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Algorithms and Programs

Sequence, selection and repetition

  • 3 things to know
  • AS Unit 1

Overview

Three basic constructs from which any algorithm can be built, sequence in an algorithm and count-controlled loop.

What you need to know

Answer each one before you reveal it.

Your recallNot started
Key term

Write an algorithm in pseudocode that reads numbers until −1 is entered and then outputs their total.

Process

What is meant by sequence in an algorithm?

Key term

Which pseudocode statement carries out selection?

Questions from this subtopic

Each one with the model answer underneath.

  1. 1

    Name the three basic constructs from which any algorithm can be built.

    Model answer

    They are sequence, selection and repetition.

  2. 2

    What is meant by sequence in an algorithm?

    Model answer

    Sequence means the statements are carried out one after another in the order they are written, each completing before the next begins.

  3. 3

    What is selection?

    Model answer

    Selection means the program tests a condition and carries out one set of statements or another depending on whether the condition is true.

  4. 4

    Which pseudocode statement carries out selection?

    Model answer

    The IF statement carries out selection, in the form IF condition THEN statements ELSE statements ENDIF.

  5. 5

    What is a CASE statement used for, and when is it preferable to nested IF statements?

    Model answer

    A CASE statement chooses between several branches on the value of one variable, and it is clearer than nested IF statements when there are many possible values to test.

  6. 6

    What is repetition?

    Model answer

    Repetition means a block of statements is executed more than once, either a fixed number of times or until a condition is met.

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