Programming in C

Pointers and Arrays 263

return sum;

}

int main (void)

{

int arraySum (int array[], const int n);

int values[10] = { 3, 7, -9, 3, 6, -1, 7, 9, 1, -5 };

printf ("The sum is %i\n", arraySum (values, 10));

return 0;

}

Program 11.11 Output

The sum is 21

Inside the arraySumfunction, the constant integer pointer arrayEndis defined and set

pointing immediately after the last element of array.A forloop is then set up to

sequence through the elements of array.The value of ptris set to point to the begin-

ning of arraywhen the loop is entered. Each time through the loop, the element of

arraypointed to by ptris added into sum.The value of ptris then incremented by the

forloop to set it pointing to the next element in array.When ptrpoints past the end

of array,the forloop is exited, and the value of sumis returned to the calling routine.

A Slight Digression About Program Optimization

It is pointed out that the local variable arrayEndwas not actually needed by the func-

tion because you could have explicitly compared the value of ptrto the end of the array

inside the forloop:

for ( ...; pointer <= array + n; ... )

The sole motivation for using arrayEndwas one of optimization. Each time through the

forloop, the looping conditions are evaluated. Because the expression array + nis

never changed from within the loop, its value is constant throughout the execution of

the forloop. By evaluating it once beforethe loop is entered, you save the time that

would otherwise be spent reevaluating this expression each time through the loop.

Although there is virtually no savings in time for a 10-element array, especially if the

arraySumfunction is called only once by the program, there could be a more substantial

savings if this function were heavily used by a program for summing large-sized arrays,

for example.

The other issue to be discussed about program optimization concerns the very use of

pointers themselves in a program. In the arraySumfunction discussed earlier, the

Program 11.11 Continued

TEAM FLY