Dynamic 2 array in C - c

Question

I am learning C and I am trying to have a virtual grid in it, where the user can put new grid elements to "activate" into the console. So for example if I am starting with nothing and the user adds (40,50), then the size is at least 40x50 with element (40,50) initialised. If (20,30) follows, it just activates the element at 20,30. But if the user then enters (500,300), it will allocate some more memory and increases the size of the array. I would like to access them easily. I would like to work (I might have to anyway), because they are new for me.
My code (at the moment) is the following:
int width = 4, height = 5;
bool **arr = (bool **) malloc(height * sizeof(bool *));
for (int x = 0; x < height; x++) {
arr[x] = (bool *) malloc(width * sizeof(bool));
}
for (int x = 0; x < height; x++) {
for (int y = 0; y < width; y++) {
*(*(arr + x) + y) = false;
}
}
*(*(arr + 3) + 2) = true;
// user puts a value bigger than (4,5) inside
int newX=10, newY = 10;
//allocate more memory
So I am using a 2D pointer with booleans and I do "malloc" the height first and afterwards make an array of them for the width.
In the last line is just an example of entering the first element at (2,3). The scan method for the user doesn't matter here.
So is there a way of increasing the size of my array afterwards or do I need a totally different concept for it?
=====
The current code:
#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>
int main() {
int width = 4, height = 5;
bool **arr = (bool **) malloc(height * sizeof(bool *));
for (int x = 0; x < height; x++) {
arr[x] = (bool *) malloc(width * sizeof(bool));
}
for (int x = 0; x < height; x++) {
for (int y = 0; y < width; y++) {
*(*(arr + x) + y) = false;
}
}
*(*(arr + 3) + 2) = true;
int newWidth = 10, newHeight = 10;
bool **narr = realloc(arr, newHeight * sizeof(bool*));
if(narr) {
arr = narr;
for(size_t i = 0; i < newHeight; i++){
bool* p = realloc(arr[i] , newWidth * sizeof(bool));
if( !p ){
perror("realloc");
}
arr[i] = p;
}
// here resize the number of elements if needed
}
else {
perror("realloc failed");
exit(EXIT_FAILURE);
}
return 0;
}

Answer 1

Yes there is a mathod called realloc. And you can use that. You can resize the jagged array completely.
bool **narr = realloc(arr , newsize * sizeof(bool*));
if( narr ) {
arr = narr;
for(size_t i = 0; i < newsize; i++){
bool* p = realloc(arr[i] , newsize1 * sizeof(bool));
if( !p ){
perror("realloc");
}
arr[i] = p;
}
// here resize the number of elements if needed
}
else {
perror("realloc failed");
exit(EXIT_FAILURE);
}
Also simplify things write a[1][2] instead of *(*(a+1)+2). The check is needed as realloc may fail - in that case instead of letting your code go awry, take appropriate step as needed.
Also note that you need to set all the newly allocated bool* to NULL. So do this:-
for(size_t i = 0; i < newHeight; i++){
if( i >= height)
arr[i] = NULL;
bool* p = realloc(arr[i] , newWidth * sizeof(bool));
if( !p ){
perror("realloc");
}
arr[i] = p;
}
This is needed because realloc expects address of memory previously allocated with *alloc functions or NULL.

Answer 2

You can use realloc to increase the size of the array
in your situation you'd do
arr = (bool **)realloc(arr, sizeof(bool*) * new_height)
for(int i = 0; i < new_height; i++){
arr[i] = (bool *)realloc(arr, sizeof(bool) * new_width);
}
Note that you have to check whether the array needs to even get bigger, realloc can also "shrink" your array so tread carefully.
Initializing the newly created memory to false:
for(int i = old_height; i < new_height; i++){
for(int j = old_width; j < new_width; j++){
arr[i][j] = false;
}
}
arr[new_height - 1][new_width - 1] = true;