範例EX4-2 利用梯行法計算(exp(-pow(x,2)/2)) / sqrt(2*3.14159) , x=-5.3 to 3.0 的積分值

範例EX4-2 利用梯行法計算(exp(-pow(x,2)/2)) / sqrt(2*3.14159)  , x=-5.3 to 3.0 的積分值

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

/* 示範梯形數值積分法  (trapeziodal.c) */
#define STEP 10

double f(double);

main()
{
    int n, k, i;
    double h, T, a, b, x;
    T = 0;
    a = -3.5;
    b = -3.0;
    for (n=1, i=0; i < STEP; ++i) {
        n = 2*n;
        h = (b-a)/n;
        x = a;
        T = f(x)/2;
        for (k=1; k<n ; ++k) {
           x = x + h;
           T = T + f(x);
        }
        T = T + f(b)/2;
        T = h*T;
        printf("T_%-5d = %17.14f.\n", n, T);
    }
}


double f(double x) {
    return ( (exp(-pow(x,2)/2)) / sqrt(2*3.14159));
}

T_8     =  0.00112060204348.                                                                                                                   

T_16    =  0.00111810278396.                                                                                                                   

T_32    =  0.00111747777712.                                                                                                                   

T_64    =  0.00111732151342.                                                                                                                   

T_128   =  0.00111728244675.                                                                                                                   

T_256   =  0.00111727268003.                                                                                                                   

T_512   =  0.00111727023835.                                                                                                                   

T_1024  =  0.00111726962793.                                                                                                                   

                                                                                                                                               

                                                                                                                                               

...Program finished with exit code 0                                                                                                           

Press ENTER to exit console.