Debugging by Thinking

208 8.5 Categorize the problem

being input to a faulty compiler. This approach considers the frequency of

use and misuse of the elements of the structure. The following elements

should be selected from the original input:

9 The elements that are least frequently used

9 The elements that are most frequently misused

9 The combinations and sequences that are least frequently used

9 The combinations and sequences that are most frequently misused

8.5

Opportunities for misusing input data elements arise out of complex

input structures. A program in a high-level programming language is an

input data set for a compiler. Examples of frequently misused input ele-

ments are goto statements, data overlays such C/C++ union, and Fortran

COMMON statements.

Yet another approach to cutting down a test input data set is to try to

reduce more than one aspect of the input at a time, keeping the others con-

stant. The aspects of a test data set include size, sequence, and values. The

size aspect includes both the number of items, as well as the number of

dimensions, in the case of arrays. The sequence aspect includes repeating

patterns and ascending or descending ordering. The values aspect includes

magnitude and the set of unique values represented.

How does reducing the required input suggest hypotheses? It focuses

your attention on just those parts of the input that are causing problems. If

a test data set doesn't include a particular feature, there is no point in form-

ing a hypothesis that the handling of that feature is the cause of the defect.

By the time you have cut down your test case input to the minimum size

required to manifest the defect, you will have eliminated a whole host of

potential hypotheses from further consideration.

Categorize the problem

8.5. I Correctness

Here is a list of questions useful in categorizing a problem with the correct-

ness of the output of a program:

9 Is there any output at all?

8.5 Categorize the problem 209

9 Is any output missing (deletion)?

9 Is there extra output (insertion)?

9 Are the individual output values correct (substitution)?

9 Are the individual output values in the correct order (transposition)?

9 Are the individual output values close to correct, but not acceptable?

8.5.2

What hypotheses do these questions suggest? Are complete elements

missing, or are individual elements (numbers, strings) only partially dis-

played? Are numerical values missing because they have been incorrectly

aggregated with others? Is an entire sequence of values missing from either

the beginning or the end of the output? Are individual random items miss-

ing, or are groups of items missing?

Are the extra values due to repetition of correct values? Is an entire

sequence of values inserted at either the beginning or the end of the output?

Are individual random items inserted, or are groups of adjacent items

inserted?

Is the same value incorrectly substituted in many places, or are there dif-

ferent values substituted in each erroneous location? Are the values substi-

tuted a progression of related values?

Are the transposed values adjacent to their correct position, or are they

some distance from where they belong? Is the distance from the correct

position a constant for all values, or is it random? Are individual random

items out of position, or are groups of adjacent items out of position, but in

position with respect to each other?

Is the difference between the actual and expected values in the least sig-

nificant digits? Are the actual values off from the expected values by a ran-

dom or a constant amount? Are the actual values off from the expected

values by a constant factor or a progression of factors?

Identifying one of these situations doesn't guarantee that it's the cause of

correctness problems. If you find one, however, it's a good candidate for an

initial hypothesis.

Completion

Here is a list of questions useful in categorizing a problem with the comple-

tion of a program:

I Chapter 8