passing whole function expression as function argument

hi everyone,

I am trying to create a function that has as an argument another function expression. I can manage to do this:

function [out1] = my_func(in1).

Then I can use a whole function expression as in1 like this:

[out1] = my_func( another_func() ).

So far so good.

My problem arises when the function another_function() has more than one outputs. So when another_func() is something like this:

function [out2 out3] = another_func(in2)

then [out1] = my_func( another_func() ) will only return the first of the outputs of another_func() which is out2.

Is there any way to use the expression [out1] = my_func( another_func() ) and get all outputs of another_func()?

thank you in advance for your help

[Stephen23]

You can solve this easily using function handles:

http://www.mathworks.com/help/matlab/function-handles.html

http://www.mathworks.com/help/matlab/matlab_prog/pass-a-function-to-another-function.html

http://www.mathworks.com/help/matlab/matlab_prog/creating-a-function-handle.html

http://www.mathworks.com/help/matlab/matlab_prog/anonymous-functions.html

Answer 1

[Walter Roberson]

Yes, but only in a very restricted context.

If you have an anonymous function A that effectively just calls another function B and B returns multiple outputs, then those multiple outputs will be returned from A.

For example,

my_anonymous_func = @() another_func(in)

then because another_func returns multiple outputs, they will all be returned from my_anonymous_func

I do not know if this is strictly the only sequence it will work in: it might also be possible in the form

function [varargout] = my_func(in1)
  [varargout{1:nargout}] = another_func(in1);

[buscuit]

Hi Walter,

thanks a lot for the input. The another_func() and my_func are both non-anonymous functions.

I tried to directly extract the outputs from another_func() and assign them in variables inside my_func() which I guess is what you are suggesting on your last piece of code but it won't work.

Furthermore the number of outputs of another_func() is known (two) so I guess I do not need to use the varargout approach (sorry, I forgot to mention that).

I am surprised that I am actually able to access the first output of the another_func() function this way but not its second one using this method...

I am running out of ideas ...

[Walter Roberson]

Just explicitly assign the outputs

function out1 = my_func(in1);
  [out2, out3] = another_func(in1);
  now do something that calculates out1

You will not be able to program something that would look like

function out1 = my_func(in2, in3)
  ...

and call it as

   my_func(another_func(in1))

hoping to have the two outputs of another_func captured as the two inputs in2 and in3 .

Remember, you can use

function out1 = my_func(func_in)
   [out2, out3] = func_in();
   now do something that calculates out1

and call it as

my_func(@() another_func(in1))

and you can even use

function out1 = my_func(in1, in2)
  if nargin == 1 && isa(in1, 'function_handle')
    [in1, in2] = in1();
  end
  now do something that calculates out1

[buscuit]

Hi Walter,

for for your comments. They have been a great help so far.

In the examples of your last comment can the another_func be of an arbitrary name? Or it has to be a specific function?

My aim is to create a function that uses as inputs two other functions. Those functions will be matlab filters e.g. butter(), cheby1 etc... So I would like if the user was able to input different filters in that function.

cheers

[Walter Roberson]

The particular form

my_func(@() another_func(in1))

can use any function provided that in1 (or the other parameters) is known before the call to my_func .

If you want a function that takes two functions as inputs, then for example

function out = my_func(data, func1, func2)
   out = func2(func1(data));

which could be called as

y = my_func(x, @butter, @cheby1)

The key is passing function handles

[buscuit]

Walter, thank you very much for all your feedback