Modulus problem...

[towed]

Problem with Modulus

I am doing fmod(-20, 360) and not getting the correct answer (340).

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

int main()
{

  float x = -20.0;
  float y = 11.0;
  float z = 390.0;

  float x_new, y_new, z_new;

  x_new = fmod(x, 360.0);
  y_new = fmod(y, 360.0);
  z_new = fmod(z, 360.0);

  printf("Using modulus, x = %f becomes x = %f.\n",
         x, x_new);
  printf("Using modulus, y = %f becomes y = %f.\n",
         y, y_new);
  printf("Using modulus, y = %f becomes y = %f.\n",
         z, z_new);

  return 0;

}

[anduril462]

You didn't say what answer you're getting, but I'm guessing it's -20. I think you just don't quite understand what fmod is supposed to do. From the standard:

The fmod functions return the value x − ny, for some integer n such that, if y is nonzero,
the result has the same sign as x and magnitude less than the magnitude of y.If y is zero,
whether a domain error occurs or the fmod functions return zero is implementation-defined

x/y is some small fraction, which, rounded to an integer, toward 0, gives n = 0. -20 - 0 * 360 = -20.

[towed]

Yeah I get -20... But how can I get 340? That is the right answer (i.e., from google).

[gardhr]

Yeah I get -20... But how can I get 340? That is the right answer (i.e., from google).

Subtract the result from 360, of course.

[quzah]

Subtract the result from 360, of course.

Don't you mean add the results to 360?

360 - -20 = 380
360 + - 20 = 340


Quzah.

[gardhr]

Don't you mean add the results to 360?

360 - -20 = 380
360 + - 20 = 340


Quzah.

[sarcasm]Hey thanks for the math lesson Quzah.[/sarcasm]

Yes, obviously I meant "add".

[grumpy]

[sarcasm]Hey thanks for the math lesson Quzah.[/sarcasm]

Yes, obviously I meant "add".

You may have meant it, but you said something else, so your post was misleading. Quzah's post correcting you was therefore justified.

[gardhr]

Quzah's post correcting you was therefore justified.

I have no problem with being corrected. What annoys me is the patronizing tone; asking someone to "confirm the contrary" followed by a graphic illustration of why it must indeed be so implies an ignorant subject - not someone who merely mixed up some words. An appropriate correction, in this case, would have simply been to address the error itself.

[grumpy]

From what evidence I can see, you are responding to a black cat in a dark room that is not there.

[gardhr]

From what evidence I can see, you are responding to a black cat in a dark room that is not there.

Maybe, maybe not. It wouldn't be the first time Quzah's exhibited passive-aggressive behavior, though. I can never be too sure...