If we have a simple task, like print the number 1 to 3, we can achieve this using

print(1)
print(2)
print(3)

However, what do we do if we change the problem to print the numbers 1 to 10,000? That’s a lot of code.

We can use an instruction to create a loop. A loop will run the same code over and over again if a condition is True.

A loop has this pattern:

while [boolean]:
   [indented code]
[code]

And here is an example

A while instruction is similar to the if statement, but at the end of the code block instead of continuing to the next line, it goes back to the while instruction and re-evaluates the boolean condition.

In our example, counter < 2 is substituted with True as the value of counter is 0. In this case we start to execute the code indented after the while instruction. We then change the value of counter, adding 1 to it's current value, then we assign that new value to counter. At the end of our indented code block, we return to the while instruction and execute the instruction again.

The value of counter is now 1 and that is still less than 2 so it is substituted with true, so the while condition is true. In this case we execute the same indented code again. At the end of the indented code block, we increase the value of counter, we add 1 to the current value of counter, and assign it to counter.

Now counter has the value of 2. We've reached the end of the indented code block so we navigate to the while instruction. This time when we evaluate the condition, 2 is not less than 2, so we substitute false. Like an if statement, if the condition is false, we skip to the end of the indented code block and continue executing instructions.

Now if we want to count to 10,000, we just change

while counter < 2

To

while counter < 10000

In fact, since we can use variables, let’s write it like this

limit = 10000
counter = 0
while counter < limit
    counter = counter + 1
    print(counter)

This is a very common pattern.

We can also split out the action from the counting. In our example, it printed the counter. This could be anything.

The following code runs but will loop forever as the boolean value is always True.

while True:
    print("goodbye")

To control the loop, we add a variable or some condition that can change. Since the substitution is always performed when the line is read, it can change. So a variable that is True, can be changed to False to exit the loop.

while isLeaving:
    print("goodbye")
    isLeaving = False