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

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"); } }

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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