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Karel — Reader (class-friendly)

Chapter 5

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Contents 1 - Meet Karel 2 - Programming 3 - New Functions 4 - Decomposition 5 - For Loops 6 - While Loops 7 - Conditionals 10 - Reference

Chapter 5: For Loops

Computers are very good at repeating actions. In this chapter we learn how to tell Karel to do the same steps several times using a for loop.

Basic for loop

Before — empty world

After — beepers placed

When you know exactly how many repetitions you need, a for loop is the right tool. The compact form we use in these examples is:

for i in range(count):
    # statements to repeat

Replace count with the number of times you want the loop to run. For example, to put 15 beepers, use for i in range(15): and call put_beeper() inside the loop body.

Matching postconditions with preconditions

The most important part of loops is ensuring that after each iteration the world is in the right state for the next iteration to start. If the state assumptions don't hold, the loop may fail or behave unexpectedly.

In the example that places beepers in each corner, each loop iteration must end with Karel positioned and facing so the next iteration's assumptions are satisfied (for instance, facing the next empty corner).

Nested loops

Loop bodies can contain other loops. These nested loops let you repeat complex patterns (e.g., making a grid or drawing multi-line shapes).

def draw_row():
    for i in range(4):
        put_beeper()

for row in range(3):
    draw_row()
    move_to_next_row()

When reading nested loops, follow the flow: the inner loop completes fully each time the outer loop iterates.

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