Loops In Python

For Loops

  • The for loop is used to iterate over items and execute code on each item.

    • It has two keywords, for and in, which are used to describe the element and the object that is being iterated over, respectively.

    • The indentation after : starts the body of the loop.

  • In the example below, the for loop is iterating over the list nums. For each item in num, it is printing the output of num + 1.

numbers = [2, 4, 6, 8, 10, 12, 14]

for number in numbers:
   print (num**2)
nums = [1, 2, 3, 4, 5]
 
for num in nums:
  print(num + 1)

This will output:

2
3
4
5
6

For loops with Range()

  • The range() function can be used with the for loop to execute a block of code multiple times. The code below iterates between numbers 0 to 2 and prints each number. 

for i in range (4) 
print (i)
for i in range(3):
  print(i)

Nested for Loops

A for loop can have nested for loops. This is particularly useful if the items you are iterating over contain subitems. In the example below, we have a list of lists called teams and we can use a nested for loop to print each name in the lists. 

fruits = [['Watermelon', 'Cherry'],['Strwaberry', 'Raspberry']]
    
   for fruit in fruits: 
            for name in fruit :
                    print (name)  
#identation is important 
teams = [['Jody', 'Abe'], ['Abhishek', 'Kim'], ['Taylor', 'Jen']]
for team in teams:
  for name in team:
    print(name)

While Loops

The while loop is used to execute code while its condition evaluates to be True. In the example below, the while loop will run and print i as long as the value of iis less than 6.

i=20 
while i<100 :
    print (i)
    i+=20
i = 1
while i < 6:
  print(i)
  i += 1

Infinite Loops

  • An infinite loop is a while loop that never terminates because the condition is always evaluated to be True.

  • This could be due to a typo or an incorrect logic in the loop. You can terminate the loop by pressing the Ctrl and c together on your keyboard.