Programming in C

The #define Statement
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And if the program is written to use MAXIMUM_DATAVALUES in all cases where the size of
the array was used, the preceding definition could be the only statement in the program
that would have to be changed.
Program Portability
Another nice use of the define is that it helps to make programs more portable from one
computer system to another. At times, it might be necessary to use constant values that
are related to the particular computer on which the program is running.This might have
to do with the use of a particular computer memory address, a filename, or the number
of bits contained in a computer word, for example.You will recall that your rotate
function from Program 12.4 used the knowledge that an int contained 32 bits on the
machine on which the program was executed.
If you want to execute this program on a different machine, on which an int contained
64 bits, the rotate function would not work correctly. 4 Study the following
code. In situations in which the program must be written to make use of machinedependent
values, it makes sense to isolate such dependencies from the program as much
as possible.The #define statement can help significantly in this respect.The new version
of the rotate function would be easier to port to another machine, even though it is a
rather simple case in point. Here’s the new function:
#include
#define kIntSize 32 // *** machine dependent !!! ***
// Function to rotate an unsigned int left or right
unsigned int rotate (unsigned int value, int n)
{
unsigned int result, bits;
/* scale down the shift count to a defined range */
if ( n > 0 )
n = n % kIntSize;
else
n = -(-n % kIntSize);
if ( n == 0 )
result = value;
4. Of course, you can write the rotate function so that it determines the number of bits in an
int by itself and, therefore, is completely machine independent. Refer back to exercises 3 and 4 at
the end of Chapter 12,“Operations on Bits.”