Thread: Pointer index wierd output:

This gives me a blank space when I compile and run... what's going on?

Code:

#include <stdio.h>
#include <stdlib.h>

int main()
{
  char *array;
  array = malloc(sizeof(char)*4);
  *array = 'a';
  *array++ = 'b';
  *array++ = 'c';
  *array++ = 'd';
  printf("%s\n", array-4);
}

You should learn how to "be the computer" and trace your code:

Code:

#include <stdio.h>
#include <stdlib.h>

int main()
{
  char *array;
  array = malloc(sizeof(char)*4);
  *array = 'a';  //original array is now "a???"
  *array++ = 'b';  //original array is now "b???"
  *array++ = 'c';  //original array is now "bc??"
  *array++ = 'd';  //original array is now "bcd?"
  printf("%s\n", array-4);  //prints the string starting one character before the actual malloced memory
}

Originally Posted by tabstop

You should learn how to "be the computer" and trace your code:

Code:

#include <stdio.h>
#include <stdlib.h>

int main()
{
  char *array;
  array = malloc(sizeof(char)*4);
  *array = 'a';  //original array is now "a???"
  *array++ = 'b';  //original array is now "b???"
  *array++ = 'c';  //original array is now "bc??"
  *array++ = 'd';  //original array is now "bcd?"
  printf("%s\n", array-4);  //prints the string starting one character before the actual malloced memory
}

I did try to trace it, I spent a long time playing around trying to fix it. I don't get why *array++ = b makes it b??? instead of ab???.
How can *array and *array++ possibly refer to the same position I mean ++ should increment it by 1?

Also why does -4 give me a blank I mean it should at least go 3 chars into the array?

It should increment it by 1 AFTER everything else has happened. (That's why the language also contains *++array.) In this case "everything else" would be assigning to the dereferenced pointer.

Originally Posted by tabstop

It should increment it by 1 AFTER everything else has happened. (That's why the language also contains *++array.) In this case "everything else" would be assigning to the dereferenced pointer.

Ok it works if I switch the ++ positions, but then I tried using (1+array) instead of (++array), now my output is just blank?

Not sure what you mean by everything else assigned to the dereferenced pointer.