Lesson 3.4: Nested Loops and Programs That Do Not Stop

A nested loop is a loop inside another loop. For every one repeat of the outer loop, the inner loop completes all of its repeats.

If an outer loop repeats 3 times and its inner loop repeats 4 times, the inner body runs 12 times. Multiply the counts when the limits do not change.

Nested loops can describe rows and columns. The outer loop can select a row while the inner loop processes each column in that row. Keep the two loop variables separate so the trace remains clear.

A program that does not stop often has a condition that never becomes false, a variable changing in the wrong direction, or a repeat count much larger than intended.

Guided practice

Build an outer repeat of 2 and an inner repeat of 3. Print scan in the inner body. Predict the number of output lines, then run and count them.

Add a movement to the inner body and a turn after the inner loop. Use Step to separate what happens per column from what happens per row. If the safety limit stops your program, inspect the smallest loop first.

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