C :: Initialize Array Values With 0s Or Assume They Will Be Initialized To 0

The following:

Code:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main(int argc, char *argv[]) {
int i;
int arr[3];

[Code] ....

Notice we didn't set a value for second index but it returns 0. Should I assume that when declaring an array with n values, those values will be initialized to 0 automatically or should I still initialize the array with all 0s doing something like this:

Code:
for(i=0;i<sizeof(arr);i++) {
arr[i]=0;
}