Repetition

Repetition (also called iteration or a loop) runs a set of statements again and again, until a condition is met. In daily life: clap five times, walk 10 steps, skip until you are tired.

When the number of repetitions is known, a counter keeps track of them. For example, to find the total of 3 marks:

• Set count = 0 and total = 0
• WHILE count < 3:
• … INPUT mark
• … total = total + mark
• … count = count + 1
• PRINT total

The counter goes 0, 1, 2, and when it reaches 3 the loop stops, after exactly three runs.

When the number of repetitions is not known in advance, the WHILE construct keeps checking a condition. For example, to add up numbers until the user enters 0:

• INPUT num
• WHILE num is not 0:
• … total = total + num
• … INPUT num
• PRINT total

Notice the two inputs. The input before the loop gives the first value to test. The input inside the loop gets the next value, so the loop can end when 0 arrives.

Every loop must change something inside it, so that its condition eventually becomes false. Otherwise, the loop would never end (an infinite loop).