Visual Basic.NET How to Program (PDF)

Chapter 7 Arrays 255
Good Programming Practice 7.1
Strive for program clarity. Sometimes, it is worthwhile to forgo the most efficient use of memory
or processor time if the trade-off results in a clearer program. 7.1
Performance Tip 7.1
Sometimes, performance considerations outweigh clarity considerations. 7.1
The For structure (lines 22–24) reads the responses from the array responses one
at a time and increments one of the 10 counters in the frequency array (frequency(1)
to frequency(10)). The key statement in the loop appears in line 23. This
statement increments the appropriate frequency counter as determined by the value of
responses(answer).
Let us consider several iterations of the For structure. When counter answer is 0,
responses(answer) is the value of responses(0) (i.e., 1—see line 14). Therefore,
frequency(responses(answer)) actually is interpreted as frequency(1),
meaning the first counter in array frequency is incremented by one. In
evaluating the expression frequency(responses(answer)), Visual Basic starts
with the value in the innermost set of parentheses (answer, currently 0). The value of
answer is plugged into the expression, and Visual Basic evaluates the next set of parentheses
(responses(answer)). That value is used as the index for the frequency
array to determine which counter to increment (in this case, the 1 counter).
When answer is 1, responses(answer) is the value of responses(1) (i.e.,
2—see line 14). As a result, frequency(responses(answer)) actually is interpreted
as frequency(2), causing array element 2 (the third element of the array) to be
incremented.
When answer is 2, responses(answer) is the value of responses(2) (i.e.,
6—see line 14), so frequency(responses(answer)) is interpreted as frequency(6),
causing array element 6 (the seventh element of the array) to be incremented
and so on. Note that, regardless of the number of responses processed in the survey, only
an 11-element array (in which we ignore element zero) is required to summarize the results,
because all the response values are between 1 and 10, and the index values for an 11-element
array are 0–10. Note that, in the output in Fig. 7.5, the numbers in the frequency
column correctly add to 40 (the elements of the frequency array were initialized to zero
when the array was allocated with New).
If the data contained out-of-range values, such as 13, the program would attempt to add
1 to frequency(13). This is outside the bounds of the array. In other languages like C
and C++ programming languages, such a reference would be allowed by the compiler and
at execution time. The program would “walk” past the end of the array to where it thought
element number 13 was located and would add 1 to whatever happened to be stored at that
memory location. This could modify another variable in the program, possibly causing
incorrect results or even premature program termination. Visual Basic provides mechanisms
that prevent accessing elements outside the bounds of arrays.
Common Programming Error 7.2
Referencing an element outside the array bounds is a runtime error. 7.2