Guys, do you know when i try to access the position like this:

M[-2][-1], DONT WORK,

But, if we try to access like:

int k = -2;

M[k][k 1], WORKS?

i'm doing a homework and i need to verify if the M position is valid on base in some randoms I and J.

    #include <stdio.h>
int main(){
  int matriz[5][5];
  int i, j;
  int cont = 0;
  for(i=0;i<5;i  ){
    for(j=0;j<5;j  ){
      matriz[i][j] = cont =1;
    }
  }
  for(i=0;i<5;i  ){
    for(j=0;j<5;j  ){
      printf("%d ",matriz[i][j]);
    }
    printf("\n");
  }
  
  int k = -2;
  //matriz[-2][-1] = 99; << dont work
  matriz[k][k 1] = 99; // << this works
  printf("\n%d",matriz[k][k 1]); << this works
  printf("\n\n");
  for(i=0;i<5;i  ){
    for(j=0;j<5;j  ){
      printf("%d ",matriz[i][j]); //<< and this dont change anything on matriz
    }
    printf("\n");
  }
  
}

CodePudding user response：

It "works" as in the compiler might allow it, but do not do this because you are changing memory that does not belong to the array and will very likely cause a crash or worse.

You can check in your function by passing in i and j, and also the number of rows and columns.

if (i >= 0 && j >= 0 && i < MAX_ROWS && j < MAX_COLS) {
    matriz[i][j] ... do something
} else {
    invalid
}