Need to write the contents of the function

Question

Have the source code. Content of the function func is unknown:

#include <stdio.h>
#include <math.h>
float func (void)
{
  // Black box
}

int main (void)
{
  float f1, f2;
  int r1, r2;

  f1 = 5.0f;
  f2 = func();

  r1 = (f1 > f2);
  r2 = (f1 <= f2);

  printf ("r1=%d r2=%d\n", r1, r2); 
  return 0;
}

Need to write the contents of the function func, to print out the message:

r1=0 r2=0

Answer:

return pow(-5.0, 0.5);

or

return 0.0/0.0;

Can't understand, why so?

Any NaN will do; see http://stackoverflow.com/questions/6235847/how-to-generate-nan-infinity-and-infinity-in-ansi-c — ecatmur, Feb 21 '14 at 19:11
@haccks dividing any number by `0` produces `Floating point exception`, not `NaN`. — ajay, Feb 21 '14 at 19:13
If `f1 > f2` is false(0) then `f1 <= f2` will be true(1) ... both of them will not be true(1) at the same time ... — Tun Zarni Kyaw, Feb 21 '14 at 19:13
@dima: Most comments are very useful, and there is not *one* correct answer. As stated by ecatmur, any comparison with `NaN` yield false, therefore any function that return `NaN` solves that "puzzle". — Martin R, Feb 21 '14 at 19:18
@ajay: Any undefined value is NaN. See: http://en.wikipedia.org/wiki/NaN — haccks, Feb 21 '14 at 19:19
@TunZarniKyaw: *Both* `f1 > f2` and `f1 <= f2` can be false if `f2` is `NaN`. — Martin R, Feb 21 '14 at 19:20
Actually the quiestion is what should i change in function func — Amazing User, Feb 21 '14 at 19:22
just put `return NAN;`. `NAN` is a macro defined in `math.h` header. — ajay, Feb 21 '14 at 19:23
@dima: I do not understand your question. First you said that *"Content of the function func is unknown"* and now *"what should i change in function func"* - So what do you really want? — Martin R, Feb 21 '14 at 19:25
@Martin R I mean what should I write, not change. The function funk is empty — Amazing User, Feb 21 '14 at 19:29
@dima read this - http://www.gnu.org/software/libc/manual/html_node/Infinity-and-NaN.html — ajay, Feb 21 '14 at 19:42

haccks · Accepted Answer · 2014-02-21T20:47:01.223

1

Regarding the answer

return 0.0/0.0;

it will return NaN. Now in this case both r1 and r2 become 0.

If you return

return pow(-5.0, 0.5);

the square root of a negative number is imaginary, and thus not representable as a real floating-point number, and so is represented by NaN. In this case both of r1 and r2 is 0.
You function would look like:

float func (void)
{
     return 0.0/0.0 // return pow(-5.0, 0.5);
}

Another option is that, just return NaN. (include <math.h>).

float func (void)
{
    return NAN;
}

edited Feb 21 '14 at 20:47

answered Feb 21 '14 at 19:25

haccks

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1

`pow(-5.0, 0.5)` returns `NaN`. `double pow(double, double)` does not return complex numbers. – Martin R Feb 21 '14 at 19:27
@MartinR; I just read this on the wiki: http://en.wikipedia.org/wiki/NaN :) – haccks Feb 21 '14 at 19:28
2

From "man pow": pow(x, y) returns a NaN and raises the "invalid" floating-point exception for finite x < 0 and finite non-integer y. – Martin R Feb 21 '14 at 19:30
The macro is `NAN`, not `NaN`. – ajay Feb 21 '14 at 19:37

Need to write the contents of the function

1 Answers1