Question

I was trying something else but suddenly stuck with this infinite loop .
Please suggest an answer with explanation that what’s going on here in the below for loop

#include<stdio.h>

int main()
{
 int x=0;
 int i;
 int array[5];
 for(i=0;i<=5;i++)
 {
  array[i]=x;
  printf("#%d value set in index %d\n",x,i);
 }

 return 0;
}

When I remove = sign in the condition of for loop It works fine.

But when I put this it goes to infinite loop , Why?
Accessing extra element (more than its limit) in array is undefined behaviour or what ?
Any help will appreciated.
Thanks in advance.

~

Answer 1

To avoid causing errors like this as easily, here’s two good rules for writing for loops over actual (local) arrays:

1. Iteration starts at index 0, since C’s arrays are 0-based.
2. Use <, always. Never <=.
3. Don’t repeat the size, let the compiler compute it using sizeof array / sizeof *array. Note the asterisk in the second term.

So, this loop should have been written:

for(i = 0; i < sizeof array / sizeof *array; i++)

and then you would have been safe.

Note that this only works for “real” arrays that have a size visible to sizeof, if you’ve let the array “collapse” into a pointer it won’t work.

Also note that sizeof is not a function, so no ()s are necessary around its argument in cases like these.