Print elements in an array

Question

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Confused about C macro expansion and integer arithmetic
A riddle (in C)

The output of the following C program is to print the elements in the array. But when actually run, it doesn't do so.

  #include<stdio.h>

  #define TOTAL_ELEMENTS (sizeof(array) / sizeof(array[0]))
  int array[] = {23,34,12,17,204,99,16};

  int main()
  {
      int d;

      for(d=-1;d <= (TOTAL_ELEMENTS-2);d++)
          printf("%d\n",array[d+1]);

      return 0;
  }

Why is that?

Is this homework? (If it is, you should tag it accordingly.) — eq-, Jun 05 '12 at 15:38
Fantastically scary macro that includes the name of the array, instead of taking it as an argument. Very confusing. — unwind, Jun 05 '12 at 15:41
See answer by Hari Mahadevan. Can you tell us what led you to setting variable d to -1 instead of zero? — Miguel Rentes, Jun 05 '12 at 15:51
[cannot-enter-in-while-statement](http://stackoverflow.com/questions/10849724/cannot-enter-in-while-statement/10849748#10849748) - [unsigned-and-signed-comparison](http://stackoverflow.com/questions/3384911/unsigned-and-signed-comparison) - [unsigned-int-and-signed-char-comparison](http://stackoverflow.com/questions/5087992/unsigned-int-and-signed-char-comparison) — gliderkite, Jun 05 '12 at 15:57
I'm curious: is this code that you wrote, and don't understand, or is this code that professor provided as a what-does-this-obscure-code-do quiz? — Robᵩ, Jun 05 '12 at 16:03

score 5 · Answer 1 · edited Jun 05 '12 at 18:02

5

sizeof() returns unsigned int and so in the <= comparison, d gets converted to unsigned int. -1 when converted to unsigned int is 0xffffff. Since 0xffffff > (7-2), for-loop body won't be executed even once.

edited Jun 05 '12 at 18:02

gliderkite

8,828
6
44
80

answered Jun 05 '12 at 15:44

Hari Mahadevan

920
7
14

1

"*-1 when converted to unsigned int is 0xffffff*" -- or `0xffff`, or `0xffffffffffffffff`, or some other number. But always a big number. – Robᵩ Jun 05 '12 at 16:05

score 4 · Accepted Answer · answered Jun 05 '12 at 15:43

This fails because sizeof is returning a value of type size_t, which is unsigned. This causes the comparison to promote the -1 to unsigned, which is generally a very large value and thus make the comparison fail.

You should receive warnings for the sign mismatch.

score 3 · Answer 3 · answered Jun 05 '12 at 15:42

3

The point is the type of value sizeof returns.

It's a size_t, which is an unsigned type, so converting -1 to that type for the comparison produces the maximal value of the type, which is far larger than 5.

answered Jun 05 '12 at 15:42

Daniel Fischer

181,706
17
308
431

When I printed out the value of TOTAL_ELEMENTS I got 7 – Levon Jun 05 '12 at 15:43
@Levon But there's a `(TOTAL_ELEMENTS-2)`. – Daniel Fischer Jun 05 '12 at 15:44

score 1 · Answer 4 · answered Jun 05 '12 at 15:45

1

for(d=-1;d <= ((int)TOTAL_ELEMENTS-2);d++)

answered Jun 05 '12 at 15:45

manuskc

784
4
14

I think your answer is better ;) The code is probably taken from an exam/test question with a conscious "defect". Both yours and mine suggestions are correct but this one says "you can't catch me". – ManieQ Jun 05 '12 at 20:52

score 1 · Answer 5 · answered Jun 05 '12 at 15:49

1

I guess this is more clear:

for(d = 0; d < TOTAL_ELEMENTS; d++)
   printf("%d\n",array[d]);

And size_t case should not be a problem anymore.

answered Jun 05 '12 at 15:49

ManieQ

330
2
8

Levon · Answer 6 · 2012-06-05T15:52:48.207

0

A more standard way to write this loop which works would be

for(d = 0; d < TOTAL_ELEMENTS; d++)
   ...

as was pointed out by correctly by several others here already, the problem lies with signed/unsigned values being compared. This construct would have avoided the problem.

edited Jun 05 '12 at 15:52

answered Jun 05 '12 at 15:40

Levon

138,105
33
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191

score 0 · Answer 7 · answered Jun 05 '12 at 15:52

Your original problem works, when I do the following:

#define TOTAL_ELEMENTS 7
int array[] = {23,34,12,17,204,99,16};

int main()
{
  int d;

  for(d=-1; d <= (TOTAL_ELEMENTS - 2); d++)
      printf("%d\n",array[d+1]);

  return 0;
}

This has something to do with comparing a negative number d=-1 with (TOTAL_ELEMENTS-2). I believe it might be sizeof, butI will leave finding out why as an exercise.

Leaving your original #define as it was --

#define TOTAL_ELEMENTS (sizeof(array) / sizeof(array[0]))

-- but using zero-based indexing also worked.

I'm not sure why your original TOTAL_ELEMENTS definition caused the loop not to print.

Print elements in an array

7 Answers7