array address do not increment correctly in C

Question

I am trying to make a recursive function to print the content of an array. The main looks like this:

#include <stdio.h>
static int s_index;
int main(void) { int even[] = {2, 4, 6, 8}; s_index = 0; print(even);}

The print function looks like this:

void print(int * array) {
    if(s_index > 3) {
        printf("\n"); return;
    }
    printf(" %d ", *array); ++s_index; print(array + s_index);
}

What I notice is:

if &even is 0x7fffffffdbf0 then (array + s_index) increments as follow with s_index:

s_index = 0 : 0x7fffffffdbf0;
s_index = 1 : 0x7fffffffdbf4;
s_index = 2 : 0x7fffffffdbfc;

it should be 0x7fffffffdbf8!!?

It is blowing my mind, could someone help with that? Thank you for your answers.

*array --> *(array+s_index) and print(array + s_index); --> print(array); — BLUEPIXY
– BLUEPIXY, Commented Nov 10, 2014 at 9:25
@Magnus Hoff: That wouldn't work. It has to be s_index that's incremented, or the if clause will break. — user1985657
– user1985657, Commented Nov 10, 2014 at 9:32

king_nak · Accepted Answer · 2014-11-10 10:35:40Z

2

You made a logical mistake in your recursive call. See what happens:

Assume &even = 0x7fffffffdbf0
First call:
array = 0x7fffffffdbf0 ; s_index = 0.
You increase s_index and pass into the second call array + s_index, which yields:
Second call:
array = 0x7fffffffdbf4 ; s_index = 1.
Again, you increase s_index and pass into the third call array + s_index, which yields:
array = (0x7fffffffdbf4) + 2 => 0x7fffffffdbfc

You should only increase array by 1 for each recursive call, and remove s_index completely (that's the point of the recursion, to get rid of global variables)

edited Nov 10, 2014 at 10:35

answered Nov 10, 2014 at 9:28

king_nak

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1 Comment

Magnus Hoff Over a year ago

Of course, getting rid of s_index would mean also introducing another termination criterion, for example a length parameter, and then calling print(array + 1, length - 1).

Gopi · Accepted Answer · 2014-11-10 09:41:40Z

1

Please get rid of the global value s_index.Change your func signature as shown below

int main(void)
{ 
   int even[] = {2, 4, 6, 8,10,12}; 
   print(even,(sizeof(even)/sizeof(int)));
}

void print(int * array,int len) {
    if( len == 0) 
    {
        printf("\n"); return;
    }
    else
    {
    printf(" %d ", *(array)); 
    print(array+1,len-1);
    }
}

edited Nov 10, 2014 at 9:41

answered Nov 10, 2014 at 9:27

Gopi

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5 Comments

Alex Davide Over a year ago

Thank you for the answer, indeed it is a logical mistake. However the else do not solve anything. I just changed it to print(++array).

Rerito Over a year ago

Fixes the problem, but the global variable, although not causing anything wrong, needs to be killed with fire

Gopi Over a year ago

@Rerito Agree but pointed out what is going wrong in the written code

Rerito Over a year ago

@Gopi Without any explanation or advice. You should make your answer more ... enlightening :)

Gopi Over a year ago

@Rerito there you have it :)

Lundin · Accepted Answer · 2014-11-10 09:52:36Z

0

Please don't do really strange and obscure things for no apparent reason, such as using recursion where simple loops are faster, safer and more readable. Because when you are doing really strange things on purpose, other equally strange things tend to happen unexpectedly.

#include <stdio.h>

void print(int* array, size_t size) 
{
  for(size_t i=0; i<size; i++)
  {
    printf("value:%d address:%p\n", array[i], (void*)&array[i]);
  }
}

int main(void) 
{ 
  int even[] = {2, 4, 6, 8}; 
  print(even, 4);
}

answered Nov 10, 2014 at 9:52

Lundin

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