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Python Loops

Updated on Jan 07, 2020

Python has only two loops:

  1. for loop
  2. while loop

For loop #

The for loop Syntax:

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for i in iterable_object:
   # do something

note:

All the statements inside for and while loop must be indented to the same number of spaces. Otherwise, SyntaxError will be thrown.

Let's take an example

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my_list = [1,2,3,4]

for i in my_list:
    print(i)

Expected Output:

1
2
3
4
1
2
3
4

Here is how for loop works:

In the first iteration i is assigned value 1 then print statement is executed. In second iteration i is assigned value 2 then once again print statement is executed. This process continues until there are no more element in the list and for loop exists.

range(a, b) Function #

The range(a, b) functions returns sequence of integers from a, a + 1, a+ 2 .... , b - 2, b - 1. For e.g:

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for i in range(1, 10):
    print(i)

Expected Output:

1
2
3
4
5
6
7
8
9
1
2
3
4
5
6
7
8
9

You can also use the range() function by supplying only one argument like this:

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>>> for i in range(10):
...        print(i)

0
1
2
3
4
5
6
7
8
9

Here range for loop prints number from 0 to 9.

The range(a, b) function has an optional third parameter to specify the step size. For e.g:

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for i in range(1, 20, 2):
    print(i)

Expected Output:

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 3
 4
 5
 6
 7
 8
 9
10
1
3
5
7
9
11
13
15
17
19

While loop #

Syntax:

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while condition:
    # do something

The while loop keeps executing statements inside it until condition becomes false. After each iteration condition is checked and if its True then once again statements inside the while loop will be executed.

Let's take an example:

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count = 0

while count < 10:
    print(count)
    count += 1

Expected Output:

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 9
10
0
1
2
3
4
5
6
7
8
9

Here while  will keep printing until count is less than 10.

break statement #

The break statement allows to breakout out of the loop.

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count = 0

while count < 10:
    count += 1
    if count == 5:
         break    
    print("inside loop", count)


print("out of while loop")

when count equals to 5 if condition evaluates to True and break keyword breaks out of loop.

Expected Output:

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inside loop 1
inside loop 2
inside loop 3
inside loop 4
out of while loop

continue statement #

When continue statement encountered in the loop, it ends the current iteration and programs control goes to the end of the loop body.

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count = 0

while count < 10:
    count += 1
    if count % 2 == 0:
           continue
    print(count)

Expected Output:

1
2
3
4
5
1
3
5
7
9

As you can see, when count % 2 == 0, the continue statement is executed which causes the current iteration to end and the control moves on to the next iteration.

In next lesson we will learn about Python mathematical function.

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