Old code

#include <iostream>
#include <algorithm>
#include <cmath>
#include <fstream>
#include <string>

#define Nstep 3200
#define A -1.
#define B 1.
#define MAXIT 100000000
#define TOL 1e-9
# define M_PI       3.14159265358979323846

double * Set1DGrid(double a,double b, double (*pf) (double));
double MyFunc(double x);
double RHS(double x);
double FirstGuess(double x);
double ExactResult(double x);
void Copy(double *dest, double *source);
void SetBC(double lower, double upper, double *solution);
void PrintGrid(double *grid);
void PrintOnGrid(double *f,double *grid);
void IterativeSolver(double *u, double *rhs, double tol);
void WriteFile(std::string filename, double *f, double *grid);
double dx = (B-A)/(Nstep-1);
int main (int argc, char *argv[]){
    double * MyGrid = Set1DGrid(A,B, MyFunc);
    double * rhs = Set1DGrid(A,B, RHS);
    double * u = Set1DGrid(A,B, FirstGuess);
    double * exact = Set1DGrid(A,B, ExactResult);
    SetBC(cos(M_PI*exp(-1)),cos(M_PI*exp(1)),u);
    IterativeSolver(u,rhs,TOL);
    WriteFile("solution.txt",u,MyGrid);
    WriteFile("exact.txt",exact,MyGrid);
    delete MyGrid;
    delete rhs;
    delete u;
    delete exact;
    return 0;
}
double * Set1DGrid(double a,double b, double (*pf)(double)){
    double * grid = new double[Nstep];
    for (int i=0;i<Nstep;++i)
        grid[i]=pf(a+i*(b-a)/(Nstep-1));
    return grid;
}
double MyFunc(double x)
{
    return x;
}
double RHS(double x){
    return -M_PI*exp(x)*(M_PI*exp(x)*cos(M_PI*exp(x))+sin(M_PI*exp(x)));
}
double FirstGuess(double x){
    return 0;
}
void SetBC(double lower, double upper, double *solution){
    solution[0]=lower;
    solution[Nstep-1]=upper;
}
void IterativeSolver(double *u, double *rhs, double tol){
    int iter=0;
    double * prevU = new double[Nstep];
    Copy(prevU,u);
    double max;
    while (iter<=MAXIT){
        max=0;
        for (int i=1;i<Nstep-1;++i){
            u[i]=(prevU[i+1]+prevU[i-1]-dx*dx*rhs[i])*0.5;
            max = fabs(u[i]-prevU[i]) > max? fabs(u[i]-prevU[i]):max;
        }
        if (max < tol){
            std::cout<<"#Solution converged in "<<iter<<" iterations with error "<<
            TOL<<std::endl;
            break;
        }
        Copy(prevU,u);
        ++iter;
    }
    delete prevU;
}
void PrintGrid(double * grid){
    for (int i=0;i<Nstep;++i)
    std::cout<<grid[i]<<" ";
    std::cout<<std::endl;
}
void PrintOnGrid(double *f,double *grid){
    for (int i=0;i<Nstep;++i)
    std::cout<<grid[i]<<" "<<f[i]<<std::endl;
}
double ExactResult(double x){
    return cos(M_PI*exp(x));
}
void Copy(double *dest, double *source){
    for (int i=0;i<Nstep;++i)
        dest[i]=source[i];
}
void WriteFile(std::string filename, double *f, double *grid){
    std::ofstream outfile;
    outfile.open(filename.c_str());
    if (!outfile.is_open())
    {
        std::cout<<"Cannot open file "<<filename<<"."<<std::endl;
        return;
    }
    for (int i=0;i<Nstep;++i)
        outfile<<grid[i]<<"  "<<f[i]<<std::endl;
    outfile.close();
}