Pointers and Arrays
Pointers and Arrays
There is a close relationship between Pointers and Arrays. Consider this program fragment:
char str[80], *p1;
p1 = str;
Here, p1 has been set to the address of the first array element in str. To access the fifth element in str, you could write
str[4]
or
*(p1+4)
Both statements will return the fifth element. Remember, arrays start at 0. To access the fifth element, you must use 4 to index str. You also add 4 to the pointer p1 to access the fifth element because p1 currently points to the first element of str. (Recall that an array name without an index returns the starting address of the array, which is the address of the first element).
The preceding example can be generalized. In essence, C/C++ provides two methods of accessing array elements: pointer arithmetic and array indexing. Although the standard array-indexing notation is sometimes easier to understand, pointer arithmetic can be faster. Since speed is often a consideration in programming, C/C++ programmers commonly use pointers to access array elements.
These two versions of putstr()— one with array indexing and one with pointers— illustrate how you can use pointers in place of array indexing. The putstr() function writes a string to the standard output device one character at a time.
/* Index s as an array. */
void putstr(char *s)
{
register int t;
for(t=0; s[t]; ++t) putchar(s[t]);
}
/* Access s as a pointer. */
void putstr(char *s)
{
while(*s) putchar(*s++);
}
Most professional C/C++ programmers would find the second version easier to read and understand. In fact, the pointer version is the way routines of this sort are commonly written in C/C++.
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