C - pointers and different results?

Question

I cannot figure this out. Perhaps it is because it's 2am. At any rate, I am at a loss here.

#include <stdio.h>

int main()
{
    char array[] = "123456789";
    char* ptr = array;

    printf("%c\n", *(ptr++));
    printf("%c\n", *ptr);

    *ptr = array[3];
    printf("%c\n", *(ptr++));
    printf("%c\n\n", *ptr);

    return 0;
}

The result is:

1
2
4
3

1. I have a pointer, which I assign to array.

2. I then print, what I thought would be the first index ( '2' ), but instead get 1. -- So, I assume that *(ptr++) actually dereferences, before it increments the pointers.

3. Then I reassign ptr the 4th index ( '4' ) and repeat step 2. This works as expected now that I see C does not calculate the parenthesis first before dereferencing.

4. Then I print the newly incremented ptr to display ( '5' ) ... and I get 3?

How is that, step 1&2 and 3&4 are identical, but I get different results?

Answer 1

Let's go through the code step-by-step:

Step 0:

char* ptr = array;

Point the char pointer to the start of array (namely position 0).

Step 1:

printf("%c\n", *(ptr++));

Dereference the pointer at position 0, print the value residing there (1) and then increment the pointer to position 1

Step 2:

printf("%c\n", *ptr);

Dereference the pointer at position 1 and print the value residing there (2)

Step 3:

*ptr = arr[3];

Dereference the pointer at position 1 and update the value pointed to with the value at position 3 of the array. This is value 4.

Step 4:

printf("%c\n\n", *(ptr++));

Dereference the pointer at position 1, print the value we just updated (4) and then increment the pointer to position 2

Step 5:

printf("%c\n", *ptr);

Dereference the pointer at position 2 and print the value there (3).

Perhaps what you actually intended is to have ptr = &arr[3]; which will assign the pointer to a new position (namely the address of arr[3]).

Note that the braces around ptr in the above are actually redundant due to operator precedence.

For the case of *(ptr++), post-increment has higher precedence than indirection therefore it will be applied before we dereference the pointer

Braces are also unnecessary around *(++ptr) too. Here even though pre-increment and indirection have the same precedence, they are evaluated right-to-left. And so the pointer will be incremented before it is dereferenced.