/**
** This is an example program that I wrote when learning about how
** to make dynamically allocated 2 dimmensional arrays in c.
** I looked at a bunch of websites, but the best site that I
** found that actually showed and explained the
** different methods was http://www.lysator.liu.se/c/c-faq/c-2.html
**
** (c) 2008 Adam J Bavier all rights reserved
*/

/** Includes */
#include
#include

/** Usage: exe foo-rows foo-columns */
int main( int argc, char** argv) {

int i, j, nrows, ncolumns;
int *array1, **array2, **array3;

/** Get the number of rows and columns and make some basic checks */
if(argc != 3) {
printf(”Usage: exe foo-rows foo-columns\n”);
return 1;
}
nrows = atoi(argv[1]);
ncolumns = atoi(argv[2]);

if(nrows<0 || ncolumns<0) {
printf(”foo-rows and foo-columns must be positive\n”);
return 1;
}
if(!nrows || !ncolumns) {
printf(”foo-rows and foo-columns must be bigger then 1\n”);
return 1;
}

/** Method 1:
** Simulate a two dimmensional array with a single,
** dynamically-allocated one-dimensional array
** Note subscript calculations must be performed by you.
** i row, j column is array1[i*ncolumns+j] not array1[i][j] */
array1 = (int *) malloc(nrows * ncolumns * sizeof(int));
if(array1 == NULL) {
printf(”array 1: Out of memory\n”);
return 1;
}

/** Method 2:
** Allocate an array of pointers.
** Then initialize each pointer to a dynamically
** allocated row. */
array2 = (int **) malloc(nrows * sizeof(int*));
if(array2 == NULL) {
printf(”array2: Out of memory\n”);
return 1;
}

for(i=0; i<nrows; i++)
array2[i] = (int *) malloc(ncolumns * sizeof(int));

/** Method 3;
** Using explicit pointer arithmetic
** the array’s contents can be kept contiguous */
array3 = (int **) malloc(nrows * sizeof(int *));
if(array3 == NULL) {
printf(”array3: Out of memory\n”);
return 1;
}

array3[0] = (int *) malloc(nrows * ncolumns * sizeof(int));
if(array3 == NULL) {
printf(”array3: Out of memory\n”);
return 1;
}

for(i=1; i<nrows; i++)
array3[i] = array3[0] + i*ncolumns;

/** Put some random values into array1 */
srand(100);
for(i=0; i<nrows; i++)
for(j=0; j<ncolumns; j++)
array1[i*ncolumns+j] = rand();

/** Put the same values into array2 */
srand(100);
for(i=0; i<nrows; i++)
for(j=0; j<ncolumns; j++)
array2[i][j] = rand();

/** Put the same values into array3 */
srand(100);
for(i=0; i<nrows; i++)
for(j=0; j<ncolumns; j++)
array3[i][j] = rand();

/** Print array1 */
printf(”array1:\n”);
for (i=0; i<nrows; i++) {
printf(”\n”);
for (j=0; j<ncolumns; j++){
printf(”%15d “, array1[i*ncolumns+j]);
}
}
printf(”\n\n”);

/** Print array2 */
printf(”array2:\n”);
for(i=0; i<nrows; i++) {
printf(”\n”);
for (j=0; j<ncolumns; j++)
printf(”%15d “, array2[i][j]);
}
printf(”\n\n”);

/** Print array3 */
printf(”array3:\n”);
for(i=0; i<nrows; i++) {
printf(”\n”);
for(j=0; j<ncolumns; j++)
printf(”%15d “, array3[i][j]);
}
printf(”\n\n”);

/** Free memory */
printf(”Free array1 in total\n\n”);
free(array1);

/** Iterate through array2 and array3 and free the rows */
printf(”Free array2 rows first\n”);
for(i=0;i<nrows;i++)
free(array2[i]);

printf(”Free array3 rows first\n”);
free(array3[0]);
printf(”\n”);

/** Then free the array of pointers */
printf(”Free array of pointers array2 then\n”);
free(array2);

printf(”Free array of pointers array3 then\n”);
free(array3);

printf(”\nDone.\n”);

return 0;
}

That is a great idea for a really common problem. I hate it when my plants lean into the sun, it’s hard enough for me to remember to water them, let alone turn them occasionally to prevent this. A great simple and elegant solution.

David