How to turn a function handle with 3 inputs (1 variable and 2 parameters) and then assign the inputs parameters and get a function handle with one input? ?

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Mohammad Shojaei Arani
回答： Jeff Miller 2022-1-15
Hello friends!
Consider the following
y=1;z=1;
f=@(x,y,z)x+y+z;
Now, I would like to create another function handle which is only a function of x. You migh suggest the following solution:
g=@(x)f(x,y,z);
but this is not what I want since I need to pass g(x) as input to another function and there I want to reduce the computational time. If I follow this solution whenever I call g, say g(1), it needs f, too plus that it does 3 arithmatics (3 sums) but I would like to have 2 arithmatics since g(x)=x+2.
Any idea?
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Torsten
Torsten 2022-1-14
But how do you think it can be possible to define a function handle for g without using the function handle for f if g is deduced from f in some way ? Sounds contradictory to me.
Mohammad Shojaei Arani
Mohammad Shojaei Arani 2022-1-14
NO. I did not say that. Of course, we need f in order to build g. But, the point in my question is how to do this in a cheap manner. If you look at my comminication with Rik, I mentioned that the following is a way to answer my question:
syms x y z
f=x+y+z;
g=subs(f,{y,z},{1,1});
g=matlabFunction(g);
But, this is computationally very expensive. In theory, Matlab has the capacity to solve my problem through function handles (not symbolic toolbox). But, it seems that matlab developers did not carefully think about this.

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回答（3 个）

Steven Lord
Steven Lord 2022-1-14
Ran in:
Let's look at the actual time difference between the two approaches.
y=1;
z=1;
f=@(x,y,z)x+y+z;
g=@(x)f(x,y,z);
h = @(x) x+2;
timeit(@() g(42))
ans = 2.8650e-06
timeit(@() h(42))
ans = 7.6423e-08
Is this really the bottleneck in your code?
I realize that this was probably just a simple example to demonstrate what you're trying to do, so can you show the output of timeit called on a more "realistic" f, g, and h functions from your actual application?
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Torsten
Torsten 2022-1-15
编辑：Torsten 2022-1-15
More and more I get the impression that the approach to your problem is not optimal or that MATLAB is not the most-suited tool for your task.
Maybe you could summarize what you are trying to do.
Matt J
Matt J 2022-1-15
编辑：Matt J 2022-1-15
Yes, what steps did you take that led you to such a monstrous 100MB mathematical expression? It sounds as if you have done nothing at all to vectorize the operations in your equations, like for example if you had rewritten a large matrix/vector operation in terms of their individual scalar components.
I suggest you show us just the first 20 lines of the expression you're trying to evaluate, so we can get a sense of the problem.

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Matt J
Matt J 2022-1-14
编辑：Matt J 2022-1-14
But, this is computationally very expensive for my super big expressions.
What if you use the optimize flag?
g=matlabFunction(f,'File','outfile','Optimize',true)
Jeff Miller
Jeff Miller 2022-1-15
Not sure I understand the situation, but this sounds to me like a case where an OO approach could be very helpful. Consider:
classdef myFunc < handle
properties
ComputedFromYandZ
end
methods
function obj = myFunc() % Constructor
obj.ComputedFromYandZ = 0;
end
function [] = SetNewYandZ(obj,Y,Z) % Compute whatever depends on Y and Z
obj.ComputedFromYandZ = Y + Z;
end
function Result = ComputeFromX(obj,X) % Compute whatever depends on X with Y and Z fixed
Result = abs( X + obj.ComputedFromYandZ ) ;
% I included abs() here to make a function for fminsearch to
% minimize.
end
end
end
You can then use a function handle to a function that depends only on X (after setting Y and Z), something like this:
myF = myFunc;
myF.SetNewYandZ(2,3);
fminsearch(@myF.ComputeFromX,20)
% ans -5
myF.SetNewYandZ(-20,-30);
fminsearch(@myF.ComputeFromX,20)
% ans 50

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