Can you answer why am I getting this error. What should Ido? The error says 'Unrecognised variable z'.

clear
clc
f=@(x) x+exp(x)-sin(x);
a=10;
b=12;
for i=1:50
    c=((a*f(b)-b*f(a))/(f(b)-f(a)));
    if abs(f(c))<0
        a=c;
        z=i;
        break;
    end
    if f(a)*f(c)>0
        b=c;
    else
        a=c;
    end
end
    fprintf('Regula Falsi Method');

Regula Falsi Method

    fprintf('Approximate Root: %.6f\n',a);

Approximate Root: 10.000000

    fprintf('Iterations: %d.6f\n',z);

Unrecognized function or variable 'z'.

Answer 1

[Dyuman Joshi]

"Can you answer why am I getting this error."

Because "z" has not been defined.

Two things - Firstly, you are assuming that the root will be found within 50 iterations, which is not guaranteed. A while loop is a better option here.

Secondly, the criteria for stopping the false position method is f(c)==0, not the absolute value less than zero (which is, quite obviously, never going to happen). So change that. And as you are dealing with floating point numbers, it is best to compare using a tolerance.

clear
clc
f=@(x) x+exp(x)-sin(x);
a=10;
b=12;
i=0;
while 1
    c=((a*f(b)-b*f(a))/(f(b)-f(a)));
    %comparing using a tolerance
    if abs(f(c))<1e-6
        a=c;
        %z is not needed as you can directly use i
        %z=i;
        break;
    end
    if f(a)*f(c)>0
        b=c;
    else
        a=c;
    end
    i=i+1;
end

Also, you can combine the fprintf() statements -

fprintf('Regula Falsi Method\nApproximate Root: %.6f\nIterations: %d\n', a, i);

Regula Falsi Method
Approximate Root: -1.234981
Iterations: 31880

Now, compare the output obtained from the Regula falsi method to the output from fzero (just for a check) -

fprintf('%0.6f', fzero(f, a))

-1.234982

Extremely close. I'd say the code is working properly now.

Answer 2

[Star Strider]

The first if block condition is not being satisfied, so ‘z’ is not defined.

The reason is easy to see, since any absolute value will always be positive, so the test that ‘abs(f( c ))<0’ will always fail.

Answer 3

[Image Analyst]

If youi knew how to debug, you wouldn't need to ask why z never got assigned. You would step through your code, like the rest of us do, and notice that you never hit the z line.

See Debugging in MATLAB | Doug's MATLAB Video Tutorials