anonymous function for if-else statements

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Patrick Mboma 2017-7-19

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评论： Patrick Mboma 2017-7-21

Hi, Is it possible to write an anonymous function or a function handle that replicates the behavior of if-else statements?

Consider the simple problem

if condition
  a=b(x);
else
  a=c(x);
end

it is possible to write the following function that will replicate that behavior

 function out=ifelse(condition,answer1,answer2)
 if condition
   out=answer1;
 else
   out=answer2;
 end

A critical difference between the first and the second pieces of code is the fact that in the second one, both answer1 and answer 2 need to be computed/evaluated before passing them to the ifelse function. For small problems this is not really a problem. However, in a situation where b(x) or c(x) are expensive to compute, it is best to avoid un-necessary operations.

One workaround would be

 function out=ifelse2(condition,input1,input2)
 if condition
   out=eval(input1);
 else
   out=eval(input2);
 end

In this case though we have to use "eval".

Is there any other way to deal with this, possibly using some kind of anonymous function or a function handle?

Thanks

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Jan 2017-7-19

Don't use eval. If you talk about expensive functions, the performance degradation by eval must be important.

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回答（4 个）

Walter Roberson 2017-7-19

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在 MATLAB Online 中打开

SelectCell = @(C, idx) C{idx}
condfunc = @(x, scalarcondition, varargin) feval(SelectCell(varargin, scalarcondition+1),x)

Example:

condfunc(x, condition, @b, @c)

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Walter Roberson 2017-7-20

When the scalarcondition is logical, then false corresponds to 0 and true corresponds to 1, and adding one to that takes it into the range 1 or 2, which is suitable for indexing a two element array. However, you might want to use 2-scalarcondition instead, to reverse the order of the tests (the first one corresponds to false in the way I wrote the code.)

Patrick Mboma 2017-7-20

My bad, I missed that possibility. I thought scalarcondition was 1 or 2. Thanks Walter.

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Sean de Wolski 2017-7-19

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In R2016b and newer you can have subfunctions in scripts as well as functions. I would recommend using them instead.

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Patrick Mboma 2017-7-20

So, the script has to be saved?

Andrei Bobrov 2017-7-20

@Patrick Mboma: yes.

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Adam 2017-7-20

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https://blogs.mathworks.com/loren/2013/01/10/introduction-to-functional-programming-with-anonymous-functions-part-1/

https://blogs.mathworks.com/loren/2013/01/24/introduction-to-functional-programming-with-anonymous-functions-part-2/

https://blogs.mathworks.com/loren/2013/02/07/introduction-to-functional-programming-with-anonymous-functions-part-3/

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Patrick Mboma 2017-7-20

Hi Adam,

Thanks for your input. Functional programming, although I am not expert in it, in large part inspires some of the things I do here. For instance the iff construct that is discussed in part-1 is similar to my ifelse function above and shares the same drawback that all alternatives have to be evaluated before the choice is made, which can be very costly.

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Jan 2017-7-19

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@(x) condition * b(x) + (~condition) * c(x)

But you see, that b(x) and c(x) are evaluated also. The best strategy is not to use anonymous functions, but normal functions. Then you have the full power and the best speed.

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Walter Roberson 2017-7-21

I would use the Symbolic Toolbox for the kind of situation you describe.

Patrick Mboma 2017-7-21

Hi Walter,

The point is not what tools you use, but rather how you handle the output. The analogy I made above was just to make a point. Essentially, some people use my codes to solve a problem I don't know anything about. But for my codes to solve the problem, my procedures will first generate some functions that are specific to the problem at hand. Those functions are the ones I want to avoid writing to an m-file.

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