Trying to change element in 2D array in C - c

Question

I'm suppose to implement in the function make2Darray to make the elements in the 2D array correspond with the row number.
The other parts of the code that prints and frees was already given, so no need to worry about that. I'm only suppose to touch the make2Darray function. However, in that function, the allocating part was also given. So the only code I am to alter is the part where I change the elements in the 2D array.
int** make2Darray(int width, int height) {
int **a;
int i = 0;
int j = 0;
/*allocate memory to store pointers for each row*/
a = (int **)calloc(height, sizeof(int *));
if(a != NULL) {
/* allocate memory to store data for each row*/
for(i = 0; i < height; i++) {
a[i] = (int *)calloc(width, sizeof(int));
if(a[i] == NULL) {
/* clean up */
free2Darray(a, height);
return NULL; /*aborting here*/
}
}
}
/* from this point down is the part I implemented, all code above was
given*/
if (height < 0 && width < 0) {
for (i = 0; i < height; i++) {
for (j = 0; j < width; j++) {
a[i][j] = j;
}
}
}
return a;
}
The elements in the 2D array is suppose to correspond to the row number
If height = 4 and width = 3
0 0 0
1 1 1
2 2 2
3 3 3
However, I always get 0s which was the default setting when I got the code
0 0 0
0 0 0
0 0 0
0 0 0

Answer 1

You have two major problems in the code.
1) The code where you initialize the 2D array shall be inside the if-block
2) if (height < 0 && width < 0) { is wrong - you want > instead of <
Try:
int** make2Darray(int width, int height) {
int **a;
int i = 0;
int j = 0;
/*allocate memory to store pointers for each row*/
a = (int **)calloc(height, sizeof(int *));
if(a != NULL) {
/* allocate memory to store data for each row*/
for(i = 0; i < height; i++) {
a[i] = (int *)calloc(width, sizeof(int));
if(a[i] == NULL) {
/* clean up */
free2Darray(a, height);
return NULL; /*aborting here*/
}
}
// Moved inside the if(a != NULL) {
/* from this point down is the part I implemented, all code above was
given*/
if (height > 0 && width > 0) { // Corrected this part
for (i = 0; i < height; i++) {
for (j = 0; j < width; j++) {
a[i][j] = j;
}
}
}
}
return a;
}
A few hint:
1) Do the check for height and width in the beginning of your function - like:
if (height <= 0 || width <= 0) return NULL;
2) The prototype make2Darray(int width, int height) seems backward to me as we normally mention row-count before column-count. I would prefer: make2Darray(int height, int width). I even prefer the term "row" instead of "height" and "column" instead "width".
3) Your current code do "all the real stuff" inside the if(a != NULL) { That's okay but the code would (to me) be more clear if you instead did if(a == NULL) return NULL;
4) No need to cast calloc
With these updates the code could be:
int** make2Darray(int rows, int columns) {
int **a;
int i = 0;
int j = 0;
if (rows <= 0 || columns <= 0) return NULL;
a = calloc(rows, sizeof(int*));
if(a == NULL) return NULL;
/* allocate memory to store data for each row*/
for(i = 0; i < rows; i++) {
a[i] = calloc(columns, sizeof(int));
if(a[i] == NULL) {
/* clean up */
free2Darray(a, rows);
return NULL; /*aborting here*/
}
}
/* from this point down is the part I implemented, all code above was
given*/
for (i = 0; i < rows; i++) {
for (j = 0; j < columns; j++) {
a[i][j] = j;
}
}
return a;
}

Answer 2

In your code, the if check seems out of place.
if(a != NULL) {
/* allocate memory to store data for each row?
for(i = 0; i < height; i++) {
a[i] = (int *)calloc(width, sizeof(int));
if(a[i] == NULL) {
/* clean up */
free2Darray(a, height);
return NULL; /*aborting here*/
}
Well, that makes no sense. In case of success, a is supposed to be not equal to NULL.
You should rather have a check for a == NULL and subsequently return NULL;, not the other way around.
That said:
if (height < 0 && width < 0) does not look very good. It's most likely wrong (in this usage context) and counter productive.
All the access for a[i][j] is invalid. You just have the space allocated for the int *s, the int *s themselves do not point to any valid memory. You need to allocate memory to them also (for example, using the width as size).