I have to make a program that calculates the integral of a function using the simpsons method and the trapezoid method. It works just fine on my computer using MinGW, but when I try to compile it in my uni's computer (GNU OS) I get:
It = 0 (should be 0.954499)
pn = 0 (should be 18)
Is = 0 (should be 0.954500)
pn = 0 (should be 6)

This is what I came up with (sorry the variables and comments are in portuguese, I'll fix it later after I get home):


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

#define eps 1.e-6
#define kmax 20

double trapezio(double (*f)(double x), double a, double b, int *n);
double simpson(double (*f)(double x), double a, double b, int *n);


#include "integral.h"
#define xmin -2
#define xmax 2

double f(double x);

int main(){
    double It,Is;
    int n = 0;
    int *pn = NULL;

    pn = &n;
    It = trapezio(f,xmin,xmax,pn)/sqrt(2*M_PI);
    printf("Pelo metodo dos trapezios a integral vale aproximadamente %lf\n", It);
    printf("O numero de iteracoes usadas foi %d\n\n",*pn);

    *pn = 0;
    Is = simpson(f,xmin,xmax,pn)/sqrt(2*M_PI);
    printf("Pelo metodo de simpson a integral vale aproximadamente %lf\n", Is);
    printf("O numero de iteracoes usadas foi %d\n",*pn);

    return 0;

double f(double x){
    return exp(-0.5*x*x); // Funcao que sera integrada


#include "integral.h"

double trapezio(double (*f)(double x), double a, double b, int *n){
    double To, Tk;
    double soma;
    int i, k = 1;
    Tk = 0.5*(f(a) - f(b))*(b - a);

    while (fabs((Tk-To)/To) > eps && k < kmax){
        soma = 0; // Resetando variavel soma
        To = Tk; // To e' T(k - 1), caso o loop se repita o ultimo Tk vira To
        for (i = 1 ; i <= (pow(2,k)-1) ; i += 2) soma += f(a + i*(b - a)/pow(2.,k));
        Tk = 0.5*To + soma*(b - a)/pow(2.,k);
        *n += 1;

    return Tk;


#include "integral.h"

double simpson(double (*f)(double x), double a, double b, int *n){
    double So, Sk = 0;
    double somaimp, somapar;
    int i, k = 1;

    while (fabs((Sk-So)/So) > eps && k < kmax){
        somaimp = 0;
        somapar = 0;
        So = Sk; // So e' S(k - 1)
        for (i = 1; i <= (pow(2,k)-1); i += 2) somaimp += f(a + i*(b - a)/pow(2.,k));
        for (i = 2; i <= (pow(2,k)-2); i += 2) somapar += f(a + i*(b - a)/pow(2.,k));
        Sk = (b - a)*(f(a) + 4*somaimp + 2*somapar + f(b))/(3*pow(2.,k));
        *n += 1;

    return Sk;

Edit: I forgot to mention that if I take the pointer out, trapezio works, but simpson still returns 0.

Edited by Arroway

5 Years
Discussion Span
Last Post by rubberman

It works for me, too (mingw32-gcc 4.7.1). What compiler versions do you have on your machine and the school one?


In function trapezio() you do not initialize the variable To, hence the result of fabs((Tk-To)/To) is NAN (Not A Number). The reason Mingw may have worked could be because it auto-initialized the automatic variables.


Ok. Did some more debugging. To is initialized to 0, but you are dividing by To (divide by zero == no-no) at the start of the loop so it doesn't go into the loop. So, your solution is to use do {} while (condition); instead of while (condition) {}; - that will work for both cases. FWIW, trapezios not returns 0.954499 and simpson returns 0.954500 as it did before.

Edited by rubberman

This topic has been dead for over six months. Start a new discussion instead.
Have something to contribute to this discussion? Please be thoughtful, detailed and courteous, and be sure to adhere to our posting rules.