How can I fix the bisection method loop when the decimal place value is changed?

[SolStis]

Hi, I've written code to resolve the routes of a function using the bisection method. The code works for the current route brackets (xb and xt) and for a decimal place value of 2. However when the decimal place value is changed the loop gets stuck. Code below:

#include <iostream>
#include <cmath>
#include <iomanip>
#include <fstream>

using namespace std;

//declaration and initiation of fixed variables
double m=5.0,L=0.6,k=1000.0,g=9.81;

//Setup of function to be solved
double f(double x){
return tan(x)-sin(x)-(m*g)/(k*L);
}

//Main function
int main(){

//Declaration of decimal places var d
int d;

//Declaration of upper, lower and middle limit vars
double xb,xt,xm,dp;

//Promt user to input no. dec places of accuracy
cout << "Please input the number of decimal places to which the calculation will be accurate to.. " << endl;
cin >> d;

//Declaration of count var i
int i;

//Setup of initial route bracket and decimal place var dp
xb=0.5;
xt=1.0;
dp=pow(0.1,d);
double ans;

do{
xm=(xb+xt)/2;
ans=f(xm);

if((f(xb)*f(xt))>0){
xb=xm;
}
else{
xt=xm;
}
i++;
}

while ((ans>dp)||(ans<-(dp)));

cout << endl << i << " " << xm << " " << ans;

}

Any resolution to this would be greatly appreciated


Thanks

Sol

 

[uart]

It's only by good luck that it even works for d=2 actually.

The error is in the line "if((f(xb)*f(xt))>0)". Here you are testing if f(xb) and f(xt) are the same sign, but you really should be comparing the sign of f(xm) and that of the end points.

 

[SolStis]

Thankyou v much