For loop in function handle

I want to creat a function with multiple variables, something like:

f = @(x) = f1(x) + f2(x) + ... + fn(x)

where x is a n-dimensional vector which can be define by user. The number of functions f1,f2,..,fn can be define by user, too.

The question is can i use a for loop in function handle to define f according to the length of vector x?

Answer 1

[Stephen23]

You can simply store function handles in a cell array of any size and use cellfun. No loop required.

>> C = {@sin,@sqrt,@pow2};
>> F = @(x) sum(cellfun(@(f)f(x),C));
>> F(3)
ans =  9.8732

compare with:

>> sin(3) + sqrt(3) + pow2(3)
ans =  9.8732

If you really want to use a for loop then you would need to write the function in an Mfile, e.g.:

function out = myfun(x)
out = 0;
C = {@sin,@sqrt,@pow2};
for k = 1:numel(C)
    out = out + C{k}(x);
end
end

[Andy Tran]

Thanks for replying me.

But my situation is a bit more complicated. The functions f1,f2,...,fn are not familier functions like sin,cos,pow2,... They are anonymous functions. For example f1 = @(x) = h1*x1+h2*x2+...+hn*xn, where h and n are n-dimentional vectors.

[Stephen23]

"But my situation is a bit more complicated. The functions f1,f2,...,fn are not familier functions like sin,cos,pow2,... They are anonymous functions"

That makes no difference, you can also use a cell array with anonymous functions:

C{1} = @(x)...;
C{2} = @(x)...;
... etc

and then use either the loop or cellfun (you might need to set its optional 'UniformOutput' to false),

[Andy Tran]

That make sense. But what if:

C{1} = @(x) h1*x1 + h2*x2 +...+ hn*xn
    

where the number of variables "x" is an input parameter. I want to clair a function with unknown number of variables.

[Walter Roberson]

http://www.mathworks.com/matlabcentral/answers/304528-tutorial-why-variables-should-not-be-named-dynamically-eval

[Andy Tran]

I think i got your point. But im not trying to create dynamical variable names. I want to create a function to calculate channel capacity acording to Shannon formular:

where p_n are variables. How can i define function C with unknown number of variables (i call it as N), others parameters are generate corresponding to N.

[Walter Roberson]

You said it yourself, you have an unknown number of variables. The only way to address an indefinite number of variables is to dynamically create the names of the variables and access the contents of the variables through the names you created. Which we firmly advise you not to do.

where p_n are variables.

Don't do that. Instead make p and h into vectors. Then

 dot(p, h) - p(n).*h(n)

provided that p(m) and h(m) are finite.

[Stephen23]

"How can i define function C with unknown number of variables (i call it as N), others parameters are generate corresponding to N."

Rather than creating lots of variables the simple and efficient MATLAB way is to use one vector/matrix/array.

Your approach is making this much more complex than it needs to be.

[Andy Tran]

Yes i know it. But further i need to find the maximum of the function C minus D (which is a linear function acording to p_n) for each p_n. The way im trying to do is using anonymous function and so that i think i need to treat p_n as a variable. Do you have other suggestion?

Thanks for helping me out.

Answer 2

[Walter Roberson]

The question is can i use a for loop in function handle to define f according to the length of vector x?

NO. Anonymous functions cannot use for .

Anonymous functions can use arrayfun() and cellfun() and can use function calls that process the resulting arrays.