16.Nested Loops

A nested loop is a loop inside another loop.The "inner loop" will be executed completely for each iteration of the "outer loop":

we can nest while loops, for loops or the combination.

Syntax

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

 

while expression: 

     while expression: 

         statement(s) 

     statement(s)

 

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

for iterating_var in sequence:

     for iterating_var in sequence: 

         statements(s) 

     statements(s) 

Note: indentation is very important to identify the code blocks

Example:

i=1

while i<=3:

   j=1

   while j<=i:

      print(j,end=' ')            # inner while loop

      j=j+1

   print()

   i=i+1

here the inner while loop will execute 'i '  times and  'i'  will very from 1 to 3 in the outer loop.So it will print

1

1 2

1 2 3

In the same way for loop can also be nested.The above program can be written using nested for loop as follows

for i in range(1,4):

    for j in range(1,i+1):

        print (j,end=' ')

    print()

Note: Nested loops are widely used for processing lists, matrix, sorting etc..