CODEWITHSUNDEEP
c++carrays
nitzs
nitzs

Reputation: 259

Array index loop variable resets to zero in C, C++

I have declared an array of length N (suppose). Now, I assign some values to the elements of this array using a loop (the loop variable is used as the array's index). The code's as follows:

int main()
{
  int arr[4], j;
  for(j=0; j<10; j++)
  {     
    printf("%d\n", j);
    arr[j] = 0;
    sleep(1);
    printf("%d\n\n", j);
  } 
return 0;
}

I expected the output to be 0 1 2 .. 9. But what actually happened was that j got reset to 0 when the assignment arr[N+2]=0 (arr[6]=0, in this case) was executed in the loop. What's actually happening here? Am I missing something?

Upvotes: 3

Views: 2901

Answers (3)

Lou Franco
Lou Franco

Reputation: 89202

Your array has 4 elements and your index is out of range so you are just stomping on memory.

Given the code here, I'd expect arr[4] to reset the array, but since you mentioned that it's of length N and N+2 is what causes it, there might be some padding in your stack. In any case, when you declare the array and j, they are put on the stack. In your case, j is in a position such that when your index is out of bounds you are accessing the memory where j is.

Upvotes: 8

Nathan Fellman
Nathan Fellman

Reputation: 127508

Your array is overflowing. You defined it like this with 4 elements:

int arr[4]

But you're trying to assign 10 elements.

j is apparently located immediately after the array, so when you assign arr[6] = 0, you're clearing j.

Upvotes: 2

dubnde
dubnde

Reputation: 4441

the behaviour of the code is undefined as it has a bug. arr has a size of 4 but you are indexing past this size with j

Upvotes: 3

Related Questions