Loops are common in all programmings languages, they help in executing tasks that are recurring in nature. Lets look at these loops in Python with few examples. The prerequisites are: knowing how to Python code through the Interpreter or as a script in a file.

While

Infinite Loop

With a string

# This is an infinite loop
# Ctrl z to stop it
while True:  # True means you dont have any condition set and you jus want to execute the loop    
    print("Hello...")

It would keep on printing ‘Hello’ until its interrupted with Ctrl C.

Hello
Hello
Hello
Hello
Hello
^CHello
Traceback (most recent call last):
  File "ex3.py", line 4, in <module>
    print("Hello")
KeyboardInterrupt

With a number

>>> # infinite loop once more
>>> i = 1
>>> while True:
...     print(i, 'hello')
...     i = i + 1
...
--TRUNCATED--
454 hello
455 hello
456 hello
^C457 hello
Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
KeyboardInterrupt

No Loop

>>> # this loop wont execute, bcause False is not the default condition
>>> while False:
...     print("Hello")
...
>>>

Finite Loop

The loop would break when the condition i <= 10 becomes false. In each iteration, we are incrementing the value of i by 1. When i becomes 11, the loop breaks.

>>> i = 1
>>> while i <= 10:
...     print(i, 'Hello')
...     i = i + 1
...
1 Hello
--TRUNCATED--
10 Hello

Break

The ‘break’ statement can also be used to break out of a loop based on certain condition.

$ cat break.py
# finite loop with the help of break
# not with the help of a condition after 'while'
i = 1
while True:
    print(i, 'hello')
    i = i + 1
    if i == 11:  # == is the equality condition
        break    # break is part of if, so it has to be indented 4 spaces under if

$ python3 break.py
1 hello
--TRUNCATED--
10 hello

Continue

The ‘continue’ statement skips rest of the statements in the current iteration, and moves the loop forward to the next iteration.

Here is a code to find odd and even numbers between 1 and 11. When its an even it would continue to the next iteration skipping the processing of rest of the statements after ‘continue’.

$ cat odd-or-even.py
i = 0
while True:
    i = i + 1
    if (i % 2 == 0):  # condition to check if its even
        print(i, 'even')
        continue # you will go forward with the rest of the loop and skip any statements below
    print(i, 'odd')
    if (i == 11):

$ python3 odd-or-even.py 1 odd 2 even 3 odd 4 even 5 odd 6 even 7 odd 8 even 9 odd 10 even 11 odd

For

in

The in statement is used with for, to loop over a list of values. Let’s try this with the range function, which is used to return a range a numbers, except the last number, for example range(1, 11) actually ranges from 1 to 10. We could loop over this range.

$ cat range.py
for i in range(0, 10):   # it will count from 0 to 9, last number is excluded
    print(i)

$ python3 ex9.py
0
--TRUNCATED--
9

nested

We can use nested blocks in python, same is true for ‘for’ as well. We are going to nest a for loop with in another for loop as follows.

$ cat nested-range.py
for i in range(1, 11):
     for j in range(11, 21):
          print(i, j)

$ python3 range.py
1 11
1 12
1 13
1 14
1 15
1 16
1 17
1 18
1 19
1 20
2 11
2 12
2 13
--TRUNCATED--
10 11
10 12
10 13
10 14
10 15
10 16
10 17
10 18
10 19
10 20

–end-of-post–