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Loops: For loop, while loop

Code can be repeated using a loop. Lines of code can be repeated N times, where N is manually configurable. In practice, it means code will be repeated until a condition is met. This condition is usually (x >=N) but it’s not the only possible condition.

Python has 3 types of loops: for loops, while loops and nested loops.
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For loop


We can iterate a list using a for loop

#!/usr/bin/python

items = [ "Abby","Brenda","Cindy","Diddy" ]

for item in items:
    print(item)

Visualization of for loop:
for loop

The for loop can be used to repeat N times too:

#!/usr/bin/python

for i in range(1,10):
    print(i)

While loop


If you are unsure how many times a code should be repeated, use a while loop.
For example,


correctNumber = 5
guess = 0

while guess != correctNumber:
    guess = int(input("Guess the number: "))
    
    if guess != correctNumber:
        print('False guess')

print('You guessed the correct number')

Nested loops


We can combine for loops using nesting. If we want to iterate over an (x,y) field we could use:

#!/usr/bin/python

for x in range(1,10):
    for y in range(1,10):
        print("(" + str(x) + "," + str(y) + ")")

Nesting is very useful, but it increases complexity the deeper you nest.

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21 thoughts on “Loops: For loop, while loop”


  1. Sagi Sulimani
    - April 22, 2017

    Hi Frank.
    There is a little mistake in the “While loop” example. It should be:

        guess = int(input("Guess the number: "))

    Otherwise, the compiler will compare between string and integer, and ‘5’!=5.
    Except it – excellent!

    1. Frank
      - April 22, 2017

      Thanks Sagi!
      I’ve updated the article.

  2. Prashant Raina
    - January 28, 2016

    whats wrong with this code not generating primes as i expected

    i=3
    num=int(input("enter the no. upto which u want the primes: "))
    arr=[2]
    print(2)
    while i<num:
        j=0
        while j<len(arr):
            if i%arr[j]==0:
                i=i+1
                j=len(arr)
            else:
                j=j+1
                print i
                arr[len(arr)]=i

    1. Frank
      - January 30, 2016

      The conditions in the while loops are never true. I fixed your code:

      num=int(input("enter the no. upto which u want the primes: "))
       arr=[2]
      print(arr)
       i = 2
      while max(arr) < num:
          prime = True
         for p in arr:
             if i % p == 0:
                 prime = False
                 break
          if prime:
             arr.append(i)
          i = i + 1  
          print(arr)
       print(arr)

      You can see the prime numbers being added to the array in the loop.

  3. Deepak V a
    - December 3, 2015

    why are you using str() in print statement i am getting even i used variable directly

    1. Frank
      - December 3, 2015

      sometimes that may lead to problems as they have different datatypes,just being cautious.

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