Tracing, altering and writing algorithms

Overview

Dry run, desk check and first step in writing an algorithm for an unfamiliar problem.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   What does tracing an algorithm by hand involve?

   Model answer

   A hand trace works through the algorithm one step at a time. The value of every variable is written down as it changes, producing a trace table.

2. 2

   Why is a trace table used?

   Model answer

   A trace table exposes logic errors that simply reading the code would miss. The point at which a value goes wrong becomes visible.

3. 3

   What is a dry run?

   Model answer

   A dry run is a manual execution of an algorithm using chosen test data. Pencil and paper take the place of the computer.

4. 4

   What is a desk check?

   Model answer

   A desk check is a careful reading of an algorithm against the requirements it must meet. Errors are found before any machine time is used.

5. 5

   The algorithm total = 0, FOR i = 1 TO 3, total = total + i runs. State the final total.

   Model answer

   The final total is 6. The loop adds 1, then 2, then 3.

6. 6

   The algorithm x = 5, WHILE x > 1, x = x − 2 runs. State the final value of x.

   Model answer

   The final value of x is 1. The loop subtracts 2 twice, giving 3 and then 1.

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