Dynamically 2D array in C using the single pointer:

Question

I'm trying to dynamically allocate memory to a 2d array using a single pointer. For that, I have 3 functions that allocate the respective memory newarray() and to store individual elements in it store(), to fetch elements from it fetch(). I don't know why I get execution errors while I test it, also I should allocate the the exact amount of memory for it, that might be the problem but I'm not sure how to do that. This probrams deals with a triangular matrix which should have the number of columns lower than the number of rows when It comes to adding elements, like I, have a 5x5 array where (4,2) and (4,4) its OK but (4,5) its NOT.

Here is the code

typedef int* triangular;

triangular newarray(int N){
    triangular mat = NULL; //pointer to integer
    //Allocate memory for row
    mat = (int *)malloc(N * N * sizeof(int));
    //Check memory validity
    if(mat == NULL)
    {
        return 1;
    }

    return mat;
}

int store(triangular as, int N, int row, int col, int val){
    if(row >= col){
        as[row * N + col] = val;
        return 1;
    }else if(row < col){
        return -1;
    }else if((row > N) ||(col > N) || (row + col > N + N))
        return -1;
}

int fetch(triangular as, int N, int row, int col){
    int value;
    value = as[row * N + col];
    if((row > N) ||(col > N) || (row + col > N + N) )
        return -1;
    else if(row < col)
        return -1;
    return value;
}

nt main()
{
    int iRow = 0; //Variable for looping Row
    int iCol = 0; //Variable for looping column
    int N;
    triangular mat = newarray(5);

    printf("\nEnter the number of rows and columns = ");
    scanf("%d",&N); //Get input for number of Row

    store(mat,N,3,2,10);
    store(mat,N,3,3,10);
     store(mat,N,4,2,111);
    store(mat,N,3,5,11);
    printf("the element at [3,5] is : %i", fetch(mat,N,3,5));
    //Print the content of 2D array

     for (iRow =0 ; iRow < N ; iRow++)
    {
        for (iCol =0 ; iCol < N ; iCol++)
        {
            printf("\nmat[%d][%d] = %d\n",iRow, iCol,mat[iRow * N + iCol]);
        }
    }

    //free the allocated memory
    free(mat);

    return 0;
}

Answer 1

int store(triangular as, int N, int row, int col, int val){
    if(row >= col){
        as[row * N + col] = val;
        return 1;
    }else if(row < col){
        return -1;
    }else if((row > N) ||(col > N) || (row + col > N + N))
        return -1;
}

in store function, first if condition is so weird. Why you dont set the value to the array when the parameters passed to function is 2(row), 3(column).

I changed your store in the following way. index and array size are different things because of that index is equal to N - 1. In your code, there are a lot of if checks I guess checking only row and col is enough to understand that they are inside boundaries.

int store(triangular as, int N, int row, int col, int val){
    int index = N - 1;

    if((row > N) ||(col > N))
        return -1;

    as[row * index + col] = val;

    return 1;
}

I changed your fetch function like below because the reason I mentioned about your store function.

int fetch(triangular as, int N, int row, int col){
    int value;
    int index = N - 1;
    
   if((row > index) ||(col > index))
        return -1;
   
   value = as[row * index + col];
    
    return value;
}

Answer 2

You are making this needlessly complicated. All those functions and manual run-time calculations aren't really necessary.

Also, you have the following problems:

• Don't hide pointers behind typedef, it just makes the code unreadable for no gain.
• Initialize the data returned from malloc or instead use calloc which sets everything to zero, unlike malloc.
• Arrays in C are zero-indexed so you can't access item [3][5] in an array of size 5x5. This is a common beginner problem since int array[5][5]; declares such an array but array[5][5] for index accessing goes out of bounds. The syntax for declaration and access isn't the same, access needs to start at 0.
• You didn't include any headers, I'm assuming you left that part out.

Here's a simplified version with bug fixes that you can use:

#include <stdio.h>
#include <stdlib.h>

int main(void)
{
  int N=5;
  int (*mat)[N] = calloc( 1, sizeof(int[N][N]) ); // allocate 2D array dynamically
  if(mat == NULL)
    return 0;

  mat[3][2] = 10;
  mat[3][3] = 10;
  mat[4][2] = 111;
  mat[3][4] = 11;

  for(int i=0; i<N; i++)
  {
    for(int j=0; j<N; j++)
    {
      printf("[%d][%d] = %d\n", i, j, mat[i][j]);
    }
  }

  free(mat);
  return 0;
}

Further study: Correctly allocating multi-dimensional arrays