relative error- can't stop it

[EDIT: 20110610 00:13 CDT - reformat - WDR]

Hi

I need help to write up a program which makes simple iterations. I don't know how to implement relative error and make it stop when error gets to 0.00001%.

here is my code:

%simple iteration method
f=input('f(x)=','s');           %you input the function, already re-arranged for calculations
tol='error tolerance=1e-5';     %error tolerance is defined by program
if length(tol)==0, tol=1e-5; end
x1=input('first guess=');
x=x1; fx=eval(f);
for i=1:1000000000
    x=fx;
    ff=eval(f);
    fx=eval(f);
    fprintf('i = %g, x = %g, fx = %g\n',i,x,fx)    
end

Thank you for help

Answer 1

[David Young]

To compute the relative error (assuming you're trying to find a stationary point of the function), after computing ff, subtract ff from x and take the absolute value. Then maybe you should divide that by the absolute value of x, but that depends on how you define relative error.

Then use a conditional statement (an "if" statement) to test whether the error is less than the tolerance, and use "break" to stop the loop if it is.

Other points to note:

• 0.00001% is 1e-7, not 1e-5.
• You can save computation time by using fx = ff instead of fx = eval(f). You do this after testing the error of course.

[jamshid]

I tried to do it, but my error is not calculated with a new value of fx. so it always gets value of 0. then I tried to make condition if abs(fx)<tol, break, end but this doesn't execute the process. it just stops.