# 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:

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 = 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.

## 19 thoughts on “Loops: For loop, while loop”

1. - 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&lt;num: j=0 while j&lt;len(arr): if i%arr[j]==0: i=i+1 j=len(arr) else: j=j+1 print i arr[len(arr)]=i```
1. - 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.

2. - December 3, 2015

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

1. - December 3, 2015

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

3. - August 31, 2015

From where I should start I can’t understand? How to save, How to edit, How to run, These are my problems.

1. - August 31, 2015

Hi Amina, start from here: https://pythonspot.com/getting-started/