Imported variable is not transferred to fzero- function

function y = fzero_optimset(~,~)
x = -5:.01:5;
disp('Please enter a value for variable a!')
a = input('a = ');
y = 1./(1+x.^2) + a * x - 1/2;
plot(x,y);
grid on;
hold on;
x0 = fzero('1./(1+x.^2) + a * x - 1/2',-1,optimset,.1);
plot(x0,0,'x');
end

[Matt J]

@Florian Roeder's comment moved here

In line 3 of my code, user is prompted to enter a value for variable a.

When I run the code, function "y=1./(1+x.^2)+a*x-1/2" is evaluated correctly. Figure is correct as well.

When function reaches line "x0 = fzero('1./(1+x.^2) + a * x - 1/2',-1,optimset,.1);" , MATLAB shows following error message:

FZERO cannot continue because user-supplied expression ==>

1./(1+x.^2) + a * x - 1/2 failed with the error below.

Error in inline expression ==> 1./(1+x.^2) + a * x - 1/2

Undefined function or variable 'a'.

Error in fzero_optimset (line 20)

x0 = fzero('1./(1+x.^2) + a * x - 1/2',-1,optimset,.1);

How can I handle this problem? I don´t know how to change the code.

Answer 1

[Matt J]

Use more modern techniques for specifying the function and passing fixed parameters to it, e.g.,

a=1;
x0 = fzero(@(x) 1./(1+x.^2) + a * x - 1/2,-1)

x0 = -0.3761

[Florian Roeder]

Thanks a lot. Is there any possibility to get it running in an editor- file? The code above was copied from an editor- file generating the error message shown above. When I type in modified code (your solution), everything is running properly. Copying the same code in the editor- file shows the error message again. Figure is drawn correctly each time, fzero- command generates the error.