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John
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Defining anonymous function using an "if" statement

Asked by John
on 5 Jan 2015
Latest activity Edited by Cristobal Montecino on 26 Jun 2019
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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3 Answers

Answer by Matt J
on 5 Jan 2015
 Accepted Answer

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 Comments

you can do like this h=@(x)max(f(x),g(x))
In this particular example, that is true. More complicated h() might require an mfile to express, however.

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Answer by Roger Stafford on 5 Jan 2015

Try this:
f = @(x) (x.^2>=x).*x.^2+(x.^2<x).*x

  5 Comments

This is a classic technique and Star Strider is right. More generally, you would do
h = @(x) L(x).*f(x) + (~L(x)).*g(x)
where L(x) is a possibly multivariate indicator function of x (i.e. =1 when f is active and =0 if g is active).
The only thing I'd point out is that it requires L(x) to be computed twice. It also requires that both f and g be computed even though you really only want to compute the one that is active. With a dedicated mfile you can often avoid that.
So, for large calculations, it has its drawbacks in CPU efficiency, but advantages in syntax efficiency.
I see. I checked the references you provided and now I understand.
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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Answer by Cristobal Montecino on 26 Jun 2019
Edited by Cristobal Montecino on 26 Jun 2019

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'.

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