Python Nested Loops

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In Python, when you write one or more loops within a loop statement that is known as a nested loop. The main loop is considered as outer loop and loop(s) inside the outer loop are known as inner loops.

Python programming language allows to use one loop inside another loop. Following section shows few examples to illustrate the concept for nested for loop and while loop.

Python Nested for Loop

The for loop having one or more inner for loops is nested for loop.

Python Nested for Loop Syntax

The syntax for a Python nested foor loop statement in Python programming language is as follows −

for iterating_var in sequence:
   for iterating_var in sequence:
      statements(s)
   statements(s)

Python Nested for Loop Example

The following program uses a nested for loop to find the prime numbers from 2 to 100 −

#!/usr/bin/python

months = ["jan", "feb", "mar"]
days = ["sun", "mon", "tue"]


for x in months:
  for y in days:
    print(x, y)

print "Good bye!"

When the above code is executed, it produces following result −

('jan', 'sun')
('jan', 'mon')
('jan', 'tue')
('feb', 'sun')
('feb', 'mon')
('feb', 'tue')
('mar', 'sun')
('mar', 'mon')
('mar', 'tue')
Good bye!

Python Nested while Loop

The while loop having one or more inner while loops are nested while loop.

Python Nested while Loop Syntax

The syntax for a nested while loop statement in Python programming language is as follows −

while expression:
   while expression:
      statement(s)
   statement(s)

A final note on loop nesting is that you can put any type of loop inside of any other type of loop. For example a for loop can be inside a while loop or vice versa.

Python Nested while Loop Example

The following program uses a nested for loop to find the prime numbers from 2 to 100 −

#!/usr/bin/python

i = 2
while(i < 100):
   j = 2
   while(j <= (i/j)):
      if not(i%j): break
      j = j + 1
   if (j > i/j) : print i, " is prime"
   i = i + 1

print "Good bye!"

When the above code is executed, it produces following result −

2 is prime
3 is prime
5 is prime
7 is prime
11 is prime
13 is prime
17 is prime
19 is prime
23 is prime
29 is prime
31 is prime
37 is prime
41 is prime
43 is prime
47 is prime
53 is prime
59 is prime
61 is prime
67 is prime
71 is prime
73 is prime
79 is prime
83 is prime
89 is prime
97 is prime
Good bye!