How can I get Matlab to use the variable value instead of name

Question

In my code, I have a line that looks like this:

f=@(test) bf{i}(5);

where bf is a cell array with functions from str2func() stored in it, i is a variable storing an integer, and the 5 is the argument to pass to the function. How can I get matlab to evaluate the line using the current value of i? Right now when I display f it outputs:

@(test)bf{i}(5)

Lets say i=1, I want it to output:

@(test)bf{1}(5)

Although technically the bf{1} should also be replaced with whatever function is stored in bf{1}. How can I force matlab to evaluate the variables in this statement?

You are aware that the function handles keep their own copy of the variables? So changing i or bf later won't change the result. — Daniel
– Daniel, Commented Aug 22, 2015 at 7:48
I guess that basically makes what I asked impossible, you can make that an answer and I will accept it if you want — Chachmu
– Chachmu, Commented Aug 22, 2015 at 7:53
It doesn't make it impossible, just wanted to know make sure you understood function handles right. — Daniel
– Daniel, Commented Aug 22, 2015 at 9:07
when you say you want it to Output @(test)bf{1}(5). Do you mean that literally as in the string representation of the function, or do you want the numerical output produced by the 1st function in array bf with input value 5 ? — Hoki
– Hoki, Commented Aug 22, 2015 at 11:04
I want to be able to plug i into function call and get out the function that is stored in that location in bf, so if bf{1} is rectMath, then I would want the final value of f to be @(test)rectMath(5) — Chachmu
– Chachmu, Commented Aug 22, 2015 at 17:24

Daniel · Accepted Answer · 2015-08-22 09:32:58Z

When you create a function handle, the workspace variables are copied and the expression is evaluated when you call the function handle (Typically not a problem in memory consumption, matlab stores only changes).

Now the problem is, to tell Matlab when to evaluate what part of the expression. If you are aiming for a better performance, pre-evaluate all constant parts of the function. Let's say your function is @(x)(g(3).*f(x)), in this case matlab would evaluate g(3) on every call.

Instead use:

f=@(x)(x.^2)
g_3=g(3)
h=@(x)(g_3.*f(x))

Now having the constant parts evaluated, you want to see the constants instead of the variabe name. I know two ways to achieve this.

You can use the symbolic toolbox, basically converting the function handle to a symbolic function, then to a function handle again. This not only displays the constants, but also substitutes f. This is not possible for all functions.

>> matlabFunction(h(sym('x')))

ans = 

    @(x)x.^2.*4.2e1

Another possibility is to use eval:

h=eval(['@(x)',sprintf('%e',g_3),'.*f(x)'])

Pre-evaluating constant parts of the expressions as I did in the first step is typically recommendable, but both solutions to get the constant visible in your function handle aren't really recommendable. The first solution using matlabFunction only applies to some functions, while the second comes with all the disadvantages of eval.

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