Thread: Modify program so that it finds maximum crest & minimum trough for combined waves

Code:

#include <stdio.h>
#include <math.h>
#define PI 3.141593

int main(void)
 {
   /*  Declare variables.  */
         int k;
         double A1, A2, freq1, freq2, height1, height2, length1, length2;
         double T1, T2, w1, w2, sum, new_period, new_height, time_incr, t;
         double maxwave=0;

   /*  Get user input from the keyboard.  */
         printf("Enter integer wave period (s) and wave height (ft) \n");
         fflush (stdout);
         printf("for wave 1: \n");
         fflush (stdout);
         scanf("%lf %lf",&T1,&height1);
         printf("Enter integer wave period (s) and wave height (ft) \n");
         fflush (stdout);
         printf("for wave 2: \n");
         fflush (stdout);
         scanf("%lf %lf",&T2,&height2);

   /*  Determine and print wavelengths.  */
        length1 = 5.13*T1*T1;
        length2 = 5.13*T2*T2;
        printf("Wavelengths (in ft) are: %.2f %.2f \n",length1,length2);
        fflush (stdout);

   /*  Determine period of combined waves.  */
        new_period = T1*T2;

   /*  Compute 200 points of the combined waves over the  */
   /*  period specified, and find the maximum height.     */
        time_incr = new_period/200;
        A1 = height1/2;
        A2 = height2/2;
        freq1 = 1/T1;
        freq2 = 1/T2;
   
    for (k=0; k<=199; k++)
   {
      t = k*time_incr;
      w1 = A1*sin(2*PI*freq1*t);
      w2 = A2*sin(2*PI*freq2*t);
      sum = w1 + w2;
      if (sum > maxwave)
         maxwave = sum;
   }
   new_height = maxwave*2;

   /*  Print new wave maximum.   */
        printf("Maximum combined wave height is %.2f ft \n",new_height);
        fflush (stdout);

   /*   Exit program.  */
   return 0;
}
/*------------------------------------------------------------*/

i need to modifi this program so that i can get the maximun crest and minimum trough for the combined waves

I'm sure there is a more direct algebraic way to find minimum and maximum of sum of sines other than going through samples at a fine resolution.

Originally Posted by nonoob

I'm sure there is a more direct algebraic way to find minimum and maximum of sum of sines other than going through samples at a fine resolution.

You could calculate the derivatives and hard code expressions into the program if you wanted. Then get the program to solve the corresponding equation. Another alternative is to numerically calculate the derivatives (i.e. calculate (delta y)/(delta t) etc) and then take smaller increments in the range when the derivative changes sign. e.g. if (delta y)/(delta t) > 0 for t = 29, but (delta y)/(delta t) < 0 for t = 30, then take a smaller range of t between 29 and 30 and repeat. Then you might find it switches again at t = 29.7, then repeat and you might find it switches at t = 29.73. The more you repeat the more decimal places you can calculate, but I'm not going to help the OP with that unless the OP asks and makes an attempt first. lol

Thanks, Swarvy. Yeah, derivatives... worst case.
Maybe something can be done with this
sin A + sin B = 2sin ((A+B)/2) cos ((A-B)/2)
but it isn't any easier that way.

I was hoping for some simpler function involving factorization - treating the sines as triangular waves for example, then iterating around the areas showing the most promise.

Sorry, I guess the problem isn't as easy as I thought initially.