Trace tables

One way to test short is to carry out a using paper. A dry run involves creating a that includes all the a program contains. Whenever the value of a variable changes, the change is indicated in the trace table.

Consider this simple program.

1 SET total TO 0 2 FOR count FROM 1 TO 3 DO 3 RECEIVE number FROM (INTEGER) KEYBOARD 4 SET total TO total + number 5 END FOR

Each instruction has been given a line number - 1 to 5. The program has three variables - total, count and number - which are put into a trace table.

Next, the program is tested using . If the numbers 5, 7 and 9 are input, the resulting total should be 21.

The instruction number is added to the trace table. If a variable changes with that instruction, the new variable value is written in the appropriate box:

Instruction	Total	Count	Number
1	0
2		1
3			5
4	5
5
2		2
3			7
4	12
5
2		3
3			9
4	21
5

Instruction	1
Total	0
Count
Number

Instruction	2
Total
Count	1
Number

Instruction	3
Total
Count
Number	5

Instruction	4
Total	5
Count
Number

Instruction	5
Total
Count
Number

Instruction	2
Total
Count	2
Number

Instruction	3
Total
Count
Number	7

Instruction	4
Total	12
Count
Number

Instruction	5
Total
Count
Number

Instruction	2
Total
Count	3
Number

Instruction	3
Total
Count
Number	9

Instruction	4
Total	21
Count
Number

Instruction	5
Total
Count
Number

At each step, the programmer is able to see if, and how, a variable is affected.

Trace tables are extremely useful because they enable programmers to compare what the value of each variable should be against what a program actually produces. Where the two differ is the point in the program where a has occurred.

Further algorithms - EdexcelTrace tables

Trace tables

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