Print 1 to 1000 with out using loop

Question

i see the question on a c++ programming context, i check for a solution and one of my friend give me this code its works perfect but i can't understand it's logic and also how it's works. i asked to him about it but he also don't know how the program is actually works, i think he is also take this solution from somewhere. Anybody can explain the logic behind this i mean in the line (&main + (&exit - &main)*(j/1000))(j+1); ?

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

void main(int j) {
  printf("%d\n", j);
  (&main + (&exit - &main)*(j/1000))(j+1);
}

Thanks in advance

Thanks for your quick replay. i try it in c++ but it will not work but in c its works perfectly.but i don't know how its works .i mean its logic — Arunprasanth K V, Nov 04 '14 at 09:34
Seems like this code is "cheating" by using a recursive function call to create the counting loop (it's not an explicit loop, but there's still a loop there). Still, nice solution. — G0BLiN, Nov 04 '14 at 10:37
@Anonymous it's a solution of a contest, I think it's meant to be full of undefined behavior :) — Ruslan, Nov 04 '14 at 12:06

score 28 · Accepted Answer · edited Nov 04 '14 at 10:08

28

It works as follows:

Performs the int division j/1000, which will return 0 always while j is smaller than 1000. So the pointer operation is as follows:

&main + 0 = &main, for j < 1000.

Then it calls the resulting function pointed by the pointer operations passing as parameter j+1. While j is less than 1000, it will call main recursively with parameter one more than the step before.

When the value of j reaches 1000, then the integer division j/1000 equals to 1, and the pointer operation results in the following:

&main + &exit - &main = &exit.

It then calls the exit function, which finishes the program execution.

edited Nov 04 '14 at 10:08

David Ranieri

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answered Nov 04 '14 at 09:35

LoPiTaL

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does the exit function must be written explicitly? or it can be deduced from the main? – antonpuz Nov 04 '14 at 09:37
2

The exit function is from the C std library, defined in stdlib.h. – LoPiTaL Nov 04 '14 at 09:39
It basically "linearly interpolates" the difference from the `MAIN` function to the `EXIT` function, with `integer-division` so that the switch from `A to B` is *instantaneous* once `1001` has been reached - and the integer is increased each time from the *recursive call* of `MAIN`. – EpicPandaForce Nov 04 '14 at 12:37

Gopi · Answer 2 · 2014-11-04T10:49:05.890

4

I go with the explanation already given but it would be easier to understand if written as below:

void main(int j) {
   if(j == 1001)
      return;
   else
   {   
      printf("%d\n", j); 
      main(j+1);
   }   
}

The above code does the same as already written code.

edited Nov 04 '14 at 10:49

answered Nov 04 '14 at 10:42

Gopi

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Realy this is more simple than the above .but logic is different – Arunprasanth K V Nov 04 '14 at 10:52
The idea is the same though. The one you have has just been obfuscated, this is the canonical way to do recursion :- though you wouldn't usually use `main` to do it as it technically isn't a valid `main` signature iirc – Baldrickk Nov 04 '14 at 10:56
1

really this is more simple method rather than above one. thanks for your replay – Arunprasanth K V Nov 04 '14 at 11:36
@ArunPrasanth np. The idea behind this is recursion and this would make you understand the code easily instead of the code which you have posted – Gopi Nov 04 '14 at 12:00

Print 1 to 1000 with out using loop

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