Source code in the c programming language

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

double int_leftrect(double from, double to, double n, double (*func)())
{
   double h = (to-from)/n;
   double sum = 0.0, x;
   for(x=from; x <= (to-h); x += h)
      sum += func(x);
   return h*sum;
}

double int_rightrect(double from, double to, double n, double (*func)())
{
   double h = (to-from)/n;
   double sum = 0.0, x;
   for(x=from; x <= (to-h); x += h)
     sum += func(x+h);
   return h*sum;
}

double int_midrect(double from, double to, double n, double (*func)())
{
   double h = (to-from)/n;
   double sum = 0.0, x;
   for(x=from; x <= (to-h); x += h)
     sum += func(x+h/2.0);
   return h*sum;
}

double int_trapezium(double from, double to, double n, double (*func)())
{
   double h = (to - from) / n;
   double sum = func(from) + func(to);
   int i;
   for(i = 1;i < n;i++)
       sum += 2.0*func(from + i * h);
   return  h * sum / 2.0;
}

double int_simpson(double from, double to, double n, double (*func)())
{
   double h = (to - from) / n;
   double sum1 = 0.0;
   double sum2 = 0.0;
   int i;

   double x;
   
   for(i = 0;i < n;i++)
      sum1 += func(from + h * i + h / 2.0);

   for(i = 1;i < n;i++)
      sum2 += func(from + h * i);

   return h / 6.0 * (func(from) + func(to) + 4.0 * sum1 + 2.0 * sum2);
}


/* test */
double f3(double x)
{
  return x;
}

double f3a(double x)
{
  return x*x/2.0;
}

double f2(double x)
{
  return 1.0/x;
}

double f2a(double x)
{
  return log(x);
}

double f1(double x)
{
  return x*x*x;
}

double f1a(double x)
{
  return x*x*x*x/4.0;
}

typedef double (*pfunc)(double, double, double, double (*)());
typedef double (*rfunc)(double);

#define INTG(F,A,B) (F((B))-F((A)))

int main()
{
     int i, j;
     double ic;
     
     pfunc f[5] = { 
       int_leftrect, int_rightrect,
       int_midrect,  int_trapezium,
       int_simpson 
     };
     const char *names[5] = {
       "leftrect", "rightrect", "midrect",
       "trapezium", "simpson" 
     };
     rfunc rf[] = { f1, f2, f3, f3 };
     rfunc If[] = { f1a, f2a, f3a, f3a };
     double ivals[] = { 
       0.0, 1.0,
       1.0, 100.0,
       0.0, 5000.0,
       0.0, 6000.0
     };
     double approx[] = { 100.0, 1000.0, 5000000.0, 6000000.0 };
          
     for(j=0; j < (sizeof(rf) / sizeof(rfunc)); j++)
     {
       for(i=0; i < 5 ; i++)
       {
         ic = (*f[i])(ivals[2*j], ivals[2*j+1], approx[j], rf[j]);
         printf("%10s [ 0,1] num: %+lf, an: %lf\n",
                names[i], ic, INTG((*If[j]), ivals[2*j], ivals[2*j+1]));
       }
       printf("\n");
     }
}