Page 88 - Programming Microcontrollers in C

Pointers 73

The following function compares two strings. It returns a negative
number if the string p is less than the string q, 0 if the two strings
are equal, and a positive number if the string p is greater than the
string q.

int strcomp(char* p, char* q)
{
while( *p++ == *q++)
if(*(p-1) == ‘\0’) /* The strings are equal */
return 0; /* zero */
return *(p-1) - *(q-1); /* The strings are
unequal, return the correct value*/
}
Here the while() statement will examine each character in the
string until the argument is not equal. Note, that the pointers p and q
are incremented after they are used. The test for equality will not
occur until after the pointers are incremented, so it is necessary to
decrement the pointer p to determine if the last equal value in the
two strings is a zero character. If the last value is a zero, the two
strings are equal, and a 0 is returned. If the last tested character is not
a zero, the two strings are not equal and the difference between the
last character in p and the last character in q is returned. This choice
will give the correct sign needed to meet the function specification.
Another approach can be used that eliminates the increments and
decrements within the program and confines them to the argument
of a for construct. Consider
int strcomp(char* p, char* q)
{
for( ; *p==*q ; p++, q++)
if( *p == ‘\0’)
return 0;
return *p - *q;
}

The pointers p and q are both incremented after the test in the
if statement. Therefore, the pointer values are correct for the if
statement as well as for calculation of the return value when the
strings are not equal.