Arrays of Pointers

Arrays of Pointers

In many cases, it's useful to declare an array of pointers and access the contents pointed to by the array by dereferencing each pointer. For instance, the following declaration declares an int array of pointers:

int *ptr_int[3];

 In other words, the variable ptr_int is a three-element array of pointers to integers. In addition, you can initialize the array of pointers. For example:

int x1 = 10;
int x2 = 100;
int x3 = 1000;
ptr_int[0] = &x1;
ptr_int[1] = &x2;
ptr_int[2] = &x3;

Listing 16.7 shows another example. Here an array of pointers is used to access arrays of strings.

TYPE
Listing 16.7. Using an array of pointers to character strings.

1:  /* 16L07.c: Using an array of pointers */
2:  #include <stdio.h>
3:  /* function declarations */
4:  void StrPrint1(char **str1, int size);
5:  void StrPrint2(char *str2);
6:  /* main() function */
7:  main()
8:  {
9:     char *str[4] = {"There's music in the sighing of a reed;",
10:                    "There's music in the gushing of a rill;",
11:                    "There's music in all things if men had ears;",
12:                    "There earth is but an echo of the spheres.\n"
13:                   };
14:    int i, size = 4;
15:
16:    StrPrint1(str, size);
17:    for (i=0; i<size; i++)
18:       StrPrint2(str[i]);
19:
20:    return 0;
21: }
22: /* function definition */
23: void StrPrint1(char **str1, int size)
24: {
25:    int i;
26:    /* Print all strings in an array of pointers to strings */
27:    for (i=0; i<size; i++)
28:       printf("%s\n", str1[i]);
29: }
30: /* function definition */
31: void StrPrint2(char *str2)
32: {
33:     /* Prints one string at a time */
34:    printf("%s\n", str2);
35: }

A piece of a poem written by Lord Byron is printed out after the executable (16L07.exe) of the program in Listing 16.7 is created and executed:

OUTPUT

C:\app>16L07
There's music in the sighing of a reed;
There's music in the gushing of a rill;
There's music in all things if men had ears;
There earth is but an echo of the spheres.
There's music in the sighing of a reed;
There's music in the gushing of a rill;
There's music in all things if men had ears;
There earth is but an echo of the spheres.
C:\app>

ANALYSIS

Let's first have a look at the array of pointers, str, which is declared and initialized in lines 9_13 inside the main() function body of the program in Listing 16.7. As you can see, str is a four-element array of pointers to a set of character strings. I have adopted four sentences of a poem written by Lord Byron and used them as four character strings in the program.

You can get access to a character string by using a corresponding pointer in the array. In fact, there are two functions, StrPrint1() and StrPrint2(), in Listing 16.7. Both of them can be called to gain access to the character strings. From the function declaration in line 4, you can see that the StrPrint1() function is passed with a pointer of pointers—that is, **str1, which is dereferenced inside the StrPrint1() function to represent the four pointers that point to the four character strings. The definition of StrPrint1() is in lines 23_29.

The StrPrint2() function, on the other hand, only takes a pointer variable as its argument, and prints out a character string referenced by the pointer. Lines 31_35 give the definition of the StrPrint2() function.

Now back to the main() function. The StrPrint1() function is called in line 16 with the name of the array of pointers, str, as the argument. StrPrint1() then displays the four sentences of Byron's poem on the screen. The for loop in lines 17 and 18 does the same thing by calling the StrPrint2() function four times. Each time, the start address of a sentence is passed to StrPrint2(). Therefore, you see all the sentences of the poem printed on the screen twice.