MATLAB Answers

Infinite loop in False postion method

4 views (last 30 days)
R Abhinandan
R Abhinandan on 27 Oct 2020
Commented: Cris LaPierre on 27 Oct 2020
Here is my code for false position method and im getting a infinite loop, i want it to stop at tol=10^-4, can someone help?
a=0.5;
b=1;
tol=0.0001;
counter=0;
f =@(x) exp(x)-3*x^2;
while abs(b-a)>tol
c= b - (f(b)* (b-a)/(f(b)-f(a)));
if f(a)*f(c)<0
b=c;
end
if f(a)*f(c)>0
a=c;
end
counter=counter+1;
er=abs(a-b);
errorcounter(counter)=er;
end
figure
hold on
plot(errorcounter)
title('Convergence of False Position')
ylabel('Error')
xlabel('Iterations')
axis ([0 5 0 9e-04])

  1 Comment

Cris LaPierre
Cris LaPierre on 27 Oct 2020
btw, hold on is unnecessary here. When used, it should always be paired with hold off.

Sign in to comment.

Answers (1)

Cris LaPierre
Cris LaPierre on 27 Oct 2020
I don't understand what your approach is, but by converting your while loop to a for loop, I can see that your error stops decreasing at 0.09. Here's a simplified version of your code.
a=0.5;
b=1;
f =@(x) exp(x)-3*x^2;
% while abs(b-a)>tol
for loop = 1:10
c= b - (f(b)* (b-a)/(f(b)-f(a)));
if f(a)*f(c)<0
b=c;
elseif f(a)*f(c)>0
a=c;
end
er=abs(a-b)
end
er = 0.1193
er = 0.0915
er = 0.0901
er = 0.0900
er = 0.0900
er = 0.0900
er = 0.0900
er = 0.0900
er = 0.0900
er = 0.0900

  2 Comments

R Abhinandan
R Abhinandan on 27 Oct 2020
I want the loop to stop at tol=10^-4, for example here is my code for bisection method which works perfectly but when i tried for false postion it doesn't
f=@(x) exp(x)-3*(x)^2;
a=0.5;
b=1;
tol=10^(-4);
counter = 0;
while abs(b-a) > tol
c = (b+a)/2;
if (f(a)*f(c)) <0
b = c;
end
if (f(a)*f(c)) >0
a = c;
end
counter = counter + 1;
err(counter) = abs(a - b);
end
disp(b)
figure
hold on
plot(err)
title('Convergence of Bisection')
ylabel('Error')
xlabel('Iterations')
axis ([0 13 0 0.0001])
Cris LaPierre
Cris LaPierre on 27 Oct 2020
I understand. Check your algorithm. You have not implemented it correctly.

Sign in to comment.

Community Treasure Hunt

Find the treasures in MATLAB Central and discover how the community can help you!

Start Hunting!

Translated by