Lesson 3.3: Counting Loops and Loop Variables

A counting loop changes a loop variable through a planned range. The loop variable tells which repeat is currently running.

A loop from 1 to 4 by 1 uses the values 1, 2, 3, and 4. That is four repeats. A loop from 0 to 4 that includes both ends uses five values. Confusing those counts causes an off-by-one error.

The step value controls how the loop variable changes. A positive step counts upward. A negative step counts downward when the start and end values are arranged for it.

Guided practice

Create a counting loop with i from 1 to 4 by 1. Print i inside the loop. Predict all four output values.

Now move forward by 1 each time and print the final scene position after the loop. Test a loop from 0 to 3 and compare its repeat count. Both examples repeat four times even though their first and last loop-variable values differ.

Use clear loop variable names when they have a meaning, such as row or itemIndex.

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