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While loops

inf2405 (11)
Hi,
I have to compare Euler, Heun's and Runge Kutta for different step sizes. I have done this by using while loops. But the problem is that I only get the result of the first while loop and zero for the others. Can't I use while loops multiple times and if so, how can I implement a loop multiple times so that I get the respective results?
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#include<iostream>
#include<iomanip>
#include<cmath>

using namespace std;

double func(double t)
{
    double a=4*(exp(0.8*t)-exp(-0.5*t))/1.3 + 2*exp(-0.5*t);
    return a;
}

double f(double t, double y)
{
    y=4*(exp(0.8*t)-exp(-0.5*t))/1.3 + 2*exp(-0.5*t);
    double a=4*exp(0.8*t)-(0.5*y);
    return a;
}

int main()
{
    double t0=0;
    double y0=2;
    double y;
    double sum11=0;
    double sum12=0;
    double sum13=0;
    double sum21=0;
    double sum22=0;
    double sum23=0;
    double sum31=0;
    double sum32=0;
    double sum33=0;
    double K1, K2, K3, K4;
    double h;
    
    //The Euler method for h=0.1
    while(t0<=9.9)
    {
        h=0.1;
        y= y0 + (h*f(t0,y0));
        y0=y;
        t0=t0+h;
        sum11=sum11+abs(func(t0)-y);
    }
    
    //The Euler method for h=0.05
    while(t0<=9.9)
    {
        h=0.05;
        y= y0 + (h*f(t0,y0));
        y0=y;
        t0=t0+h;
        sum12=sum12+abs(func(t0)-y);
    }
    
    //The Euler method for h=0.025
    while(t0<=9.9)
    {
        h=0.025;
        y= y0 + (h*f(t0,y0));
        y0=y;
        t0=t0+h;
        sum13=sum13+abs(func(t0)-y);
    }
    
    //Heun's method for h=0.1
    while(t0<=9.9)
    {
        h=0.1;
        K1=f(t0,y0);
        K2=f(t0+h, y0+h*K1);
        y = y0+ (h*(K1+K2))/2;
        y0=y;
        t0=t0+h;
        sum21=sum21+abs(func(t0)-y);
    
    }
    
    //Heun's method for h=0.05
    while(t0<=9.9)
    {
        h=0.05;
        K1=f(t0,y0);
        K2=f(t0+h, y0+h*K1);
        y = y0+ (h*(K1+K2))/2;
        y0=y;
        t0=t0+h;
        sum22=sum22+abs(func(t0)-y);
        
    }
    
    //Heun's method for h=0.025
    while(t0<=9.9)
    {
        h=0.025;
        K1=f(t0,y0);
        K2=f(t0+h, y0+h*K1);
        y = y0+ (h*(K1+K2))/2;
        y0=y;
        t0=t0+h;
        sum23=sum23+abs(func(t0)-y);
        
    }
    
    //4-stage Runge-Kutta method for h=0.1
    while(t0<=9.9)
    {
        h=0.1;
        K1= h*f(t0,y0);
        K2=h*f(t0+ h/2, y0+ K1/2);
        K3=h*f(t0+ h/2, y0+ K2/2);
        K4=h*f(t0+h, y0+ K3);
        y = y0 + (K1+ 2*K2+ 2*K3+ K4)/6;
        y0=y;
        t0=t0+h;
        cout << "Values of t are: " << t0 << endl;
        cout << "The approximate values are:" << y << endl;
        cout << "The actual values are: " << func(t0) << endl;
        cout << "The respective error is: " << abs(func(t0)-y)<< endl;
        sum31=sum31+abs(y-func(t0));
    }
    cout << "The cumulative error is:" << sum31 << endl;
    
    //4-stage Runge-Kutta method for h=0.05
    while(t0<=9.9)
    {
        h=0.05;
        K1=h*f(t0,y0);
        K2=h*f(t0+ h/2, y0+ K1/2);
        K3=h*f(t0+ h/2, y0+ K2/2);
        K4=h*f(t0+h, y0+ K3);
        y = y0 + (K1+ 2*K2+ 2*K3+ K4)/6;
        y0=y;
        t0=t0+h;
        sum32=sum32+abs(func(t0)-y);
    }

    
    //4-stage Runge-Kutta method for h=0.025
    while(t0<=9.9)
    {
        h=0.025;
        K1=h*f(t0,y0);
        K2=h*f(t0+ h/2, y0+ K1/2);
        K3=h*f(t0+ h/2, y0+ K2/2);
        K4=h*f(t0+h, y0+ K3);
        y = y0 + (K1+ 2*K2+ 2*K3+ K4)/6;
        y0=y;
        sum33=sum33+abs(func(t0)-y);
    }
    setprecision(20);
    cout << fixed;
    cout << "                         " << "h" << "                     " << endl;
    cout << "                 0.1" << "       " << "   0.05" << "         " << "  0.025" << endl;
    cout <<"  Euler    " <<     sum11 <<"            " <<       sum12 << "               " <<     sum13 << endl;
    cout <<"  Heun       " <<  sum21 << "           "  <<   sum22 << "                " <<     sum23 << endl;
    cout <<"Runge Kutta   " <<  sum31 << "          " <<     sum32 << "              " <<     sum33 << endl;

    return 0;
}
salem c (3686)
Yeah, look at your t0 values at the start of each while loop.

You should have had a function called euler
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double euler ( double t0, double t1, double h ) {
  double result = 0.0;
  double y0 = 2, y = 0.0;
  while ( t0 <= t1 ) {
        y= y0 + (h*f(t0,y0));
        y0=y;
        t0=t0+h;
        result=result+abs(func(t0)-y);
    }
  return result;
}


Then your main would have been
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    //The Euler method for h=0.1
    sum11 = euler(0,9.9,0.1);
    //The Euler method for h=0.05
    sum12 = euler(0,9.9,0.05);
    //The Euler method for h=0.025
    sum13 = euler(0,9.9,0.025);


You copy/pasted yourself into a hole.
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