problem with 2d array mallocation (segmentation fault) - c

Question

I tried to malloc a 2d array, but it gdb gives me Segmentation fault in line 8. It seems like something is wrong with if (A[i+x][j+y]!=A[i][j].
By the way: This program should print out the dimensions of the maximum square-sized sub-matrix of an input matrix, such that all its digits are equal matrix, such that all its digits are equal.
#include <stdio.h>
#include <stdlib.h>
#include <math.h>
int same_surrounding(int **A, int i, int j, int size){
for (int x=0; x<size; x++){
for (int y=0; y<size; y++){
if (A[i+x][j+y]!=A[i][j])
return 0;
}
}
return 1;
}
int main(){
int n, i, j, sub, max_sub;
int **A;
scanf("%d", &n);
A=malloc(sizeof*A*n);
for (i=0; i<n; i++){
A[i]=malloc(sizeof**A*n);
}
for (i=0; i<n; i++){
for (j=0; j<n; j++){
scanf("%d", &A[i][j]);
}
}
for (i=0; i<n; i++){
for (j=0; j<n; j++){
sub=1;
while (sub<n&&same_surrounding(A, i, j, sub)){
if (sub>max_sub)
max_sub=sub;
sub++;
}
}
}
printf("%d", max_sub);
for (i=0; i<n; i++){
free(A[i]);
}
free(A);
return 0;
}

Answer 1

There are mainly 2 things wrong with this code:
Your program is checking for submatrices beyond the dimension of input matrix n. for example, if n=5,i=3,j=0 and sub=3 you can easily see that your code is checking for numbers in A[i+x][j+y] i.e. upto A[5][2] which is out of bound of A. You need to check this before sending it to same_surrounding(). For that all you need to do is slightlt change the while loop:
while (sub<=n && i+sub<n && j+sub<n && same_surrounding(A, i, j, sub)){...}
Always initialize your variables. Here you didn't initialize max_sub variable. When you don't initialize your variables, it contains random garbage value. For example if the garbage value is 478231 and you are looking for largest square matrix of 5x5 matrix, you'll never reach the condition if (sub>max_sub) and as a result max_sub will never update. That's why always initialize your variable with a safe value, for example, max_sub=0.
Also, as a warning, you should use sizeof() instead of explicitly writing the number of bytes necessary for memory allocation. Because they might change depending on computer architecture.
A=(int**) malloc(sizeof(int*)*n);
for (i=0; i<n; i++){
A[i]=(int*)malloc(sizeof(int)*n);
}
Here's the working version of your code. All I did was some { } cleanup. If you have just one statement under for, while, if etc., you don't really need the braces.