Tracing code and test data

Overview

Trace table, off-by-one error and three classes of test data.

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 is a trace table?

   Model answer

   It is a table with one column per variable and one row per step, filled in by hand as an algorithm is followed, showing how the values change.

2. 2

   What does it mean to dry run an algorithm?

   Model answer

   It means working through the algorithm by hand with chosen data, recording the values, to check the logic before or instead of running it on a computer.

3. 3

   A fragment sets total ← 0 then repeats total ← total + i for i taking each value from 1 to 5. What is the final value of total?

   Model answer

   The final value of total is 15.

4. 4

   A variable x starts at 10 and a loop subtracts 3 from x while x is greater than 0. How many times does the loop body run, and what is the final value of x?

   Model answer

   The body runs four times and x finishes at −2.

5. 5

   A counting loop runs from 0 to 9 inclusive. How many times does its body execute?

   Model answer

   The body executes ten times.

6. 6

   An array has eight elements with indices 0 to 7. What happens if the program reads element 8?

   Model answer

   The index is outside the array, so the program either reports an out of range error and stops or reads unrelated data, depending on the language.

Other ways to work this

The same content, four other ways.

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Six ways to work the same material.