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Programming

Sequence, selection and iteration

  • 3 things to know
  • Paper 1

Overview

The three basic constructs, definite and indefinite iteration, WHILE, REPEAT…UNTIL and DO…WHILE loops, nested loops and nested selection, and the output of short loops.

What you need to know

Answer each one before you reveal it.

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Key term

How does DO…WHILE NotSolved differ from REPEAT…UNTIL Solved?

Key term

What is output by the code n ← 1, then WHILE n < 20, n ← n * 3, ENDWHILE, then OUTPUT n?

Key term

What are the three basic constructs used to build programs?

Questions from this subtopic

Each one with the model answer underneath.

  1. 1

    What are the three basic constructs used to build programs?

    Model answer

    The three constructs are sequence, selection and iteration. Every imperative program is built by combining them.

  2. 2

    What is sequence?

    Model answer

    Sequence means statements are carried out one after another in the order they are written. Changing the order of statements can change the result.

  3. 3

    What is selection?

    Model answer

    Selection chooses which statements to run, depending on whether a condition is true. Selection is written with IF, ELSE IF and ELSE.

  4. 4

    What is iteration?

    Model answer

    Iteration repeats a block of statements. The block runs a set number of times or until a condition is met.

  5. 5

    What is definite iteration?

    Model answer

    Definite iteration repeats a block a fixed number of times, known before the loop starts. Definite iteration is also called count-controlled iteration.

  6. 6

    What is indefinite iteration?

    Model answer

    Indefinite iteration repeats a block until a condition is met, so the number of repetitions is not known in advance. Indefinite iteration is also called condition-controlled iteration.

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