i have problem with acessing 2d array via function

Question

i am learning pointers and i write these code to add two martix via function, it compile well and give correct answer for 2x2 matrix but for more than two row and col it fails when i try it with 3x3 matrix [0][0] element for the first matrix it automaticaly become 6 and [1][0] it became 9 every time, i can't figure out why this happen

#include <stdio.h>
//-----------------Function------------------
void add(int (*p1)[10], int (*p2)[10], int (*p3)[10], int r, int c);
void get(int (*p1)[10], int r, int c);
void print(int (*p1)[10], int r, int c);

//---------------Main Program----------------
int main()
{
    //declartion
    int i, j, r, c;
    printf("\n\tenter the row and column of matrix\n\t");
    scanf("%d %d", &r, &c);
    int m1[r][c], m2[r][c], m3[r][c];
    printf("\n\tenter the element of first matrix\n\t");
    get(m1, r, c);
    print(m1, r, c);
    printf("\n\tenter the element of second matrix\n\t");
    get(m2, r, c);
    print(m2, r, c);
    add(m1, m2, m3, r, c);
    printf("\n");
    print(m3, r, c);
    return(0);
}
//-------------Define Function---------------
//get()
void get(int (*p1)[10], int r, int c)
{
    int i, j;
    for(i = 0; i < r; i++)
    {
        printf("\n\t");
        for(j = 0; j < c; j++)
        {
            scanf("%d", (*(p1+i)+j));
        }
    }
}
//--------------------X----------------------
//add()
void add(int (*p1)[10], int (*p2)[10], int (*p3)[10], int r, int c)
{
    int i, j;
    for(i = 0; i < r; i++)
    {
        for(j = 0; j < c; j++)
        {
            //printf("\n%d %d = %d & %d", i, j, *(*(p1+i)+j), *(*(p2+i)+j));
            *(*(p3+i)+j) = *(*(p1+i)+j) + *(*(p2+i)+j);
        }
    }
}
//--------------------X----------------------
//print()
void print(int (*p1)[10], int r, int c)
{
    int i, j;
    for(i = 0; i < r; i++)
    {
        printf("\n");
        for (j = 0; j < c; j++)
        {
            printf("\t%d", *(*(p1+i)+j));
        }
    }
}
//--------------------X----------------------

Your functions expects pointers to arrays with 10 integers but your main allows different sizes — 4386427
– 4386427, Commented Aug 24, 2021 at 15:07
Alternatively, if you're enamoured of the explicit pointer to array style then void get(int r, int c, int (*p1)[c]) would work too. As long as VLAs are being used in the first place, however, I think WeatherVane's suggestion is clearer. — John Bollinger
– John Bollinger, Commented Aug 24, 2021 at 15:18
This p1[i][j] would be more readable than your *(*(p1+i)+j) — Weather Vane
– Weather Vane, Commented Aug 24, 2021 at 15:23
scanf("d", (*(p1+i)+j)); -> scanf("%d", (*(p1+i)+j)); (But agree with previous comment...) scanf("%d", p1[i][j])... — ryyker
– ryyker, Commented Aug 24, 2021 at 15:30

Lundin · Accepted Answer · 2021-08-25 06:10:54Z

1

Drop the array pointer notation and the fixed size. Use variable-length arrays based on the parameters.
Remove useless comments like // get ... void get...
Don't use unreadable de-referncing with *(arr+i) when you could be writing readable arr[i].
Print new line after each row, not before each row.

Here's a cleaned up program which compiles. I haven't tested it.

#include <stdio.h>

void add   (int r, int c, int p1[r][c], int p2[r][c], int p3[r][c]);
void get   (int r, int c, int p1[r][c]);
void print (int r, int c, int p1[r][c]);

int main (void)
{
    int i, j, r, c;
    printf("\n\tenter the row and column of matrix\n\t");
    scanf("%d %d", &r, &c);
    int m1[r][c], m2[r][c], m3[r][c];
    printf("\n\tenter the element of first matrix\n\t");
    get(r, c, m1);
    print(r, c, m1);
    printf("\n\tenter the element of second matrix\n\t");
    get(r, c, m2);
    print(r, c, m2);
    add(r, c, m1, m2, m3);
    printf("\n");
    print(r, c, m3);
    return(0);
}

void get (int r, int c, int p1[r][c])
{
    for(int i = 0; i < r; i++)
    {
        for(int j = 0; j < c; j++)
        {
            scanf("%d", &p1[i][j]);
        }
    }
}

void add (int r, int c, int p1[r][c], int p2[r][c], int p3[r][c])
{
    for(int i = 0; i < r; i++)
    {
        for(int j = 0; j < c; j++)
        {
            p3[i][j] = p1[i][j] + p2[i][j];
        }
    }
}

void print (int r, int c, int p1[r][c])
{
    for(int i = 0; i < r; i++)
    {
        for (int j = 0; j < c; j++)
        {
            printf("\t%d", p1[i][j]);
        }
        printf("\n");
    }
}

edited Aug 25, 2021 at 6:10

answered Aug 24, 2021 at 15:53

Lundin

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1 Comment

Ian Abbott Over a year ago

There is a slight typo in one of those print() calls. I guess the second one should be printing the m2 matrix?

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