Value of tan(90) in c?

Question

the value it give is 557135813.94455. does the value will remain same every time?? why its not showing infinity??

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

#define PI 3.14159265

int main ()
{
  double param, result;
  param = 90.0;
  result = tan ( param * PI / 180.0 );
  printf ("The tangent of %f degrees is %f.\n", param, result );
  return 0;
}

that `90.0 * pi / 180.0 `is so confusing. Why not just `pi / 2`? Edit: Nevermind didn't even notice it was a degree to radian convertion — Dleep, Jul 20 '15 at 17:33
Do not define PI on your own, use M_PI, M_PI or M_PIl from math.h — nsilent22, Jul 20 '15 at 17:41
On my system the output of your program is 557135115.020977. — 4566976, Jul 20 '15 at 18:04
@nsilent22 `M_PI` is not in standard `` See http://stackoverflow.com/a/5008376/2410359 — chux - Reinstate Monica, Sep 22 '15 at 03:36

score 7 · Answer 1 · answered Jul 20 '15 at 17:33

7

You are not passing the value of Pi/2, you are passing 90.0 * 3.14159265 / 180.0, an approximation.

answered Jul 20 '15 at 17:33

chux - Reinstate Monica · Answer 2 · 2016-01-11T22:54:55.120

Code is not asking for the tangent of 90°, but the tangent of a number, in radians, close to 90°. The conversion to radians is not exact since π/2 radians is not representable exactly as a double.

The solution is to perform degrees range reduction first and then call tan(d2r(x)).

#include <math.h>

static double d2r(double d) {
  return (d / 180.0) * ((double) M_PI);
}

double tand(double x /* degrees */) {
  if (!isfinite(x)) {
    return tan(x);
  } else if (x < 0.0) {
    return -tand(-x);
  }
  int quo;
  double x45 = remquo(fabs(x), 90.0, &quo);
  //printf("%d %f ", quo & 3, x45);
  switch (quo % 4) {
    case 0:
      return tan(d2r(x45));
    case 1:
      return 1.0 / tan(d2r(- x45));
    case 2:
      return -tan(d2r(-x45));
    case 3:
      return -1.0 / tan(d2r(x45));
  }
  return 0.0;
}

#define PI 3.14159265

int main(void) {
  double param, result;
  param = 90.0;
  result = tan(param * PI / 180.0);
  printf("Angle  %.*e radian\n", DBL_DECIMAL_DIG - 1, param * PI / 180.0);
  printf("Pi/2 = 1.5707963267948966192313216916398...\n");
  printf("The tangent of %f degrees is %f.\n", param, result);
  int i;
  for (i = -360; i <= 360; i += 30) {
    printf("The tangent method 1 of %.1f degrees is  %.*e\n", 
        1.0*i, DBL_DECIMAL_DIG - 1, tan(d2r(-i)));
    printf("The tangent method 2 of %.1f degrees is  %.*e\n", 
        1.0*i, DBL_DECIMAL_DIG - 1, tand(-i));
  }
  return 0;
}

OP's output

Angle  1.5707963250000001e+00 radian
Pi/2 = 1.5707963267948966192313216916398...
The tangent of 90.000000 degrees is 557135183.943528.

Better results

The tangent method 1 of -360.0 degrees is  -2.4492935982947064e-16
The tangent method 2 of -360.0 degrees is  0.0000000000000000e+00
The tangent method 1 of -330.0 degrees is  -5.7735026918962640e-01
The tangent method 2 of -330.0 degrees is  -5.7735026918962573e-01
The tangent method 1 of -300.0 degrees is  -1.7320508075688770e+00
The tangent method 2 of -300.0 degrees is  -1.7320508075688774e+00
The tangent method 1 of -270.0 degrees is  5.4437464510651230e+15
The tangent method 2 of -270.0 degrees is  -inf
The tangent method 1 of -240.0 degrees is  1.7320508075688752e+00
The tangent method 2 of -240.0 degrees is  1.7320508075688774e+00
The tangent method 1 of -210.0 degrees is  5.7735026918962540e-01
The tangent method 2 of -210.0 degrees is  5.7735026918962573e-01
The tangent method 1 of -180.0 degrees is  -1.2246467991473532e-16
The tangent method 2 of -180.0 degrees is  0.0000000000000000e+00
...

score 1 · Answer 3 · answered Jul 20 '15 at 17:41

Floating point arithmetic is not an exact arithmetic. You cannot even compare two floating point numbers using ==; e.g. 0.6 / 0.2 - 3 == 0 should be true but on most systems it will be false. Be careful when you perform floating point calculations and expect exact results; this is doomed to fail. Consider every floating point calculation to only return an approximation; albeit a very good one, sometimes even an exact one, just don't rely on it to be exact.

Value of tan(90) in c?

3 Answers3

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