Plot integral with fplot

Hi,

I have this code:

string='int(x^2,x)';
y=str2func(['@(x)' string]);

Then I try to plot it with fplot

fplot(y)

But the problem is that axes remain white, without an error or anything else.

I've already solved the problem with symbolic toolbox

syms y(x)
y(x)=eval(string)
fplot(y)

But this solution is terribly slow and for the application that I'm developping it is important a solution as fast as possibile. If I use str2func with functions that require only one input (cos,sin,...) it works perfectly but with function with 2 or more input axes remain white. Have you a solution to suggest me? Thanks in advance.

Answer 1

[Star Strider]

Using str2func is not the correct approach for dealing with symbolic expressions, and especially those that involve symbolic functions (such as int) that only operate in the symbolic context.

Try this instead:

string = 'int(x^2,x)';
y = matlabFunction(str2sym(string))

fplot(y)

That is still significantly inefficient, however it has the one redeeming feature of producing the result you apparently want, using the character vector you began with.

[Federico Spada]

It is definitly faster but now I have another problem to overcome: if

string='1';

I have the same problem as before: axes remain white while i would want a line in y=1. Is there a way to fix this? Thanks

[Star Strider]

That may also throw a ‘Too many input arguments’ error, and will require an if block to trap it.

This works:

string = '1';
y = matlabFunction(str2sym(string))

if nargin(y) < 1
    fplot(@(x)y())
else
    fplot(y)
end

Experiment to get the result you want with your functions.

[Federico Spada]

Ok, perfect! Thank you!

[Star Strider]

As always, my pleasure!