CODEWITHSUNDEEP
cfunctiontrigonometry
Trying to program
Trying to program

Reputation: 15

Creating a sin formula in C

The program compiles but when running it I am not getting the correct values back from my inputs. I have looked for a way to make the sine formula and have found this one but I don't believe it was right.

Is my formula correct? I think just running the sin function in C is giving me the wrong value as well.

Update: I'm still getting the wrong values with these changes if I input 1.5 and 4. I'm getting 0.000 and a random integer

 /*
  * Function mySin is using a sin formula to get the sin of x
  * the function then returns the sin value computed 
  * Parameters -for function mySin are "x" is the value for sin
  *            -"n"is the number of series to run
  *
  *The program also uses the sin(x) function to get the real sin
  * 
  */ 

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

 int mySin(double x, int n){

    int i=0;
    double sinx=0;

    for(i=1; i<=n; i+=2){
        sinx=(pow(x,i)+sinx); // 
    }

    return sinx;
    }


    int main(double sinx){

    double realSin=0;

    double x=0;
    int n=0;

    printf("Please input x and n:");
    scanf("%lf",&x);
    scanf("%d",&n);

    mySin(x,n);

    realSin=sin(x);

    printf("mySin= %f\n sin=%d\n",sinx,realSin); 
 }

Upvotes: 0

Views: 2966

Answers (4)

sunmoon
sunmoon

Reputation: 1498

mySin should return double. You are returning int.

sin values can be calculated with the Fourier series.

Upvotes: 1

phuclv
phuclv

Reputation: 41805

One more incorrect point in your code is that you're calculating power by **

There's no power operator in C. The compiler is treating x**i as x * (*i). But since i is not a pointer, it returns the error you saw. You must use pow() instead

Upvotes: 1

user2357112
user2357112

Reputation: 280857

Your mySin function is wrong in at least 5 separate ways. Using ** to represent exponentiation (to avoid confusion with xor), the correct formula is

sin(x) = x - x**3/3! + x**5/5! - x**7/7! + x**9/9! ...

Your implementation fails because

  1. It discards the previous terms on every iteration of the loop.
  2. It doesn't alternate signs.
  3. It includes even-exponent terms.
  4. It doesn't compute a factorial in the divisor.
  5. The return type is int, discarding the fractional component of the computed result.

Furthermore, main is wrong in several more ways. The return value of mySin is completely ignored. Instead, sinx is the first argument to main, which is the number of command-line arguments the program received (including the name it was run as). Also, %d is used for all numbers in format strings, regardless of type, when it's only meant for int.

To fix mySin, have i only go over odd values, and have every iteration of the loop compute x**i/i! and add it into the current value of sinx.

To fix main, declare a local sinx variable in main and assign sinx = mySin(x, n) instead of declaring sinx as an argument. Also, use %lf for reading doubles with scanf and %f for writing doubles with printf.

Upvotes: 5

Alaeddine
Alaeddine

Reputation: 1687

x is double so you have to use the %lf in scanf and %f in the printf

    double x;
    scanf("%lf", &x);
    printf ("%f", x);

Upvotes: 1

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