22. For a while

When we want to repeat some instructions until a certain condition is satisfied, Python gives us a simpler way to write this using a new keyword: while. For example, suppose we want to have Reeborg keep moving until it reaches a wall. Previously, we might have done something like the following:

def move_until_wall():
    if front_is_clear():
        move()

repeat 42:
    move_until_wall()

and hoped that 42 would have been a number of repetitions sufficient to reach a wall. Using while, we can write the following:

while front_is_clear():
    move()

That’s it! No more guessing and asking something to be performed a large number of time just to ensure that it will be enough.

Here’s a flowchart for this simple program:

22.1. How to think about while

Suppose we have the following:

while condition():
    do_1()
    do_2()
    do_3()

You can think of this as being equivalent to:

if condition():
    do_1()
    do_2()
    do_3()
if condition():
    do_1()
    do_2()
    do_3()
if condition():
    do_1()
    do_2()
    do_3()
if condition():
    do_1()
    do_2()
    do_3()
....

which is to say that the block of code is repeated as long as the condition remains True. So, what happens if the condition is always True? The block of code is repeated for ever and the program never ends.

This is bad.

Instead of using this description of repeated blocks of code, programmers describe this as a loop: that is, you start with the first instruction (do_1()) inside the code block, continue with all the others until you reach the last instruction (do_3()), then *loop* back, or go back, to the test just before the beginning of the block and see if the condition is satisfied; if so, you repeat once again the cycle. If the condition never becomes False, you keep repeating and end up with an infinite loop.

Conclusion: you want to make sure that the condition will become False at some point.

22.2. Back to hurdles!

Your turn!

Go back to Hurdles 1, Hurdles 2 and Hurdles 3, and write a single program for all three world, using while and without using repeat.

Hint

Your program might look like the following:

def jump_over_hurdle():
    # suitable definition

def run_or_jump ():
    # suitable definition

while not at_goal():
    run_or_jump()

That’s it! No more arbitrary repetitions! From now on, you should use repeat only when you know exactly how many times a given function must be repeated.