Bisection method. Vector<> global variable? function pointers?

[evilcman]

I have this simple code, it should search for a root of function f with the bisection method:

#include <iostream>
#include <vector>
#include <cstdlib>
#include <cmath>

using namespace std;

int n=30;

vector<double> lambda;

double f(double alpha)
{
  double sum;

  for(int i=0;i<n;i++){
    sum += lambda[i]/(lambda[i]+alpha);
  }

  return -0.1*alpha+sum;

}

double bisect(double * a, double * b , double (*f)(double)){
  double c;

  if(f(a[0]) * f(b[0])>=0.0){
    cerr << "bisect error: f(a)*f(b)>=0.0\n";
    return -123456789;
  }

  int itermax=20;
  double EPS=1e-5;
  cout << "iter\n";
  for(int i=0;i<itermax;i++)
  {
    c=((a[0])+(b[0]))/2.0;

    cout << " f(" << a[0] << ")= " << f(a[0])
         << " f(" << b[0] << ")= " << f(b[0])
         << " f(" << c << ")= " << f(c) << endl;

    if(abs(f(c))<EPS){ break;}
    else{
      if(f(a[0])*f(c)<0.0){
      b[0]=c;}
    else{
      a[0]=c;}
    }
 }

 return c;
}

int main(void){

for(int i=0;i<n;i++){
double l = 100((double)rand()/((double)RAND_MAX)-1.0);
lambda.push_back(exp(l));
cout << "lambda[" << i <<"]= " << exp(l) << endl;
}

double a=1e-5;
double b=1e5;
double x;
cout << "bisect\n";
x = bisect(&a,&b,&f);
cout << "\n\nsolution= " << x << endl;

return 0;

}

Here are two lines from the output:
f(24.4141)= 1.61311 f(48.8281)= -0.864139 f(36.6211)= 0.338324
f(36.6211)= -3.51471 f(48.8281)= -4.75959 f(42.7246)= -4.16748
It looks like the function f is not deterministic! Really strange.

If I change to code so that lambda is an additional argument of both the f and bisect functions than the program finds the root correctly. But, this is part of a bigger code, and it would really help me if i could use lambda as a global variable, and I am seriously out of ideas on why this doesn't work as I think it should.

I also tried checking if something changes lambda by writing to stdout the components at each call of f. They remain unchanged.

What the hell is the problem?

[viliml]

try to replace line 14 with double sum=0.0;. I think that the hell is the problem.

[evilcman]

You're right. I'm an idiot... :S

[viliml]

:D