I am reading C The Complete Reference 4th edition by Herbert Schildt. In Chapter 4 - Arrays and Strings, it mentions "One major reason for the addition of variable-length arrays to C99 is to support numeric processing".
My question is: What is numeric processing? And how do variable-length arrays support numeric processing?
There are three places where variable-length arrays can appear:
Automatic local variables can be problematic — they occupy stack space, and there's no portable way to find out whether there's enough stack space left to allocate the array you want to create. If there isn't enough space, the program will halt unilaterally; there is no way to recover from the error. This has many people up in arms against VLAs.
Dynamically allocated arrays are not problematic — though the notation is not singularly convenient. You can tell when an allocation fails and react appropriately.
Function parameters are where the flexibility comes into play — especially with 2D arrays (and higher dimensions too).
Prior to the introduction of VLAs, you had to either treat the array as a 1D vector and calculate the indexes manually or you had to write different functions for each different size of matrix — or, at least, for each different number of columns.
You could use:
enum { NCOLS = 8 };
static void dump_array(const char *tag, size_t nrows, int array[][NCOLS])
{
printf("%s: (%zux%d)\n", tag, nrows, NCOLS); // Note %zu vs %d!
for (size_t r = 0; r < nrows; r++)
{
for (size_t c = 0; c < NCOLS; c++)
printf(" %5d", array[r][c]);
putchar('\n');
}
}
That's fine as far as it goes, but if you want matrices with different numbers of columns, you have to write (and call) different functions.
An alternative was to use a large 1D vector and calculate the 2D subscripts in your code:
static void dump_array(const char *tag, size_t nrows, size_t ncols, int array[])
{
printf("%s: (%zux%zu)\n", tag, nrows, ncols);
for (size_t r = 0; r < nrows; r++)
{
for (size_t c = 0; c < ncols; c++)
printf(" %5d", array[r * ncols + c]);
putchar('\n');
}
}
With VLA notation for the arguments, though, you can write a single function to print any 2D matrix (of a given base type) and use direct subscript notation for maximum convenience:
static void dump_array(const char *tag, size_t nrows, size_t ncols, int array[nrows][ncols])
{
printf("%s: (%zux%zu)\n", tag, nrows, ncols);
for (size_t r = 0; r < nrows; r++)
{
for (size_t c = 0; c < ncols; c++)
printf(" %5d", array[r][c]);
putchar('\n');
}
}
There isn't much difference in the notation, but the gain in flexibility is enormous. You can write a single matrix multiplication function that will work for any sizes of matrix, for example, whereas before VLAs were available, only the 1D version could deal with arbitrary sizes of array.
The dynamic allocation is reasonably simple, though not entirely obvious:
/* SO 7531-3779 */
#include <stdio.h>
#include <stdlib.h>
static void dump_array(const char *tag, size_t rows, size_t cols, int array[rows][cols])
{
printf("%s (%zux%zu):\n", tag, rows, cols);
for (size_t r = 0; r < rows; r++)
{
printf("%2zu:", r);
for (size_t c = 0; c < cols; c++)
printf(" %5d", array[r][c]);
putchar('\n');
}
}
int main(void)
{
size_t rows = 13;
size_t cols = 8;
int (*array)[8] = malloc(sizeof(int [rows][cols]));
for (size_t r = 0; r < rows; r++)
{
int sign = -1;
for (size_t c = 0; c < cols; c++)
{
array[r][c] = sign * ((r + 1) * 100 + c + 1);
sign = -sign;
}
}
dump_array("Array 1", rows, cols, array);
free(array);
return 0;
}
Output:
Array 1 (13x8):
0: -101 102 -103 104 -105 106 -107 108
1: -201 202 -203 204 -205 206 -207 208
2: -301 302 -303 304 -305 306 -307 308
3: -401 402 -403 404 -405 406 -407 408
4: -501 502 -503 504 -505 506 -507 508
5: -601 602 -603 604 -605 606 -607 608
6: -701 702 -703 704 -705 706 -707 708
7: -801 802 -803 804 -805 806 -807 808
8: -901 902 -903 904 -905 906 -907 908
9: -1001 1002 -1003 1004 -1005 1006 -1007 1008
10: -1101 1102 -1103 1104 -1105 1106 -1107 1108
11: -1201 1202 -1203 1204 -1205 1206 -1207 1208
12: -1301 1302 -1303 1304 -1305 1306 -1307 1308
And the VLA notation can be used with fixed-size arrays — global or local — too. That is, ordinary arrays can be passed to functions that accept a VLA as long as you describe the arrays accurately:
/* SO 7531-3779 */
#include <stdio.h>
#include <stdlib.h>
static void dump_array(const char *tag, size_t rows, size_t cols, int array[rows][cols])
{
printf("%s (%zux%zu):\n", tag, rows, cols);
for (size_t r = 0; r < rows; r++)
{
printf("%2zu:", r);
for (size_t c = 0; c < cols; c++)
printf(" %5d", array[r][c]);
putchar('\n');
}
}
/* Created by: gen_matrix -r 9 -c 5 -L -9999 -H +9999 -x -n matrix1 -w5 -E */
/* Random seed: 0x7F3F1FA0 */
int matrix1[9][5] =
{
{ 9337, 5320, 5059, 1115, 14, },
{ -7514, -1643, 8461, 1613, 6968, },
{ 2469, 8307, -8045, 2327, -7862, },
{ 8174, -7062, 666, -3480, 1836, },
{ -2400, -7863, -1859, 2436, -6840, },
{ 5819, -4112, -2037, 9005, -9748, },
{ 823, -9687, 1245, -2074, 3741, },
{ 4812, -9254, -6365, -1263, -9265, },
{ -9400, -5479, -3756, -7417, -5726, },
};
enum { MATRIX1_ROWS = 9, MATRIX1_COLS = 5 };
int main(void)
{
/* Created by: gen_matrix -E -r 13 -c 8 -H 999 -L -999 -i -w4 -n matrix2 */
/* Random seed: 0x64347CFE */
int matrix2[13][8] =
{
{ -27, -268, 73, 112, -148, 407, -411, 418, },
{ -782, 368, -306, -830, -851, 9, 505, 33, },
{ -558, -979, 471, 376, -290, -270, -910, 812, },
{ -374, 201, 454, 966, -39, 653, -747, -664, },
{ 322, 385, -141, -326, 37, 941, -298, -281, },
{ 529, 68, -995, -30, -942, -670, 563, -244, },
{ 773, 46, -315, -363, 732, 218, 230, 536, },
{ 566, -164, -493, 568, -256, -196, -635, -387, },
{ 452, -348, 79, 103, -416, -756, 688, -473, },
{ -294, -641, 530, -307, 508, 878, -786, -745, },
{ 427, 462, -229, 253, 116, -804, -72, -35, },
{ -776, 290, 158, 154, 662, -621, 576, 388, },
{ 999, -684, -207, -506, 708, -949, 149, -969, },
};
enum { MATRIX2_ROWS = 13, MATRIX2_COLS = 8 };
dump_array("Matrix 1", MATRIX1_ROWS, MATRIX1_COLS, matrix1);
dump_array("Matrix 2", MATRIX2_ROWS, MATRIX2_COLS, matrix2);
return 0;
}
Output:
Matrix 1 (9x5):
0: 9337 5320 5059 1115 14
1: -7514 -1643 8461 1613 6968
2: 2469 8307 -8045 2327 -7862
3: 8174 -7062 666 -3480 1836
4: -2400 -7863 -1859 2436 -6840
5: 5819 -4112 -2037 9005 -9748
6: 823 -9687 1245 -2074 3741
7: 4812 -9254 -6365 -1263 -9265
8: -9400 -5479 -3756 -7417 -5726
Matrix 2 (13x8):
0: -27 -268 73 112 -148 407 -411 418
1: -782 368 -306 -830 -851 9 505 33
2: -558 -979 471 376 -290 -270 -910 812
3: -374 201 454 966 -39 653 -747 -664
4: 322 385 -141 -326 37 941 -298 -281
5: 529 68 -995 -30 -942 -670 563 -244
6: 773 46 -315 -363 732 218 230 536
7: 566 -164 -493 568 -256 -196 -635 -387
8: 452 -348 79 103 -416 -756 688 -473
9: -294 -641 530 -307 508 878 -786 -745
10: 427 462 -229 253 116 -804 -72 -35
11: -776 290 158 154 662 -621 576 388
12: 999 -684 -207 -506 708 -949 149 -969