Unsized Arrays
As you've seen, the size of a dimension is normally given during the declaration of an array. It means that you have to count each element in an array. It could be tedious to do so, though, especially if there are many elements in an array.
The good news is that the C compiler can actually calculate a dimension size of an array automatically if an array is declared as an unsized array. For example, when the compiler sees the following unsized array:
int list_int[] = { 10, 20, 30, 40, 50, 60, 70, 80, 90};
it will create an array big enough to store all the elements.
TYPE
Likewise, you can declare a multidimensional unsized array. However, you have to specify all but the leftmost (that is, the first) dimension size. For instance, the compiler can reserve enough memory space to hold all elements in the following two-dimensional unsized array:
char list_ch[][2] = {
`a', `A',
`b', `B',
`c', `C',
`d', `D',
`e', `E',
`f', `F',
`g', `G'};
The program in Listing 12.7 initializes a one-dimensional character unsized array and a two-dimensional unsized integer array, and then measures the memory spaces taken for storing the two arrays.
Listing 12.7. Initializing unsized arrays.1: /* 12L07.c: Initializing unsized arrays */
2: #include <stdio.h>
3:
4: main()
5: {
6: char array_ch[] = {`C', ` `,
7: `i', `s', ` `,
8: `p', `o', `w', `e', `r',
9: `f', `u', `l', `!', `\0'};
10: int list_int[][3] = {
11: 1, 1, 1,
12: 2, 2, 8,
13: 3, 9, 27,
14: 4, 16, 64,
15: 5, 25, 125,
16: 6, 36, 216,
17: 7, 49, 343};
18:
19: printf("The size of array_ch[] is %d bytes.\n", sizeof (array_ch));
20: printf("The size of list_int[][3] is %d bytes.\n", sizeof (list_int));
21: return 0;
22: }
OUTPUT
The following output is obtained by running the executable 12L07.exe:
C:\app>12L07
The size of array_ch[] is 15 bytes.
The size of list_int[][3] is 42 bytes.
C:\app>
ANALYSIS
A character unsized array, array_ch, is declared and initialized in lines 6_9. In
lines 10_17, a two-dimensional unsized integer array, list_int, is declared and initialized too.
The statement in line 19 measures and prints out the total memory space (in bytes) taken by the array array_ch. The result shows that the unsized character array is assigned 15 bytes of memory to hold all its elements after compiling. When you calculate the total number of the elements in the character array manually, you find that there are indeed 15 elements. Because each character takes one byte of memory, the character array array_ch takes a total of 15 bytes accordingly.
Likewise, the statement in line 20 gives the total number of bytes reserved in the memory for the unsized two-dimensional integer array list_int. Because there are a total of 21 integer elements in the array, and an integer takes 2 bytes, the compiler should allocate 42 bytes for the integer array list_int. The result printed out by the printf() function in line 20 proves that there are 42 bytes reserved in the memory for the two-dimensional integer array. (If the size of int or char is different on your machine, you may get different values for the sizes of the arrays in the program of Listing 12.7.)
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