range(start, stop, step)

• start - The start value of the Python range() function is 0 by default if not specified.
• stop - The stop value of the Python range() function is the number before which the sequence of numbers will generate. The last value of the number's sequence is always stopped value -1. For example range(1, 4) will generate number as 1, 2, and 3.
• step - The step value of the Python range() function specifies the increment or decrement order of the generated numbers. By default the step value is 1 if not specified. For example range(1, 10, 2) will generate number as 1, 3, 5, 7 and 9 with an increment value of 2 between the numbers.

Let’s see some examples for more understanding.

for x in range(1, 11):
    print(x)

1
2
3
4
5
6
7
8
9
10

In the above example, we have run a for loop from 1 to 11 using the range() function. Each time when the loop runs, we print the value of the variable x on the screen on a separate line using the print() function. The loop ends when the value of x is more than 10.

for x in range(10, 21, 2):
    print(x)

10
12
14
16
18
20

In the above example, we have run a for loop from 10 to 21 using the range() function. Each time when the loop runs, we print the value of the variable x on the screen on a separate line using the print() function and then the value of x is incremented by 2. The loop ends when the value of x is more than 20.

for x in range(10, 0, -1):
    print(x)

10
9
8
7
6
5
4
3
2
1

In the above example, we have run a for loop from 10 to 0 in reverse order using range() function. Each time when the loop runs, we print the value of the variable x on the screen on a separate line using the print() function and then the value of x is decremented by -1. The loop ends when the value of x is less than 1.

Nested for Loop

Python programming allows using one for loop inside another for loop. This is known as nested for loop.

for x in sequence:
    for y in sequence:

for x in range(1,6):
    for y in range(1, x+1):
        print(y,end='')
    print()

1
12
123
1234
12345

In the above example, we have run two loops, one outer loop and another inner loop. The outer loop runs from 1 to 5, and the inner loop runs from 1 to the current value of the outer loop + 1 (for example, when the value of x is 1, the inner loop runs from 1 to 2. When the value of x is 2, the inner loop runs from 1 to 3 and so on). The program ends when the value of x is more than 5.

Python Infinite for Loop

Python infinite for loop is a loop that never ends. It keeps on executing unless and until we terminate the loop using the break statement. In Python, we need to import the itertools module to use an infinite loop.

import itertools

for x in itertools.repeat(1):
    # body of the infinite for loop

import itertools

i=0
for x in itertools.repeat(1):
    print('Hello World')

    i=i+1
    if i==5:
        break       # terminate the infinite loop when the value of i is 5

Hello World
Hello World
Hello World
Hello World
Hello World

In the above example, we have run an infinite for loop using the itertools.repeat(1) statement. This statement will repeat the value of x to 1 always, which results in an infinite for loop that never ends. To terminate an infinite for loop we have to use a break statement as shown above in the example.