How can I compare a number with other numbers without writing them all out in C?

Question

Problem: I have IF statement if (x == 180 || x == 360 || x == 540 and so on

How can I keep that list going without writing 180 + 180 * n all out?

Extra info: i want to print "doesnt exist" if sin(x * M_PI / 180) is 0. It is 0 when sin(180), sin(360) and so on.

EDIT: i've tried sin(x2 * M_PI / 180) == 0 but it doesnt work (probably because its close to 0 but not 0)

If `x` an integer or floating point number? What if the value is -180, -360,...? — chux - Reinstate Monica, Oct 03 '16 at 15:04

score 9 · Accepted Answer · answered Oct 03 '16 at 12:50

9

use modulo operand : %

if (x>0 && x%180==0) {..}

answered Oct 03 '16 at 12:50

P.Bra

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score 6 · Answer 2 · answered Oct 03 '16 at 12:52

Look into the mod % operator. It gives you the remainder when doing a divisions. So

(x % 180)

will be 0 for any integer multiple of 180 (assuming you're not so high that you get some odd wrap-around, say over 4 billion for an unsigned number.)

So you could use.

if ((x % 180) == 0)

score 3 · Answer 3 · answered Oct 03 '16 at 12:49

3

if (180 <= x && x % 180 == 0)

should do it. Why are you using M_PI ?

answered Oct 03 '16 at 12:49

Tom's

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If he wants to do it the complicated way with sin, that's what must be done to convert to radians. – kalj Oct 03 '16 at 12:52

rocambille · Answer 4 · 2016-10-03T13:08:32.373

2

If x is an integer, you can use remainder operator in your IF statement:

if((x%180) == 0)
{
    // ...
}

Or, if x is not an integer, you can use fmod:

if(fabs(fmod(x, 180.0)) < DBL_EPSILON)
{
    // ...
}

edited Oct 03 '16 at 13:08

answered Oct 03 '16 at 12:50

rocambille

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score 1 · Answer 5 · answered Oct 03 '16 at 12:52

1

A sloppy but practical test is

if ( fabs(sin(a)) < FLT_EPSILON ) 
   treataszero();

answered Oct 03 '16 at 12:52

Malcolm McLean

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2

When comparing against zero, [epsilon method might not be what you want](https://randomascii.wordpress.com/2012/02/25/comparing-floating-point-numbers-2012-edition/). – user694733 Oct 03 '16 at 13:01
No, it's sloppy. But usually dividing by a very small value is almost as bad as a division by zero, because the floating point unit can't cope and accuracy is lost. Generally using FLT_EPSILON as "effectively zero" is good enough, but not for serious numerical work. – Malcolm McLean Oct 03 '16 at 13:06

score 1 · Answer 6 · edited May 23 '17 at 12:19

What appears to be OP's higher level problem "tried sin(x2 * M_PI / 180) == 0 but it doesnt work (probably because its close to 0 but not 0)" is the goal is to take the sine of x as expressed in degrees.

Rather than x%180, which is a problem if x is floating point, the idea is to reduces the sin/cos argument to a range of 90 degrees first before converting to radians. No need to give up some precision with an EPSILON comparison.

See c++ Sin and Cos to get the exact value for many cases n*90 degrees and other improved accuracies.

How can I compare a number with other numbers without writing them all out in C?

6 Answers6