I am trying to store function handles in array by running a loop and then adding them.
for i = 1:n^2
x1 = x_coord(elements(i,1));
x2 = x_coord(elements(i,2));
x3 = x_coord(elements(i,3));
x4 = x_coord(elements(i,4));
y1 = y_coord(elements(i,1));
y2 = y_coord(elements(i,2));
y3 = y_coord(elements(i,3));
y4 = y_coord(elements(i,4));
SF(elements(i,1)) = #(x,y)((x-x3)*(y-y3)/((x1-x3)*(y1-y3)) + SF(elements(i,1)(x,y))
end
Here I am saving function handles in array SF and then adding them inside the loop.
But I am getting this error:
Error: ()-indexing must appear last in an index expression.
PLEASE HELP.
When trying something this complicated, break it down and try smaller pieces. For example, when I run this at the command line:
F(1) = #(x,y)(x+y);
F(2) = #(x,y)(x+y+2);
I get the following error message: Nonscalar arrays of function handles are not allowed; use cell arrays instead.
Oops. I guess you need this instead:
F{1} = #(x,y)(x+y);
F{2} = #(x,y)(x+y+2);
And now, sure enough, F{1}(1,2) works as expected.
What's not going to work is the overall logic of the program, as you're trying to recursively define SF(elements(i,1)) in terms of itself, which doesn't make any sense. Try some more smaller pieces, like I show above, to experiment with function handles defined in terms of other function handles. You're likely to find some surprises.
Related
Is it possible to get the 'nth' return value from a function without having to create dummy variables for all n-1 return values before it?
Let's say, I have the following function in MATLAB:
function [a,b,c,d] = func()
a = 1;
b = 2;
c = 3;
d = 4;
Now suppose, I'm only interested in the third return value. This can be accomplished by creating one dummy variable:
[dummy, dummy, variableThatIWillUse, dummy] = func;
clear dummy;
But I think this is kind of ugly. I would think that you might be able to do something like one of the following things, but you can't:
[_, _, variableThatIWillUse, _] = func;
[, , variableThatIWillUse, ] = func;
variableThatIWillUse = func(3);
variableThatIWillUse = func()(3);
Are there any elegant ways to do this that do work?
So far, the best solution is to simply use the variableThatIWillUse as a dummy variable. This saves me from having to create a real dummy variable that pollutes the work-space (or that I would need to clear). In short: the solution is to use the variableThatIWillUse for every return value up until the interesting one. Return values after can simply be ignored:
[variableThatIWillUse, variableThatIWillUse, variableThatIWillUse] = func;
I still think this is very ugly code.
With MATLAB Version 7.9 (R2009b) you can use a ~, e.g.,
[~, ~, variableThatIWillUse] = myFunction();
Note that the , isn't optional. Just typing [~ ~ var] will not work, and will throw an error.
See the release notes for details.
This is somewhat of a hack, but it works:
First a quick example function:
Func3 = #() deal(1,2,3);
[a,b,c]=Func3();
% yields a=1, b=2, c=3
Now the key here is that if you use a variable twice on the left-hand side of a multiple-expression assignment, an earlier assignment is clobbered by the later assignment:
[b,b,c]=Func3();
% yields b=2, c=3
[c,c,c]=Func3();
% yields c=3
(Just to check, I also verified that this technique works with [mu,mu,mu]=polyfit(x,y,n) if all you care about from polyfit is the third argument.)
There's a better approach; see ManWithSleeve's answer instead.
If you wish to use a style where a variable will be left to fall into the bit bucket, then a reasonable alternative is
[ans, ans, variableThatIWillUse] = myfun(inputs);
ans is of course the default junk variable for MATLAB, getting overwritten often in the course of a session.
While I do like the new trick that MATLAB now allows, using a ~ to designate an ignored return variable, this is a problem for backwards compatibility, in that users of older releases will be unable to use your code.
I generally avoid using new things like that until at least a few MATLAB releases have been issued to ensure there will be very few users left in the lurch. For example, even now I find people are still using an old enough MATLAB release that they cannot use anonymous functions.
Here's another option you can use. First make a cell array to capture all the outputs (you can use the NARGOUT function to determine how many outputs a given function returns):
a = cell(1,3); % For capturing 3 outputs
% OR...
a = cell(1,nargout(#func)); % For capturing all outputs from "func"
Then call the function as follows:
[a{:}] = func();
Then simply remove the element from a that you want, and overwrite a:
a = a{3}; % Get the third output
I wrote a kth out function:
function kth = kthout(k, ffnc, varargin)
% kthout: take the kth varargout from a func call %FOLDUP
%
% kth = kthout(k, ffnc, varargin)
%
% input:
% k which varargout to get
% ffnc function to call;
% varargin passed to ffnc;
% output:
% kth the kth argout;
[outargs{1:k}] = feval(ffnc, varargin{:});
kth = outargs{k};
end %function
You can then call
val_i_want = kthout(3, #myfunc, func_input_1, func_input_2);
You could also wrap up the function like:
func_i_want = #(varargin)(kthout(3, #myfunc,varargin{:})); % Assuming you want the third output.
After which you use
val_i_want = func_i_want(func_input_1, func_input_2);
Note that there is overhead associated with using anonymous functions like this, and this is not something I would do in code that would be called thousands of times.
In MATLAB 2010a, I found a neat way of doing what you are asking for.
It is simply to use the character "~" (without the quotes of course) as your dummy variable (as many as you want when returning multiple parameters). This also works for input parameters to functions if the functions are designed to handle missing data.
I don't know if this existed in previous versions, but I just came across it recently.
You can make a function (or anonymous function) that only returns selected outputs, e.g.
select = #(a,b) a(b);
Then you can call your function like this:
select(func,2);
select(func,1:3);
Or you can assign the output to a variable:
output(1,2:4) = select(func,1:3);
I don't see any reason not to use ans(n). Like this:
size(rand([5 10 20 40]));
b = ans(2);
It gives b = 10, and this way would be compatible with all MATLAB versions. Note that size() here is just used to represent any function that has multiple return variables.
Furthermore, this works to get the second output argument when you don't know how many arguments there will be! Whereas, if you do this:
[~, b] = size(a);
Then b = 8000! (You need to end with ~, to catch more arguments!)
total newbie here. I'm having problems looping a function that I've created. I'm having some problems copying the code over but I'll give a general idea of it:
function[X]=Test(A,B,C,D)
other parts of the code
.
.
.
X = linsolve(K,L)
end
where K,L are other matrices I derived from the 4 variables A,B,C,D
The problem is whenever I execute the function Test(1,2,3,4), I can only get one answer out. I'm trying to loop this process for one variable, keep the other 3 variables constant.
For example, I want to get answers for A = 1:10, while B = 2, C = 3, D = 4
I've tried the following method and they did not work:
Function[X] = Test(A,B,C,D)
for A = 1:10
other parts of the code...
X=linsolve(K,L)
end
Whenever I keyed in the command Test(1,2,3,4), it only gave me the output of Test(10,2,3,4)
Then I read somewhere that you have to call the function from somewhere else, so I edited the Test function to be Function[X] = Test(B,C,D) and left A out where it can be assigned in another script eg:
global A
for A = 1:10
Test(A,2,3,4)
end
But this gives an error as well, as Test function requires A to be defined. As such I'm a little lost and can't seem to find any information on how can this be done. Would appreciate all the help I can get.
Cheers guys
I think this is what you're looking for:
A=1:10; B=2; C=3; D=4;
%Do pre-allocation for X according to the dimensions of your output
for iter = 1:length(A)
X(:,:,iter)= Test(A(iter),B,C,D);
end
X
where
function [X]=Test(A,B,C,D)
%other parts of the code
X = linsolve(K,L)
end
Try this:
function X = Test(A,B,C,D)
% allocate output (it is faster than changing the size in every loop)
X = {};
% loop for each position in A
for i = 1:numel(A);
%in the other parts you have to use A(i) instead of just A
... other parts of code
%overwrite the value in X at position i
X{i} = linsolve(K,L);
end
end
and run it with Test(1:10,2,3,4)
To answer what went wrong before:
When you loop with 'for A=1:10' you overwrite the A that was passed to the function (so the function will ignore the A that you passed it) and in each loop you overwrite the X calculated in the previous loop (that is why you can only see the answer for A=10).
The second try should work if you have created a file named Test.m with the function X = (A,B,C,D) as the first code in the file. Although the global assignment is unnecessary. In fact I would strongly recommend you not to use global variables as it gets very messy very fast.
I find it impossible to write MATLAB code without creating a huge number of superfluous, single-use variables.
For example, suppose function foo returns three column vectors of exactly the same size. Say:
function [a, b, c] = foo(n)
a = rand(n, 1);
b = rand(n, 1);
c = rand(n, 1);
end
Now, suppose that bar is a function that expect as imput a cell array of size (1, 3).
function result = bar(triplet)
[x, y, z] = triplet{:};
result = x + y + z;
end
If I want to pass the results of foo(5), I can do it by creating three otherwise-useless variables:
[x, y, z] = foo(5);
result = bar({x, y, z});
Is there some function baz that would allow me to replace the two lines above with
result = bar(baz(foo(5)));
?
NB: the functions foo and bar above are meant only as examples. They're supposed to represent functions over which I have no control. IOW, modifying them is not an option.
You can replace the three variables by a cell array using a comma-separated list:
vars = cell(1,3); % initiallize cell array with as many elements as outputs of foo
[vars{:}] = foo(5); % comma-separated list: each cell acts as a variable that
% receives an output of foo
result = bar(vars);
Not possible. baz in baz(foo(5)) will only take the first output of foo, the other two would be ignored. The plain two-line variant is not that awkward. And this is not a common situation. You don't generally work with cell arrays where normal numerical arrays would do.
You could of course just write your own wrapper for foo that returns whatever you need (i.e. containing similar two lines), in case you need to use it frequently.
As nirvana-msu said, it is not possible to do the task without creating temporary variables. But it is possible to handle it within a function and even with varargin and varargout. Inspired by this answer on my question, you can define and use the following function:
function varargout = redirect(F1, F2, count, list, varargin)
output = cell(1, count);
[output{:}] = F2(varargin{:});
varargout = cell(1, max(nargout, 1));
[varargout{:}] = F1(output(list));
end
For instance, in your example you can write result = redirect(#bar, #foo, 3, [1:3], 5);
The issue is you are converting the cell triplet{} into an array[], so a conversion is the method you want. Although this method will perform the inverse transformation, I know of no method that will perform the transformation you want, likely due to the relative complexity of the cell data structure. You may have some luck further digging into the API.
EDIT: EBH kindly pointed out this method, which does what you are looking for.
EDIT2: The above method will not perform the action OP asked for. Leaving this up because the API often has great solutions that are hidden by bad names.
I'm trying to use arrayfun() to map a function over a cell array. The following is happening:
>> arrayfun(solveFunc, equArray)
Error using arrayfun
First input must be a function handle.
Error in solve>genGuess (line 33)
funcVals = abs(arrayfun(inFunc, xValues));
Error in solve (line 8)
x = genGuess(inFunc, varargin{1}, varargin{2});
Error in makeSolveFunc>#(func)solve(func,start,stop) (line 3)
sFunc = #(func) solve(func, start, stop);
But, the first input IS a function handle. Also... if I manually apply the function to each element of the provided cell array, everything works fine:
>> solveFunc(equArray{1})
ans =
4.7335
>> solveFunc(equArray{2})
ans =
4.7356
Does anyone know why this would be happening? I assumed that if I could manually apply the function to each element of my array, and the return type of the function was consistent and one of the allowed types (you can't for example have arrayfun return an array of function handles... I already tried doing that), it should work. Perhaps that is not the only requirement.
Here is some code that generates this error:
solve.m
function solution = solve(inFunc, start, stop)
%SOLVE solve an equation using Newton's Method
x = genGuess(inFunc, start, stop);
for i = 1:100
m = getSlope(inFunc, x);
x = (m*x - inFunc(x))/m;
end
solution = x;
end
function slope = getSlope(inFunc, x)
%SLOPE calculate the slope at a given point
inc = 1e-5;
if x ~= 0
inc = inc * x;
end
slope = (inFunc(x + inc) - inFunc(x - inc))/(2*inc);
end
function guess = genGuess(inFunc, start, stop)
%GENGUESS get an initial guess to the solution
xValues = linspace(start, stop, 101);
funcVals = abs(arrayfun(inFunc, xValues));
[~, minIndex] = min(funcVals);
guess = xValues(minIndex);
end
charEqu.m
function equ = charEqu(a)
%CHAREQU create a KP model characteristic equation with provided p
equ = #(x) x + a;
end
makeSolveFunc.m
function sFunc = makeSolveFunc(start, stop)
%MAKESOLVEFUNC return a function that solves an equation
sFunc = #(func) solve(func, start, stop);
end
test.m
pArray = 1:5;
equArray = cell(1,arrayLen);
for i = 1:5
equArray{i} = charEqu(pArray(i));
end
solveFunc = makeSolveFunc(1.1*pi, 2*pi);
alphaAArray = arrayfun(solveFunc, equArray);
I have narrowed down the error to something in genGuess(). For some reason, in the line funcVals = abs(arrayfun(inFunc, xValues)); the variable inFunc is a 1x1 cell array containing a function handle. I have no idea why that would be the case. I traced this back to the anonymous function call #(func) solve(func, start, stop); in the makeSolveFunc() function. There it is still a 1x1 cell array containing a function handle. I'm not really sure where that cell array is coming from as that function is getting called from arrayfun().
Background information on what I'm trying to do in case someone wants to suggest a better way:
I'm trying to solve equations using Newton's method. I have written a function that can solve an equation given an initial guess range. This function is the solve() function you can see in the first error message. It takes a function, and the guess range and returns a function that I'm calling solveFunc(). solveFunc() takes a function and solves it using the initial guess range previously provided.
Maybe I'm just too used to functional programming and should just use a loop.
If the arguments passed to the function handle are contents of elements of a cell array, you need to use cellfun instead of arrayfun:
cellfun(solveFunc, equArray)
This is equivalent to
for i=1:length(equArray)
out(i) = solveFunc(equArray{i});
end
since solveFunc is already a function handle.
Check where the error comes from. This line causes the error:
funcVals = abs(arrayfun(inFunc, xValues));
The first input argument is a 1x1 cell containing one function handle. This is caused because equArray is a cell, thus use cellfun as Jonas already mentioned:
pArray = 1:5;
equArray = cell(1,arrayLen);
for i = 1:5
equArray{i} = charEqu(pArray(i));
end
solveFunc = makeSolveFunc(1.1*pi, 2*pi);
alphaAArray = cellfun(solveFunc, equArray);
I have the following function:
function x = dataParser(y)
%// Importing list of places
places = textread('UKPlaceNames.txt' ,'%s');
%// Picking two places
place = char(places(y)); %// Converting them to an array of chars
placeInt = place - 'A' + 1;
x = placeInt;
end
In a separate function, I create:
myVector = 1:6
and then call
myVectorB = arrayfun(dataParser, myVector)
However, MATLAB throws an error on:
place = char(places(y));
stating that there are not enough parameters, where
places = textread('test.txt' ,'%s');
I am a bit confused as to where the error is, since the function takes one paramater and I am providing it...
Thanks!
You should do the following in your main script/function:
myVectorB = arrayfun(#dataParser, myVector)
Note the # in front of dataParser: this passes a function handle of dataParser to arrayfun, instead of evaluating the function dataParser() and passing the result to arrayfun.