Defining anonymous function using an "if" statement

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John
John 2015-1-5
评论： Oscar Raya i Casanova 2020-1-29
I'm trying to define an anonymous function using an "if" statement. Here is a minimum "not working" example (my code is more convoluted, but this illustrates the issue):
clc;
clear all;
f=@(x)x^2;
g=@(x) x;
if f(x)>=g(x)
h=@(x) f(x)
else h=@(x) g(x)
end
The point is that I don't want to create an m file and have to recompute both f and g in it (my actual problem would require recomputing many more). So the question is:
1-) Is it possible to create an anonymous function using "if" statements (fixing the code above)?
2-) Is it possible to define function h on a separate m file, without having to recompute f and g inside of it?

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采纳的回答

Matt J
Matt J 2015-1-5
You can also do things like the following
f=@(x)x^2;
g=@(x) x;
h=@(x) fg_mfile(x,f,g);
function out=fg_mfile(x,f,g) %put this in mfile
fval=f(x);
gval=g(x);
if fval>=gval
out=fval;
else
out= gval;
end
end
  2 个评论
Matt J
Matt J 2018-8-24
编辑：Matt J 2018-8-24
In this particular example, that is true. More complicated h() might require an mfile to express, however.

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更多回答（2 个）

Roger Stafford
Roger Stafford 2015-1-5
Try this:
f = @(x) (x.^2>=x).*x.^2+(x.^2<x).*x
  5 个评论
显示 3更早的评论
John
John 2015-1-6
I see. I checked the references you provided and now I understand.
Alec Jacobson
Alec Jacobson 2016-12-16
This style/hack has two (more) "gotchas".
1) Suppose L(x) is true, but g(x) produces nan/inf. Then h(x) = nan rather than f(x).
2) Suppose L(x) is true and g(x) is very expressive to compute or has side-effects (admittedly rare for matlab code), g(x) will still be called. This is very different behavior from:
if L(x)
return f(x)
else
return g(x)
I really wish matlab had full-citizen anonymous functions...

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Cristobal Montecino
编辑：Cristobal Montecino 2019-6-26
I use:
call = @(fun) fun();
cellget = @(cell, index) cell{index};
iff = @(test, truefn, falsefn) call(cellget({falsefn, truefn}, test + 1));
And:
arrayget = @(arr, index) arr(index);
ifv = @(test, trueval, falseval) arrayget([falseval, trueval], test + 1);
%or
cellget = @(cell, index) cell{index};
ifv = @(test, trueval, falseval) cellget({falseval, trueval}, test + 1);
Examples:
iff(true, @() 2, @() this_throw_an_error)
% out: 2
iff(false, @() 2, @() this_throw_an_error)
% out: Undefined function or variable 'this_throw_an_error'.
ifv(true, 2, 3)
% out: 2
ifv(true, 2, this_throw_an_error)
% out: Undefined function or variable 'this_throw_an_error'.
  1 个评论
Oscar Raya i Casanova
Oscar Raya i Casanova 2020-1-29
Nice tip. As improvement for the first option:
call = @(fun,par) fun(par{:});
cellget = @(cell, index) cell{index};
iff = @(test, truefn, falsefn, truePar, falsePar) call( ...
cellget({falsefn, truefn}, test + 1), ... % functions
cellget({falsePar, truePar}, test + 1) ... % params
);
Examples:
f1 = @(a,b) a + b;
f2 = @(a,b,c) a*b + c;
f3 = @() 7;
iff(true,f1,f2,{1,2},{1,2,3});
% out: 3
iff(false,f1,f2,{1,2},{1,2,3});
% out: 5
iff(false,f1,f3,{1,2},{});
% out: 7
This way you can use functions with different number of parameters.

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