Thread: Cannot understand the output.

I was browsing through some output questions in C language, where I found the following one.

Code:

#include<stdio.h>

int main()
{
  char arr[]  = "geeksforgeeks";
  char *ptr  = arr;
 
  while(*ptr != '\0')
      ++*ptr++;
  printf("%s %s", arr, ptr);
 
  getchar();
  return 0;
}

I ran the above code, and the output came out to be 'hffltgpshfflt'. Now the expression ++*ptr++ is quite confusing and it's difficult to understand what's exactly happening there. Any help regarding that would be appreciable. (Note- I do remember precedence rules/associativity but how that expression will get evaluated is my problem).

I'm not 100% sure on this so I'll just misinform you and then someone can come correct me.

If I recall correctly the post increment operator actually has very high precedence, it is just evaluated last.

So ++*ptr is ++(*ptr) because of the order of operands.
*ptr++ is *ptr; ptr++ because the ++ is higher precedence but evaluated last.

I always keep a C operator precedence chart handy: C Operator Precedence Table. I mostly have it memorized by now, but it's nice to reassure myself for complicated stuff.

Note that postfix ++ has the highest precedence, followed by * (dereference) and prefix ++. * and prefix ++ have equal precedence, but right-to-left associativity, meaning the right-most of those gets evaluated first.

DeadPlanet is right:

*ptr is not '\0' (it's 'g'), so:
1. ptr++ is evaluated first, but the value of that expression is just ptr, and the increment happens last (at the next sequence point, which is the end of the statement here, i.e. the ; on line 9)
2. We apply * to ptr (the value before the increment happens), to dereference and get the char there (e.g. 'g' the first iteration)
3. We then apply the prefix ++ to *ptr, turning 'g' into 'h' (assuming a charset where letters are adjacent and in alphabetical order).
4. The increment of ptr from the postfix ++ happens, moving ptr to the next spot in the string.
5. Go back to the top of the loop, and check *ptr again.

Thanks for the detailed reply anduril462, surely helped a lot to get things clear.