How to define a piecewise anonymous function

Hello everyone,

the example code

syms x
continuous_function = x^2+x;
matlabFunction(continuous_function,'Vars',x)

gives me the anonymous function

 @(x)x+x.^2

I would like a similar result for piecewise functions. However, the code

syms x
piecewise_function = piecewise( 0<x<1, x, 'otherwiseVal', 0 );
matlabFunction(piecewise_function,'Vars',x)

does not work.

Thanks for any help!

Answer 1

[Star Strider]

You need to use ‘logical indexing’:

piecewise_function = @(x) (x.^2+x) .* ((0<x) & (x<1));

x = linspace(-1, 2);
figure
plot(x, piecewise_function(x))

producing:

[Stephan]

Thank you!

Do I understand you correctly that there is no function that converts

piecewise_function = piecewise( 0<x<1, x^2+x, 'otherwiseVal', 0 );

to

@(x) (x.^2+x) .* ((0<x) & (x<1))

[Star Strider]

Yes.

This code:

syms x
piecewise_function = piecewise( 0<x<1, x^2+x, 'otherwiseVal', 0 );
matlabFunction(piecewise_function)

throws this error:

Error using symengine
Unable to generate code for piecewise for use in anonymous functions.

(and some others).

The construction I use here appears to be the only way. Various such expressions can be combined in one function to create piecewise functions across various intervals, for example:

piecewise_function = @(x) (x.^2+x) .* ((0<x) & (x<1)) + (x+sqrt(x)).*((x>=1) & (x<1.5));

So all is not lost.

[Stephan]

Thank you for this useful remark!

[Star Strider]

As always, my pleasure!

[ElPerroVerde]

THANK YOU SO MUCH!!!!!!!!!!!! I've been looking for something like this for months!!!

[Star Strider]

ElPerroVerde — My pleasure!

A Vote would be appreciated!

.

Answer 2

[Walter Roberson]

If you use matlabFunction with 'file' option, then it will convert piecewise() into if/else in the generated code.

Note: because it uses if/else instead of logical indexing, the generated code will not be vectorized on that variable.