error: invalid type argument of unary ‘*’ (have ‘int’) *(*(m+r)+c) = s; //save it in the resulting matrix

Question

int main(void)
{
        int r,c,i,j,k,s;
        printf("Enter the numbers of columns of the matrices: "); //asking the user about the rows and columns
        scanf("%d%d",&r,&c);
        int *arr = (int *)malloc(r * c * sizeof(int));  //dynamically alloting the array 1
        int *a = (int *)malloc(r * c * sizeof(int));    //dyanamically alloting the array 2
        int *m = (int *)malloc(r*c*sizeof(int));        //dynamically alloting the resulting array
        printf("Enter the elements for the first matrix: \n"); //entering the elements of array 1
        for(i=0;i<r;i++)
        {
                for(j=0;j<c;j++)
                {
                        scanf("%d", (*( arr + r ) + c));
                }
                printf("\n");
        }
        printf("Enter the elements for the second matrix: \n"); //entering the elements of array 2
        for(i=0;i<r;i++)
        {
                for(j=0;j<c;j++)
                {
                        scanf("%d", (*( a + r ) + c));
                }
                printf("\n");
        }
        for(i=0;i<r;i++)  //multiplying the matrices
        {
                for(j=0;j<c;j++)
                {
                s=0;  //alloting an element and it's value as zero
                        for(k=0;k<c;k++)
                        {
                                s += (*(arr+r)+k) * (*(a+k)+ c);
                        }
                *(*(m+r)+c) = s; //save it in the resulting matrix
                }
        }
        printf("The multiplied matrix is: \n"); //printing the multiplied matrix
        for(i=0;i<r;i++)
        {
                for(j=0;j<c;j++)
                        printf("%d ",(*( m + r) + c));
                printf("\n");
        }
        free(arr);
        free(a);
        free(m);
        return 0;

Answer 1

There are multiple problems in your code, I'm guessing from misunderstandings about how pointers and arrays works.

First of all you need to learn that for any pointer (or array) m and index r, the expression *(m + r) is exactly equal to m[r]. If we do this substitution in your expression *(*(m + r) + c) then it would be *(m[r] + c) which it seems you want to be m[r][c].

There are at least three problems with it:

1. The first is that m[r] is an int, so m[r] + c is also an int, and that can't be referenced.

2. The second is that you use the array sizes in your expression, not the i and j positions.

3. The third problem is that while arrays of arrays can be emulated using one single-dimension array, they don't work like pointers to pointers, and the arithmetic to get an element in them doesn't work using plain addition only.

While the first two points should be pretty easy to understand, the last could be a little harder. First one need to know how an array of arrays is laid out in memory, so lets take a simple array:

int m[2][3];

This will look something like this in memory:

+---------+---------+---------+---------+---------+---------+
| m[0][0] | m[0][1] | m[0][2] | m[1][0] | m[1][1] | m[1][2] |
+---------+---------+---------+---------+---------+---------+

To get to m[i][j] we can't just add i and j together to get the offset. For example lets take the case of i == 0 and j == 1, then i + j would be 1. This clashes with i == 1 and j == 0 where again the sum of i + j would also be 1. So both cases (i == 0 && j == 1 and i == 1 && j == 0) would use the same element, which we don't want.

Instead we need to use multiplication for the "row" number (i in this case), and addition for the column (j). And we need to multiply with the row-size 2. So to get m[i][j] we would use m + (i * 2) + j (using the example array defined above).

So in your code, whenever you attempt to use a single cell in the "matrix" you need to use m[i * r + j].

Answer 2

The pointer arr (and other pointers in your program) declared like

 int *arr = (int *)malloc(r * c * sizeof(int));

has the type int *. So dereferencing the pointer

scanf("%d", (*( arr + r ) + c));

you get an object of the type int while the function scanf expects a pointer..

Moreover in these loops

    for(i=0;i<r;i++)
    {
            for(j=0;j<c;j++)
            {
                    scanf("%d", (*( arr + r ) + c));
            }
            printf("\n");
    }

the expression (*( arr + r ) + c) depend neither on i nor on j.

You should write at least like

    for(i=0;i<r;i++)
    {
            for(j=0;j<c;j++)
            {
                    scanf("%d", arr + i * c + j );
            }
            printf("\n");
    }

Another approach is indeed to allocated a two-dimensional variable length array like

 int ( *arr )[c] = (int *)malloc( sizeof( int[r][c] ) );

and to use two indices to access elements of the array.

Pay attention to that the matrices have incorrect dimensions for the multiplication operation. That is if the first matrix has dimensions r and c then the second matrix should have dimensions c and r and the result matrix will have dimensions r and r.