How to use anonymous functions

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Ken 2022-3-5

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1664204-how-to-use-anonymous-functions

评论： Ken 2022-3-12

采纳的回答： KSSV

在 MATLAB Online 中打开

I am trying to understand anonymous functions. What exactly does this do:

pTransition = [ 
    0   0   0   0   0
    0  .05  .1 .05  0
    0   .1  .4  .1  0
    0  .05  .1 .05  0
    0   0   0   0   0  ];
getPLocalTransition = @(localCoordinate) ...
    pTransition(localCoordinate(1) + 3, localCoordinate(2) + 3);

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Walter Roberson 2022-3-5

What exactly does this do

We do not know exactly what it does. The details of how function handles are created and processed internally are MATLAB proprietary details, hidden in code that people outside Mathworks do not have access to.

I could probably spend more than an hour writing up and providing references to how it probably works internally, in order to be consistent with actions we can test and documentation we have access to, but unless you are into Compiler Construction, and Denotation Semantics, that would likely be a waste of my time, compared to the possibility that you could instead ask a specific question that you are trying to resolve in your mind.

When you get into an elevator and you ask "How exactly does this work?" are you asking about multi-phase motor theory, or are you asking about the material science behind the precise choice of compounds used in the emergency brakes, or are you asking about how the emergency phone works, or are you asking about how capacitance sensing works for the push buttons?

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采纳的回答

KSSV 2022-3-5

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1664204-how-to-use-anonymous-functions#answer_910534

The function needs a localCoordinate as input and it should be a 1X2 array. The array can have values 0,1,2.

Depedning on the values given, to the array 3 is added, these are the indices and the respective values are extracted from pTransition.

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Ken 2022-3-7

编辑：Torsten 2022-3-7

在 MATLAB Online 中打开

I am trying the Markov localization prediction algorithm in MATLAB. My robot moves in global coordinates depicted in a square grid. I know its previous stae x_t-1. I give it an input u_twhich is a 2-element integer offset. I now try to predict the state x_t where the robot has moved.For this, I have a transition 5X5 grid called pTransition. This has its probabilities in each square of the grid. I try to calculate state x_t (probability) of where the robot has moved to. I use an anonymous function calculatePTransition for this purpose which has x_t,x_t-1 and u_t as its arguments. This is as shown below.

pTransition = [ 
    0   0   0   0   0
    0  .05  .1 .05  0
    0   .1  .4  .1  0
    0  .05  .1 .05  0
    0   0   0   0   0  ];
getPLocalTransition = @(localCoordinate) ...
    pTransition(localCoordinate(1) + 3, localCoordinate(2) + 3);
%%%%%%%%%%% PLEASE DON'T CHANGE ANYTHING ABOVE THIS LINE %%%%%%%%%%%
calcPTransition = @(x_t, u_t, x_tm1) ...;
plotPredictionUpdate(calcPTransition);

Walter Roberson 2022-3-10

在 MATLAB Online 中打开

In the code you posted, the (second) line

plotPredictionUpdate(calcPTransition);

was after the function definition for get_direction()

In MATLAB, once you start your first function definition, everything else to the bottom of the file has to be function definitions.

The code you had in https://www.mathworks.com/matlabcentral/answers/1664204-how-to-use-anonymous-functions#comment_2030754 would not define an anonymous function, then call plotPredictionupdate(), then define the function get_direction, and then switch back to the original script to call plotPredictionUpdate a second time. MATLAB is just not designed that way.

In MATLAB, once you have hit your first function line, you have a couple of possibilities. You can have a structure that looks like

some code before any function
function first_function
    code for first_function
end
function second_function
    code for second_function
end

This defines first_function and second_function independently inside a script. Notice that all the executable code is either before the first function or else is inside a function/end pair.

Or you can have

some code before any function
function first_function
    code for first_function
    function second_function
        code for second_function
    end
end

This defines second_function to be private to first_function, and cannot (normally) be called anywhere except from first_function. Notice that all executable code is either before the first function or else is inside a function/end pair

Or you can have

some code before any function
function first_function
    code for first_function
    function second_function
        code for second_function
    end
    more code for first_function
end

This defines second_function to be private to first_function, and cannot (normally) be called anywhere except from first_function. The "code for first function" would be sort-of executed before attempting to define second_function, and after second_function was defined, you would return to defining first function with the "more coe for first_function". Notice that all executable code is either before the first function or else is inside a function/end pair.

But you cannot do

some code before any function
function first_function
    code for first_function
end
more code outside any function

Once you get to the first function then all code must be within a function/end pair.

The rules are a bit different for files that do not have any "script" at the top -- files that immediately start with a function line. In such files, you can use the function/end pair like above, or you can define all functions with no end for functions at all:

function first_function
    code for first_function
function second_function
    code for second_function

If you were to put in an end matching any function statement in such a file, you would get an error message. Either every function statement must have a matching end statement, or else no function statement can have an end statement. But inside a script file (a file for which the first executable word is not function or classdef ) then matching function/end is the only option.

Ken 2022-3-10

在 MATLAB Online 中打开

Thanks. Tried:

pTransition = [ 
    0   0   0   0   0
    0  .05  .1 .05  0
    0   .1  .4  .1  0
    0  .05  .1 .05  0
    0   0   0   0   0  ];
getPLocalTransition = @(localCoordinate) ...
    pTransition(localCoordinate(1) + 3, localCoordinate(2) + 3);
%%%%%%%%%%% PLEASE DON'T CHANGE ANYTHING ABOVE THIS LINE %%%%%%%%%%%
calcPTransition = @(x_t, u_t, x_tm1) getPLocalTransition(get_direction(x_t, u_t, x_tm1));
plotPredictionUpdate(calcPTransition);
function dir = get_direction(x_t, u_t, x_tm1)
dir = [0;0]; % do your calculations on as many lines as you desire
    x_t=u_t+x_tm1;
    x_t=max(-2,min(u_t+x_tm1));
 
end
Got error:calcPTransition evaluates to a wrong value for x_t=[1 1], u=[-4 -4], 
x_tm1=[0 0] : 0.4 (0 expected)
the goal is to find an expression for the transition probabilities of the robot ending up in 
cell x_t, which is given by x_tm1+u_t plus some noise as defined by pTransition. To lookup the 
probability for a given x_t from pTransition, you need to transform x_t into local indices 
centered around x_tm1+u_t.
For example, in case where x_t = x_tm1 + u_t (no noise), the index of the local transition matrix 
should be [0, 0].

Walter Roberson 2022-3-11

在 MATLAB Online 中打开

function dir = get_direction(x_t, u_t, x_tm1)
dir = [0;0]; % do your calculations on as many lines as you desire

That is a constant. No matter what parameters are passed, you always assign [0;0] to dir .

dir is also your return variable. Unless another value is assigned to dir before the end of the function, this constant will be what you return.

x_t=u_t+x_tm1;

That overwrites that x_t that was passed in. It is not obvious why you bother passing in x_t if you are just going to overwrite it, but perhaps you have Good Reasons.

x_t=max(-2,min(u_t+x_tm1));

Okay, you limit the x_t you calculated. That is fine so far.

end

But now you have completed the function. You have not assigned anything else to dir so it is always going to return [0;0] . You assigned something internally into x_t but that is confined to the function workspace, and will not affect anything outside the function.

To emphasize: the change you make to x_t here will not change x_t in the calling function.

As far as MATLAB is concerned, your function is effectively

function dir = get_direction(~, ~, ~)
    dir = [0;0];
    if nargin < 3; error('insufficient arguments'); end
    now waste a bit of time
end

I would suggest to you that it is highly likely that your output variable, dir should be set to something calculated based upon at least one of the input variables.

Ken 2022-3-12

Got a response:

"You now have 2 issues left, the first one is about the MATLAB function syntax: When you define a function with function dir = get_direction(x_t, u_t, x_tm1), the variable dir is what gets returned in the end, so your code has always returned [0,0] so far.

The second is about the vector that you are calculating. The matrix pTransition describes the probability to move onto a neighbouring cell of x_tm1+u_t, further getPLocalTransition(...) expects a direction vector that is relative to this location. Since you use u_t+x_tm1 as input, which is relative to wold origin (0,0), this will most likely result to a probability of 0 (because the map is much bigger than the 5x5 of pTransition)."

Ken 2022-3-12

I tried this i.e. did not use dir=[0;0]. Then I used the foll sentence to convert to local coordinates. Still no luck. getPLocalTransition(pTransition(localCoordinate(1) + 3, localCoordinate(2) + 3)); function dir = get_direction(x_t, u_t, x_tm1); temp = u_t + x_tm1 ; dir = getPLocalTransition(max(-2,min(2,temp))); dir= getPLocalTransition(pTransition(localCoordinate(1) + 3, localCoordinate(2) + 3)); end

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