Pseudocode and Flowcharts

A trace table is a table used to record the value of each variable at every step of an algorithm, so that the algorithm can be hand-executed and checked for correctness.

Trace tables are essential for finding logic errors, where a program runs without crashing but produces the wrong output. By stepping through the algorithm with specific test data, the programmer can spot the exact step where the variables go wrong.

How to set up a trace table

1. List the variables the algorithm uses as column headings. Include any output as the last column.
2. Number the rows (optional, but useful for long algorithms).
3. Walk through the algorithm line by line, updating the relevant column whenever a variable changes.
4. Note any output in the OUTPUT column when an OUTPUT statement is reached.
5. Continue until the algorithm finishes (the loop terminates or the program reaches END).

Example — Trace the following algorithm with input data 4, 3, 7, 1, 8, 3, 6, 9, 12, 10 (ten numbers, one at a time). The algorithm finds the largest number entered.

Count    ← 0
Highest  ← 0
INPUT Number
Highest  ← Number
Count    ← 1
REPEAT
   INPUT Number
   IF Number > Highest THEN
      Highest ← Number
   ENDIF
   Count ← Count + 1
UNTIL Count = 10
OUTPUT Highest

The trace table:

After ten inputs, the algorithm outputs 12 (the largest number entered). The trace table makes it easy to see that Highest was updated each time a larger value arrived, and that the loop ran exactly ten times.

Why trace tables are useful

• Finding logic errors: if Highest does not update when it should, the trace table reveals the exact step where the bug is.
• Verifying loop termination: walking through the loop counter line by line shows that the loop ends when it is meant to.
• Understanding unfamiliar algorithms: a trace table forces a careful, step-by-step reading and often makes the purpose of the algorithm obvious.
• Picking suitable test data: tracing with boundary and abnormal data (topic 18) checks the program's edge cases.