Ways to increment Iterator from inside the For loop in Python

For loops, in general, are used for sequential traversal. It falls under the category of definite iteration. Definite iterations mean the number of repetitions is specified explicitly in advance. But have you ever wondered, what happens, if you try to increment the value of the iterator from inside the for loop. Let’s see with the help of the below example.
Example:

Python3

lis = [1, 2, 3, 4, 5]

for i in range(len(lis)):

    print(lis[i])
    i += 2

Output:

1
2
3
4
5

The above example shows this odd behavior of the for loop because the for loop in Python is not a convention C style for loop, i.e., for (i=0; i<n; i++) rather it is a for in loop which is similar to for each loop in other languages. However, there are few methods by which we can control the iteration in the for loop. Some of them are –

• Using While loop: We can’t directly increase/decrease the iteration value inside the body of the for loop, we can use while loop for this purpose.
  Example:
  

Python

# Using while loop

lis = [1, 2, 3, 4, 5]
i = 0

while(i < len(lis)):
    print(lis[i], end = " ")

    # Changing the value of
    # i inside the loop will
    # change it's value at the
    # time of checking condition
    i += 2

Output: 
 1 3 5

Time complexity: O(n/2) = O(n), where n is the length of the list.

Auxiliary space: O(1), as we are not using any extra data structure, only one variable (i) is being used.

• Using another variable: We can use another variable for the same purpose because after every iteration the value of loop variable is re-initialized.
  Example:
  

Python

# Using for loop

lis = [1, 2, 3, 4, 5]

i = 0

for j in range(len(lis)):

    # Terminating condition for i
    if(i >= len(lis)):
        break

    print(lis[i], end = " ")
    i += 2

• Output: 
  

1 3 5

• Using Range Function: We can use the range function as the third parameter of this function specifies the step.
  Note: For more information, refer to Python range() Function.
  Example:
  

Python3

# Using for loop

lis = [1, 2, 3, 4, 5]

for i in range(0, len(lis), 2):

    print(lis[i], end = " ")

• Output:
  

1 3 5