Step 12: While¶

Tutorial¶

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:

Your Turn¶

Open Step 12 on the Reeborg environment.

One of Reeborg’s household jobs is to take out the compost. There is, however, a different amount of compost in the container in the house every time Reeborg needs to bring it outside.

Create a program to have Reeborg take out the compost, then return to the house. Reeborg needs to pick up as many rotten apples as are in the compost pail, bring them to the compost container outside, then return to the goal (7, 8). You will need to use while loops in your solution.

If You’re Having Trouble (a more detailed explanation)¶

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 not, you repeat the cycle once again. 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.