Trace tables and finding errors

A dry run means working through an algorithm by hand, line by line, without a computer.

A trace table records the dry run. It has one column for each variable, and one for the output.

Write a new value in a column only when that variable changes. Move to a new row when the loop goes round again.

This algorithm reads numbers until the user types -1. A value that only means 'stop' is called a rogue value.

Input data: 12, 7, 20, 9, -1. Follow each row of the table below and check you agree.

Then use the toy. Pick Counting (WHILE), a similar rogue-value loop, and press Step to run it one line at a time.

DECLARE Value : INTEGER
DECLARE Total : INTEGER
DECLARE Count : INTEGER
DECLARE Highest : INTEGER
Total ← 0
Count ← 0
Highest ← 0
INPUT Value
WHILE Value <> -1 DO
  Total ← Total + Value
  Count ← Count + 1
  IF Value > Highest
    THEN
      Highest ← Value
  ENDIF
  INPUT Value
ENDWHILE
OUTPUT Total / Count, " ", Highest

total = 0
count = 0
highest = 0
value = int(input())
while value != -1:
    total = total + value
    count = count + 1
    if value > highest:
        highest = value
    value = int(input())
print(total / count, highest)

There is something to try for this step in the IGCSE library.

A syntax error breaks the rules of the language, like a spelling mistake in a keyword. The program will not run.

A logic error means the program runs but gives the wrong answer. Examples: `<` instead of `>`, or a total that starts at 1.

Exam questions mostly test logic errors. Trace tables and test data are how you find them.

The lesson page has the rest of the explanation, step by step:

• Worked example: find four errors
• The corrected algorithm

Open the lesson page