Programming in C

272 Chapter 11 Pointers
Program 11.14
Output
A string to be copied.
So is this.
Operations on Pointers
As you have seen in this chapter, you can add or subtract integer values from pointers.
Furthermore, you can compare two pointers to see if they are equal or not, or if one
pointer is less than or greater than another pointer.The only other operation that is permitted
on pointers is the subtraction of two pointers of the same type.The result of subtracting
two pointers in C is the number of elements contained between the two pointers.
So, if a points to an array of elements of any type and b points to another element
somewhere farther along in the same array, the expression b – a represents the number
of elements between these two pointers. For example, if p points to some element in an
array x, the statement
n = p - x;
has the effect of assigning to the variable n (assumed here to be an integer variable) the
index number of the element inside x to which p points. 4 Therefore, if p is set pointing
to the hundredth element in x by a statement such as
p = &x[99];
the value of n after the previous subtraction is performed is 99.
As a practical application of this newly learned fact about pointer subtraction, take a
look at a new version of the stringLength function from Chapter 10.
In Program 11.15, the character pointer cptr is used to sequence through the characters
pointed to by string until the null character is reached. At that point, string is
subtracted from cptr to obtain the number of elements (characters) contained in the
string. The program’s output verifies that the function is working correctly.
Program 11.15 Using Pointers to Find the Length of a String
// Function to count the characters in a string – Pointer version
#include
int stringLength (const char *string)
{
const char *cptr = string;
4.The actual type of signed integer that is produced by subtracting two pointers (for example,
int, long int, or long long int) is ptrdiff_t, which is defined in the standard header file
.