C adding to values for no reason [closed]

Question

    int sum1 = aboveMain(matrice);
    printf("%d nice\n", sum1);
    int sum2 = underMain(matrice);
    printf("%d nice\n", sum2);

For some reason this works perfectly fine, prints the intended values 11 and 19. Yet the moment i delete the printf line after the int sum1 declaration like so...

    int sum1 = aboveMain(matrice);
    int sum2 = underMain(matrice);
    printf("%d nice\n", sum2);

It returns 30, which means its been adding them both. Yet this seems to make 0 sense to me, can anybody please explain to me why this is happening? thanks

heres the entire code btw

#include <stdio.h>
#define ROWS 3
#define COLS 3


int aboveMain(int matrice[][COLS]);
int underMain(int matrice[][COLS]);
    
void main() {
    
    int matrice[3][3] = {{1,2,3},
                         {4,5,6},
                         {7,8,9}};

    int sum1 = aboveMain(matrice);
    printf("%d nice\n", sum1);
    int sum2 = underMain(matrice);
    printf("%d nice\n", sum2);
}


int aboveMain(int matrice[][COLS]) {
    int sum, i, j;
    for(i=0;i<3;i++) {
        for(j=0;j<3;j++) {
            if(i < j) 
                sum+= matrice[i][j];    
            
        }
    }
    return sum;
}



int underMain(int matrice[][COLS]) {
    int sum, i, j;
    for(i=0;i<3;i++) {
        for(j=0;j<3;j++) {
            if(i > j) 
                sum+= matrice[i][j];    
            
        }
    }
    return sum;
}

Answer 1

sum is not initialized to 0 in your functions,with the C language the initial value of local variables is undetermined. Therefore the bahaviour of your code is undefined.

int aboveMain(int matrice[][COLS]) {
  int sum = 0;             // you need to initialize sum to 0 here
  int i, j;   
  for (i = 0; i < 3; i++) {
    for (j = 0; j < 3; j++) {
      if (i < j)
        sum += matrice[i][j];

    }
  }
  return sum;
}