How to extract an element from a ''vector'' of function handle

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Luca Amicucci 2022-4-1

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评论： Steven Lord 2022-4-2

采纳的回答： Steven Lord

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Hi guys,

I'm having some troubles managing handle functions.

What I would like to do is to extract an element of an "handle" array like this below.

f = @(x1,x2,x3)[x1*2-x3;x3*x1-x2;x1-x2^2]

And I would like to compute "something" like that.

f(1) = x1*2-x3;
f(2) = x3*x1-x2;
f(3) = x1-x2^2;

So, like a vector.

How can I do that ??

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Answer 1

Steven Lord 2022-4-1

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If you've already created your f function you could do this using one helper function. I vectorized your f function.

f = @(x1,x2,x3)[x1*2-x3;x3.*x1-x2;x1-x2.^2];
getRow = @(data, rowNum) data(rowNum, :); % Helper function

Now if you want a function handle that retrieves the third row in the output of f:

g3 = @(x1, x2, x3) getRow(f(x1, x2, x3), 3);

Check the full output versus the row 3 output:

y123 = f(1:5, 6:10, 11:15)
y123 = 3×5
    -9    -8    -7    -6    -5
     5    17    31    47    65
   -35   -47   -61   -77   -95
y3 = g3(1:5, 6:10, 11:15)
y3 = 1×5
   -35   -47   -61   -77   -95

You could theoretically eliminate the getRow function by explicitly calling subsref yourself inside g3, but using a separate getRow function makes the code much clearer IMO.

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Steven Lord 2022-4-1

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fun = @(x1,x2,x3)[x1*2-x3;x3.*x1-x2;x1-x2.^2];
getRow = @(data, rowNum) data(rowNum, :); % Helper function
g1 = @(x1,x2,x3) getRow(fun(x1, x2, x3), 1);
g2 = @(x1,x2,x3) getRow(fun(x1, x2, x3), 2);
g3 = @(x1,x2,x3) getRow(fun(x1, x2, x3), 3);
% Recombine
h = @(x1, x2, x3) [g1(x1, x2, x3); g2(x1, x2, x3); g3(x1, x2, x3)];
y123_fun = fun(1:5, 6:10, 11:15)
y123_fun = 3×5
    -9    -8    -7    -6    -5
     5    17    31    47    65
   -35   -47   -61   -77   -95
y1 = g1(1:5, 6:10, 11:15)
y1 = 1×5
    -9    -8    -7    -6    -5
y2 = g2(1:5, 6:10, 11:15)
y2 = 1×5
     5    17    31    47    65
y3 = g3(1:5, 6:10, 11:15)
y3 = 1×5
   -35   -47   -61   -77   -95
y123_h = h(1:5, 6:10, 11:15)
y123_h = 3×5
    -9    -8    -7    -6    -5
     5    17    31    47    65
   -35   -47   -61   -77   -95

But obviously you're not going to want to do this splitting if the output of f has, say, 100 rows. In that case rather than having y1, y2, y3, ..., y100 have one function.

yfun = @(n, x1, x2, x3) getRow(fun(x1, x2, x3), n);
isequal(yfun(2, 1:5, 6:10, 11:15), ...
          g2(   1:5, 6:10, 11:15)) % true
ans = logical
   1

Luca Amicucci 2022-4-2

在 MATLAB Online 中打开

Ok but even so,

h = @(x1, x2, x3) [g1(x1, x2, x3); g2(x1, x2, x3); g3(x1, x2, x3)];

It's not a vector.

I would like to have a vector 3x1 [g1;g2;g3], but maybe it is not possible to convert an handle function to an array of size 3x1.

Or do you have an idea ?

Steven Lord 2022-4-2

在 MATLAB Online 中打开

Years ago MATLAB did allow you to have a vector of function handles, but that functionality was removed when we introduced anonymous functions (to avoid ambiguity: would f(3) be a call to the function with the input 3 or a request for the third element of the array?) If you want an array of function handles it must be a cell array.

Q = {@sin, @cos, @tan};

Now Q{2} is unambiguous and so is Q{2}(pi).

f = Q{2}(pi)
f = -1

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Answer 2

Voss 2022-4-1

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It seems like you want a cell array of function handles:

f = {@(x1,x2,x3)x1*2-x3; @(x1,x2,x3)x3*x1-x2; @(x1,x2,x3)x1-x2^2}
f = 3×1 cell array
    { @(x1,x2,x3)x1*2-x3}
    {@(x1,x2,x3)x3*x1-x2}
    { @(x1,x2,x3)x1-x2^2}
f{1}
ans = function_handle with value:
    @(x1,x2,x3)x1*2-x3
f{2}
ans = function_handle with value:
    @(x1,x2,x3)x3*x1-x2
f{3}
ans = function_handle with value:
    @(x1,x2,x3)x1-x2^2

Then you can evaluate the functions one at a time:

f{1}(10,13,15)
ans = 5
f{2}(10,13,15)
ans = 137
f{3}(10,13,15)
ans = -159

Or all at once:

cellfun(@(x)x(10,13,15),f)
ans = 3×1
     5
   137
  -159

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Luca Amicucci 2022-4-1

Thank you guys for the answers.

But the function handle comes from some calculus and I cannot arrange as a cell array.

I made some calculus in syms "mode" and then, with the matlabFunction, I converted the result in a handle function.

The problem is that this result in handle function contains 3 functions

results = @(x1,x2,x3) [f1;f2;f3]

and so i wanted to extract f1 f2 f3 apart.

Voss 2022-4-1

在 MATLAB Online 中打开

I see. Then maybe something like this would work, depending on what f1, f2 and f3 actually are:

% a function handle you have
results = @(x1,x2,x3) [f1;f2;f3]
results = function_handle with value:
    @(x1,x2,x3)[f1;f2;f3]
% make it a char vector:
str = func2str(results)
str = '@(x1,x2,x3)[f1;f2;f3]'
% parse the char vector into argument list and functions f1, f2, f3:
f = regexp(str,'(@\(.+\))\[(.+);(.+);(.+)\]','tokens');
f = [f{:}]
f = 1×4 cell array
    {'@(x1,x2,x3)'}    {'f1'}    {'f2'}    {'f3'}
% make a cell array of function handles out of that argument list and each
% function (f1, f2, f3):
f = cellfun(@(x)str2func([f{1} x]),f(2:end),'UniformOutput',false)
f = 1×3 cell array
    {@(x1,x2,x3)f1}    {@(x1,x2,x3)f2}    {@(x1,x2,x3)f3}

Now you have a cell array of function handles to those individual functions.

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How to extract an element from a ''vector'' of function handle

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