The prototype of variable-length array (VLA), the declaration position, and the essence of variable-length array, vla prototype

The prototype of variable-length array (VLA), the declaration position, and the essence of variable-length array, vla prototype

Variable-length array is a newly added content in the C99 standard of C language. The so-called "variable" does not mean that the size of the array of this type can be modified after it is created, the size of the variable-length array cannot be modified after it is created. So why is it "variable length"? It is because the dimension of the Variable Length array can be specified by variables. This feature makes up for the deficiency that variable dimensions before C99 can only be specified by constants or constant expressions.

There are some limitations on variable-length arrays. variable-length arrays can only be declared inside the function or as function parameters. And cannot be initialized in the Declaration.

Let's look at an example:

Write a program, initialize a 3x5 double array, and copy the array to another two-dimensional array using a function based on the variable length array. You also need to write a function based on a variable-length array to display the content of the two arrays. These two functions should be able to process any NxM array.

1 #include <stdio.h>
 2 
 3 void copy (int row, int col, double arr [] [col], double tar [row] [col]);
 4 void show (int, int, double arr [*] [*]);
 5
 6 int main (void) {
 7
 8 double num [3] [5] = {
 9 {2.3,1.2,5.2,6.1,1.1},
10 {4.2,3.2,6.2,1.7,7.4},
11 {8.5,6.2,4.7,2.8,2.1}
12};
13
14 double rain [5] [12] = {
15 {4.3,4.3,4.3,3.0,2.0,1.2,0.2,0.2,0.4,2.4,3.5,6.6},
16 {8.5,8.2,1.2,1.6,2.4,0.0,5.2,0.9,0.3,0.9,1.4,7.3},
17 {9.1,8.5,6.7,4.3,2.1,0.8,0.2,0.2,1.1,2.3,6.1,8.4},
18 {7.2,9.9,8.4,3.3,1.2,0.8,0.4,0.0,0.6,1.7,4.3,6.2},
19 {7.6,5.6,3.8,2.8,3.8,0.2,0.0,0.0,0.0,1.3,2.6,5.2}
20};
twenty one     
22 double tar [3] [5] = {0};
23 double tar2 [5] [12] = {0};
twenty four     
25 copy (3,5, num, tar);
26 show (3,5, tar);
27
28 copy (5,12, rain, tar2);
29 show (5,12, tar2);
30
31 return 0;
32}
33
34 void copy (int row, int col, double arr [] [col], double tar [row] [col]) {
35 int i, j;
36 for (i = 0; i <row; i ++) {
37 for (j = 0; j <col; j ++) {
38 tar [i] [j] = arr [i] [j];
39}
40}
41}
42
43 void show (int row, int col, double arr [row] [col]) {
44 int i, j;
45 for (i = 0; i <row; i ++) {
46 for (j = 0; j <col; j ++) {
47 printf ("%. 1f", arr [i] [j]);
48 if (j! = Col-1) printf (",");
49}
50 printf ("\ n");
51}
52 printf ("-------------------------------------- \ n \ n");
53}
In this example, given two two-dimensional arrays, a 3X5 array and a 5X12 array, and in the example, the two arrays use the same function to process the copy or use the same function to display.

Which function is used for display:

void show (int row, int col, double arr [row] [col])
Is a variable-length array definition method, declare the integer representing the dimension before using the variable-length array of the dimension. Because variable names can be omitted in the prototype, the prototype can be written as:

void show (int, int, double arr [*] [*])
 

 

To be continued ......