CODEWITHSUNDEEP
carrayspointersinitialization
Colin Ferguson
Colin Ferguson

Reputation: 23

Two-dimensional array full of garbage data... sometimes

I've been working on this simple coding challenge at HackerRank. I decided to implement my solution as a C function because any doing it in another language or in the main function itself seemed trivial. I assumed that I'd muck up the array/pointer syntax a few times doing it this way and I'd have a chance to learn something.

This is my function : 

int sum_diagonal(int n, int (*arr)[n]) {
   int lD, rD, sum;
   for(int i = 0; i < n; ++i) {
       for(int j = 0; j < n; ++j) {
           if (i == j) {
                rD += arr[i][j];
           }
           if ((i+j) == (n-1)) {
                lD += arr[i][j];
           }
       }
   }
   sum = abs(lD - rD);
   return sum;
}

And this is the main() function that calls it (the code to input the matrix was provided) :

int main(){
   int n; 
   scanf("%d",&n);
   int a[n][n];
   for(int a_i = 0; a_i < n; a_i++){
      for(int a_j = 0; a_j < n; a_j++){
         scanf("%d", &a[a_i][a_j]);
      }
   }
   //printf("why does this make it work?");
   int sum = sum_diagonal(n,a);
   printf("%d", sum);
   return 0;
}

Sometimes it works exactly as intended, and sometimes I get garbage output. Stranger still, though, is the fact that if I uncomment that random printf() call in the main function it seems to work every time. What is going on?

Upvotes: 2

Views: 122

Answers (2)

xy36
xy36

Reputation: 325

You are trying to use a non-constant value (n) to setup the size of your array. Thats not working in C. If you need a flexible size of your array, you should use malloc() to allocate memory. 

int n; 
scanf("%d",&n); 
int a[n][n]; //n is a non-constant value.

Otherwise you could use a fix size of your array by using #define for example:

#define SIZE_OF_ARRAY 5

int a[SIZE_OF_ARRAY ][SIZE_OF_ARRAY ];

Upvotes: -1

Sourav Ghosh
Sourav Ghosh

Reputation: 134346

In your code, you have defined

 int lD, rD, sum;

without initializing and then, you wrote

 rD += arr[i][j];

and

lD += arr[i][j];

rD and lD are automatic local variable and are not initialized. In the very first iteration itself, you're using them as to read the values from them (i.e., LHS of +=).

At that point of time, their content (values) are indeterminate. Using them, as you've done, invokes undefined behavior.

Solution: Initialize your local variables always, like

 int lD = 0, rD = 0, sum= 0;

Upvotes: 6

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