Problem using str2func for creating a function handle

Hello!

I have two functions:

%---------

function [ handle_res ] = handle_fun_creation_2( File_String,Type_identifier,Value )

     if Type_identifier == 0
         handle_res = str2func(['@' File_String]);
     elseif Type_identifier == 1
         handle_res = str2func(['@(t,y)' File_String '(t,y,Value)'] );
     end

end

%-------------

and:

%-------------

function [ y ] = test_3( t,x,Var_opt )

     y=x+Var_opt(1)+Var_opt(2);

end

%---------------

Could anyone explain me why the following code doesn't work:

%---------------------------

Var_opt = [30 40]';

[handle_4] = handle_fun_creation_2('test_3',1,Var_opt);

[t6,y6] = ode15s(handle_4,[0:1],0);

Undefined function or variable 'Value'.

Error in @(t,y)test_3(t,y,Value)

%---------

But this one does?

%---------

handle_5 =  @(t,y)test_3(t,y,Var_opt);

[t7,y7] = ode15s(handle_5,[0:1],0);

I think I'm missing some detail on how str2func works maybe? Thanks in advance.

[Stephen23]

This is just a very indirect and inefficient approach to parameterize a function. Simpler and more efficient:

https://www.mathworks.com/help/matlab/math/parameterizing-functions.html

Answer 1

[Walter Roberson]

str2func() does not execute the content of any strings it is passed so it has no idea that the string 'Value' should relate to the parameter named Value.

if isnumeric(Value)
  valstr = sprintf('%g', Value);
else
  valstr = char(Value);
end
handle_res = str2func(sprintf('@(t,y)%s(t,y,%s)', File_String, valstr));

[Pedro]

Many thanks for the answer.

Do you have by any chance an idea of how to build a function such as the 'handle_fun_creation_2'? (input: parameter, function name,variable) and output the correspondent function handle?

I know I can always create a set of if clauses to do that but thats not very flexible... Thanks in advance.

[Walter Roberson]

What is its purpose? Perhaps something like

if isa(File_String, 'function_handle')
  fhan = File_String;
else
  fhan = str2func(File_String);
end
if Type_identifier == 0
  handle_res = fhan;
else
  handle_res = @(t,y) fhan(t,y,Value);
end

[Pedro]

Not exactly, nonetheless I believe I found a solution using eval and sprintf:

%------------

function [ handle_res ] = handle_fun_creation_2(File_String,Type_identifier,Value )

     if Type_identifier == 0

         handle_res = sprintf('@%s',File_String);

     elseif Type_identifier == 1

         handle_res = sprintf('@(t,y)%s(t,y,Value)',File_String);
     end

%----------------

[sprint_4] = handle_fun_creation_2('test_3',1,Var_opt);

handle_4 = eval(sprint_4);

[t6,y6] = ode15s(handle_4,[0:1],0);

%------------------

Although I'm not sure about this code's efficiency, it works. Many thanks for your input!

[Stephen23]

"Although I'm not sure about this code's efficiency, it works"

It is not efficient, nor does it work as you think it does.

For example, although you pass the input argument Value to handle_fun_creation_2 it remains totally unused, it is only when you later EVAL the string in the caller workspace that MATLAB will look for a function or variable named Value. You can easily test this by calling handle_fun_creation_2 with complete nonsense for the third input argument and everything will work (as long as the correct Value is defined in the caller workspace!):

sprint_4 = handle_fun_creation_2('test_3',1,{'hello world'}) % 3rd arg = silly cell array

sprint_4 = '@(t,y)test_3(t,y,Value)'

Value = [pi,2/3]; % <----------- must be defined in the calling workspace...
handle_4 = eval(sprint_4) % <--- because this is actually where Value is included.

handle_4 = function_handle with value:
    @(t,y)test_3(t,y,Value)

[t6,y6] = ode15s(handle_4,0:1,0); % no errors

If Value is not defined in the caller workspace then your code will not work, it will throw an error about Value being undefined (see bottom of this comment). It is clear that my silly Hello World cell array actually does nothing.

But this just seems to be a very indirect and inefficient approach to parameterize a function:

https://www.mathworks.com/help/matlab/math/parameterizing-functions.html

As the documentation shows, a much simpler and more efficient approach is to use an anonymous function:

fn1 = @(x,y)test_3(x,y,Value);
[t7,y7] = ode15s(fn1,0:1,0);
isequal(t6,t7)

ans = logical
   1

isequal(y6,y7)

ans = logical
   1

and if the function name really is provided as text (suboptimal design, but still simpler and more direct than a special "creation" function returning a string and evaluating it to get a function handle):

fns = 'test_3';
fn2 = str2func(fns);
fn3 = @(x,y)fn2(x,y,Value);
[t8,y8] = ode15s(fn3,0:1,0);
isequal(t6,t8)

ans = logical
   1

isequal(y6,y8)

ans = logical
   1

And finally, lets try your code when Value is not defined in the caller workspace:

clearvars Value
handle_4 = eval(sprint_4) % looks the same... but is it?

handle_4 = function_handle with value:
    @(t,y)test_3(t,y,Value)

[t6,y6] = ode15s(handle_4,0:1,0); % nope.

Unrecognized function or variable 'Value'.

Error in solution>@(t,y)test_3(t,y,Value)

Error in odearguments (line 90)
f0 = feval(ode,t0,y0,args{:}); % ODE15I sets args{1} to yp0.

Error in ode15s (line 152)
odearguments(FcnHandlesUsed, solver_name, ode, tspan, y0, options, varargin);