Finding out the size of x (so how many xi we have) in a function handle

Hello, if i have fore example the following function

f1 = @(x) x(1)^2+x(2)^2;

can we somehow find out the "size" of x? So with size of x i mean how many x Variables we have so in this example 2: x=(x(1),x(2))

f1 = @(x) x(1)^2+x(2)^2+x(2)+x(3);

And in this example the size of x would be three.

Thank you.

Answer 1

[Matt J]

One possibility,

f1 = @(x) x(1)^2+x(2)^2+x(2)+x(3);

str = extractBetween( func2str(f1),'x(',')');

dimension=max(str2double(str))

dimension = 3

[Daniela Würmseer]

Thanks to all.

This is a quiete clever way to do it.

Answer 2

[Steven Lord]

If you're doing this so you can dynamically generate the body of f that computes the sum of the squares of the elements of the input, you don't need to.

f = @(x) sum(x.^2);
f([1 2])

ans = 5

f([3 4 5])

ans = 50

f([6 7 8 9])

ans = 230

If you wanted f to have multiple inputs:

g = @(varargin) sum([varargin{:}].^2)

g = function_handle with value:
    @(varargin)sum([varargin{:}].^2)

g(1, 2)

ans = 5

g(3, 4, 5)

ans = 50

g(6, 7, 8, 9)

ans = 230

Answer 3

[Matt J]

No, not from the function handle itself. Note that your f1 and f2 work for vectors of any length>3, e.g.,

f1 = @(x) x(1)^2+x(2)^2;

f1(1:5)

ans = 5

f1(rand(1,7))

ans = 0.0223

so f1 does cannot know the size of the vector you're giving it.

Answer 4

[David Hill]

g=@(x)f1(x);
g([1 2 3])
g([1 5])

function y=f1(x)
  if length(x)==2
      y=x(1)^2+x(2)^2;
  elseif length(x)==3
      y=x(1)^2+x(2)^2+x(2)+x(3);
  end
end