Python 101 - Part 5 - Repetition Structures - While

Hi!

Let’s keep talking about Basic Programming in Python! In case you haven’t seen the previous tutorials, we already have:

• Introduction to Python
• Python Code Blocks
• Conditional Structures in Python - 1
• Conditional Structures in Python - 2

Today we are going to talk about one of the most important structures: the while.

The while is a statement that tells a block of code to be executed while a condition is satisfied. Thus, it allows execution loops to be created, as we have in games and applications with graphical interfaces.

while is a very useful statement, but it can be dangerous, because if we don’t handle the stopping criterion correctly, the loop may never end, and the program doesn’t do what it should.

Let’s see an example of how while works in Python:

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x = 100
while x > 0:
    print("x > 0")
    x = x - 1

In this code, the message x > 0 will be displayed until x is equal to zero. When the value of x goes to 0, the check x>0 will return false, the program exits the loop.

Now let’s look at this code snippet:

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i = 0
while True:
    print("I will never stop!")
    i = i + 1
    if i > 100:
        break

Here we have a loop that would run forever (while True:). However, inside it, we do a check, and when the i variable is storing a value greater than 100, the break command will be executed. The break command literally breaks the loop, so at that instant, the program exits the loop.

If just below this code we did a print(i), the value displayed would be 100.

So, along with the while command, we have now seen the break command which can be very useful for breaking loops outside the standard comparison.

Now let’s look at the following snippet:

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i=0
while i < 100:
    i = i + 1
    if i % 2 == 0:
        continues
    print(i)

Initially, i is assigned the value 0, and is compared to 100. As 0 is less than 100, the loop starts. Soon after, after incrementing the value of i, we have a comparison: if 1%2 == 0:. here we check if i is a multiple of 2. If i is a multiple of two, there is a continue command. continue causes this iteration of the loop to end and the program moves on to the next iteration. This way, the program ignores everything after continue.

In case i is not a multiple of 2, the program does not execute continue, and prints the value of i on the screen.

If a continue were outside the if, the program would always execute this continue, and the one after it would never be executed. Thus, continue controls the execution of the loop without breaking it.

That’s it for now.

Our next post will talk about the for statement.

Be sure to post questions and opinions in the comments below!

See you in the next post!