Jingwen Zheng
  • Feb 07, 2019

Python: while loop

This blog presents how to acheive a while loop by python. Except for the basic using, you will also comprehend the infinite loop, nested while loop, if-else statement with while loop and loop of a ...

while loops, like the for loop, are used for repeating sections of code - but unlike a for loop, the while loop will not run n times, but until a defined condition is no longer met. If the condition is initially false, the loop body will not be executed at all. In this blog, I’ll talk about how to achieve while loops about:

  • syntax
  • flow diagram
  • basic using
  • infinite loop
  • nested while loop
  • if-else statement with while loop
  • loop of a list

Syntax

while expression:
    statement(s)

Here, statement(s) may be a single statement or a block of statements. The condition may be any expression, and true is any non-zero value. The loop iterates while the condition is true. When the condition becomes false, program control passes to the line immediately following the loop.

Flow diagram

Key point of the while loop is that the loop might not ever run. When the condition is satisfied, the loop body will be executed; however, when the condition is tested and the result is false, the loop body will be skipped and the first statement after the while loop will be executed.

Basic using

>>> count = 0
>>> while (count < 7):
...    print('The count is:', count)
...    count = count + 1
...
The count is: 0
The count is: 1
The count is: 2
The count is: 3
The count is: 4
The count is: 5
The count is: 6

We initially set count with value “0”. Then we continue to while loop - when count is smaller than 7, then print count value and assign “count” with its value plus one; so when count is equal to 0, the script prints “The count is: 0”, count’s new value is 1 (0+1); and so forth until count’s value is 6.

The infinite loop

>>> i = 7
>>> while (i < 10):
...    print ('Infinite loop')
...
Infinite loop
Infinite loop
Infinite loop
Infinite loop
Infinite loop
Infinite loop
Infinite loop
Infinite loop
Infinite loop
Infinite loop
... # infinite "Infinite loop"

Here, the value of i will always be smaller than 10, so it will print “Infinite loop” without stopping.

Nested While Loops

Besides the basic using and the infinite loop, we can nest loops inside each other. This is also true for while loops where we can create multiple levels of while statements as long as proper indentation rules are followed.

The syntax is as follows:

while condition1:
    do_something
    while condition2:
        do_something
        while condition3:
            do_something

Let’s look at the following example:

>>> x = -7
>>> y = 15
>>> while x <= y:
...     print('X is now: ', x)
...     x = x + 1
...     while x <= 0:
...         print('X is negative')
...         x = x + 1
...
X is now:  -7
X is negative
X is negative
X is negative
X is negative
X is negative
X is negative
X is negative
X is now:  1
X is now:  2
X is now:  3
X is now:  4
X is now:  5
X is now:  6
X is now:  7
X is now:  8
X is now:  9
X is now:  10
X is now:  11
X is now:  12
X is now:  13
X is now:  14
X is now:  15

The initial values of x and y are -7 and 15, respectively. When x is smaller than or equal to y, print x’s value and assign its value with x plus one, then if x is also smaller than or equal to 0, print “X is negative” and reassign with x plus one, until x is greater than y.

If-else statement with while loop

You can include an else conditional statement without the corresponding if statement in while loops as well.

The syntax is as follows:

while condition1:
    do_something
else:
    do_something_else

Let’s look at the following example:

>>> count = 0
>>> while count < 6:
...     print('The current count is: ', count)
...     count += 1
... else:
...     print('The final count is: ', count)
...
The current count is:  0
The current count is:  1
The current count is:  2
The current count is:  3
The current count is:  4
The current count is:  5
The final count is:  6

In this example, count is initially equal to 0. When count is smaller than 6, print current count’s value and reassign its value with count plus one, until count equals to 6, we print “The final count is: 6”

Loop of a list

>>> names = ['John', 'Jerome', 'Paul', 'George', 'Andy', 'Michael']
>>> i = 0
>>>
>>> while i < len(names):
...     print(names[i])
...     i += 1
...
John
Jerome
Paul
George
Andy
Michael

List “names” contains 6 elements, i’s initial value is 0. Now we will start while loop. When i is smaller than names’ length (6), print names’ i-location value and reassign i with i plus one, until i equals to 5, which corresponds ‘Michael’.

Conclusion

In this blog, we talked about syntax and basic using of while loop, and its supplementary applications: the infinite loop, nested while loop, if-else statement with while loop and loop of a list. Hope it’s useful for you :)

Reference

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