Thread: trapezoidal rule

Hi
i'm new here so sorry if i put in the code the wrong way
In class on friday I we were doing the trapezoidal rule and the lecturer told us to create a c programe which takes in five values for x and five values for y and then work out the answer.
The equation to is f(x) = .5*h*(f₀ + 2*f₁ + 2*f₂ + ... + 2*f_n-1 + f_n). (where h is just x₁ - x₀₎
I have created a programe that does it but i want to change it so it can be used for any sized sample.
I have created a matrix that asks how many y values do you want to put in and then puts in the y values.
What i would like some help with is how do i adjust the equation so it can calculate any number of y's instead of just 5?
The code i have below gets you to put in how many y's and but the equation will still only work out 5 y values
Any help would be great

Code:

 
#include <stdio.h>

double trapezoidal (double arrayy[], double arrayx[], double h);
int arraysize;
int main (void)
{
    int a,b;
    double f,arrayx[2],arrayy[100], ans, h,j,g;
    
    printf("Enter arraysize: ");                                  
    scanf ("%d", &arraysize);                   
    
    for (a= 0; a < 2; a++)
    {
        printf ("Enter a value x%d:", a);                      
        scanf("%lf", &arrayx[a]) ;                               
    }
    printf("\n");
    
    for (b= 0; b < arraysize; b++)
    {
        printf ("Enter a value y%d:", b);                      
        scanf("%lf", &arrayy[b]) ;                               
    }
    
    ans = trapezoidal (arrayy,arrayx,h);                           
    
    printf ("The answer is:%lf\n", ans);                      
    
    system ("pause");
    return 0;

}

double trapezoidal (double arrayy[], double arrayx[], double h)
{
    double f,ans;
    int a,b;
    h= arrayx[1] - arrayx[0];                                   
    printf("Value for h is:%lf",h);
    
    system ("pause");
    ans =(.5)*h*(arrayy[0] + 2*arrayy[1] + 2*arrayy[2] + 2*arrayy[3] + 2*arrayy[4] + 2*arrayy[5] + arrayy[arraysize]);   // works out the answer
    
    
    return ans;                                                      
}

Hope it is better now

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Have another go at posting your code, so it isn't on one line.
Try editing it first, rather than posting again.

Originally Posted by nonoob

+ arrayy[arraysize-1]

Minus one.


Quzah.