Sams Teach Yourself C in 21 Days

Pointers: Beyond the Basics 397

15

The second function,printarray_2(), takes a different approach. It too is passed a

pointer to an array of four integers, but, in addition, it is passed an integer variable that

specifies the number of elements (the number of arrays of four integers) that the multidi-

mensional array contains. With a single call from line 31,printarray_2()displays the

entire contents of multi.

Both functions use pointer notation to step through the individual integers in the array.

The notation (int *)ptrin both functions (lines 43 and 54) might not be clear. The

(int *)is a typecast, which temporarily changes the variable’s data type from its

declared data type to a new one. The typecast is required when assigning the value of

ptrtopbecause they are pointers to different types (pis a pointer to type int, whereas

ptris a pointer to an array of four integers). C doesn’t let you assign the value of one

pointer to a pointer of a different type. The typecast tells the compiler, “For this state-

ment only, treat ptras a pointer to type int.” Day 20, “Working with Memory,” covers

typecasts in more detail.

DOremember to use the double indirec-

tion operator (**) when declaring a

pointer to a pointer.

DOremember that a pointer increments

by the size of the pointer’s type (usually

what is being pointed to).

DON’Tforget to use parentheses when

declaring pointers to arrays.

To declare a pointer to an array of char-

acters, use this format:

char (*letters)[26];

To declare an array of pointers to charac-

ters, use this format:

char *letters[26];

DO DON’T

Working with Arrays of Pointers ........................................................................

Recall from Day 8, “Using Numeric Arrays,” that an array is a collection of data storage

locations that have the same data type and are referred to by the same name. Because

pointers are one of C’s data types, you can declare and use arrays of pointers. This type

of program construct can be very powerful in certain situations.

Perhaps the most common use of an array of pointers is with strings. A string, as you

learned on Day 10, “Working with Characters and Strings,” is a sequence of characters

stored in memory. The start of the string is indicated by a pointer to the first character (a

pointer to type char), and the end of the string is marked by a null character. By declar-

ing and initializing an array of pointers to type char, you can access and manipulate a

large number of strings using the pointer array. Each element in the array points to a dif-

ferent string, and by looping through the array, you can access each of them in turn.

25 448201x-CH15 8/13/02 11:13 AM Page 397