To execute statement/s multiple times, loops are used. Python loops:

• Python While Loop
• Python For Loop
• Python Nested Loops

While Loop

• Condition is tested before executing the loop body.
• The condition may be any expression, and any non-zero value is considered as true
• The loop iterates while the condition is true
• When the condition becomes false, program control passes to the line immediately next to the loop

Syntax

while expression:
   statement/s

Ex:

counter = 5
while (counter > 0):
   print("The counter value is: ", counter)
   counter = counter - 1
print("Tata")

OUTPUT:

The counter value is:  5
The counter value is:  4
The counter value is:  3
The counter value is:  2
The counter value is:  1
Tata

In the above program the condition is true until counter value becomes 1, so it enters into the loop body until counter value is > 0. Once counter=0 it executes statement next to the loop body i.e. print(“Tata”).

For Loop

Executes statement/s for fixed number of times.

• If a sequence contains an expression list, it is evaluated first
• Then, the first item in the sequence is assigned to the iterating variable (iter_var: please refer Syntax below)
• Then, the statement/s are executed.
• Each item in the list is assigned to iterating variable (iter_var), and the statement/s are executed until the entire sequence is exhausted

Syntax

for iter_var in sequence:
   statements/s

Here, sequence can be:

• String
• List
• List of List
• Range

Ex:

print("From String as sequence Loop")
for character in "Python":     # String as sequence
   print("Current character in string is: ", character)

list1 = ["Jonny", "Bablu", "Raju"]
print("From List as sequence Loop")
for boy in list1:             # List as sequence
   print(" Current list item is: ", boy)

list2 = ["Jonny", "Bablu", "Raju"]
print("From Iterating by sequence index")
for index in range(len(list2)):            # Iterating by sequence index
   print(" Current list item is: ", list2[index])

list_of_lists = [ [1, 2, 3], [4, 5, 6], [7, 8, 9]]
print("From List of Lists as sequence Loop")
for list1 in list_of_lists:    # List of Lists as sequence
    for item1 in list1:        # Nested for loop
        print(item1)
         
print("From range as sequence Loop")
for num in range(0, 3):          # range as sequence
    print("Hello world %d", num)

print("Tata")

OUTPUT:

From String as sequence Loop
Current character in string is:  P
Current character in string is:  y
Current character in string is:  t
Current character in string is:  h
Current character in string is:  o
Current character in string is:  n
From List as sequence Loop
Current list item is:  Jonny
Current list item is:  Bablu
Current list item is:  Raju
From Iterating by sequence index
Current list item is:  Jonny
Current list item is:  Bablu
Current list item is:  Raju
From List of Lists as sequence Loop
1
2
3
4
5
6
7
8
9
From range as sequence Loop
Hello world %d 0
Hello world %d 1
Hello world %d 2
Tata

Nested Loops

Python allows to use nested loops. One or more loop can be used inside any another while or for loop.

• Nested For Loop
  Syntax:

  for iter_var in sequence:
     for iter_var in sequence:
        statement/s
     statement/s
  

  Ex:

  list_of_lists = [ [1, 2, 3], [4, 5, 6], [7, 8, 9]]
  for list1 in list_of_lists:    
      for item1 in list1:        # Nested for loop
          print(item1)
  

• Nested While Loop
  Syntax:

  while expression:
     while expression:
        statement/s
     statement/s
  

• Nested For and While Loop
  Syntax:

  while expression:
        statement/s
        for iter_var in sequence:
           statement/s
        statement/s
  statement/s
  

• Nested While and For Loop
  Syntax:

  for iter_var in sequence:
       statement/s
       while expression:
          statement/s
       statement/s 
  statement/s