Is there a way in matlab to force a function to output a certain number of arguments? For example this is what matlab does:
function [a,b,c] = practice
if nargout >=1
a =1;
end
if nargout >=2
b=2;
end
if nargout ==3
c = 3;
end
end
d(1:3) = practice()
% d = [1 1 1]
I would want:
d(1:3) = practice()
% d = [1 2 3]
Can I get this behavior without needing to say
[d(1),d(2),d(3)] = practice()
There is an option to let your function output everything when only a single output argument is used:
function varargout=nargoutdemo(x)
varargout{1}=1;
varargout{2}=2;
varargout{3}=3;
if nargout==1
varargout={[varargout{:}]};
end
end
For non uniform return data, it might be necessary to switch to a cell
If you wish not to change the function, you could use this a little bit more generic code:
out=cell(1,3)
[out{:}]=practice
Please not, that this returns a cell, not an array. That's because array to comma separated list conversion is not directly possible.
Related
I have a task to write a function that has an optional output argument.
Let's say I have a function y = fun(a,b). From what I understood, depending on whether the user needs the value y, it will EITHER calculate y OR draw some diagram.
So I think it means that if user calls my function like this: z = fun(1,2), then it calculates y and returns it, but if he calls it like this: fun(3,4);, then it won't return anything and draw a diagram instead.
Is there any way to check how my function has been called inside it? If yes, then how?
You can use nargout here:
function y = q61527462(a,b)
if nargout > 0
% Calculate y
y = a + b;
else
% Plot
plot(a,b)
end
end
so when you call the function as:
>> y = q61527462(1,2)
you get:
y =
3
and when you call with:
>> q61527462(1,2)
you get the plot
Have a look at nargout, that roughly translates to number of argument to output (there is also a nargin to check the number of input arguments).
However, the function will return the return-value anyway if your just check for nargin. You will also need to use varargout (variable argument output) to make your function return something only if the output will be assigned to some external variable.
So you just write in your function
function varargout = fun()
if nargout % implicit cast to a logical. It is equivalent to nargout > 0
% return output
varargout{1} = true; % note that this requires to wrap your output in a cell
else
% do plotting
end
EDIT:
It is even not necessary to use varargout. If you don't assign a value to a return-variable, it won't appear. So MATLAB can manage a not-assigned return-variable (something I was surprised to learn myself^^). This even works with multiple outputs!
In Matlab some functions adapt their output to the number of output variables in the call. For example I can do:
A=[[1 2 3];[4 5 6]];
s=size(A);
And I get
s=[2, 3];
But if I want to handle independently width and height I can do:
[h, w]=size(A);
And I get:
h=2;
w=3;
Now, if I have a function that always output a vector of let's say 3 elements. Is there a way to assign each element to a different variable?
I mean to avoid an scenery like this:
pos=getPosition();
X=pos(1);
Y=pos(2);
Z=pos(3);
I hope I have explained what I mean.
I've had the same problem. Mostly with functions handling coordinates as in your example. My solution was to create the following function:
function varargout = dealOneByOne(vector)
% Assign each column of vector to each variable on the output variables
%
for i=1:size(vector,2)
varargout{i}=vector(:,i);
end
end
Then you can just do
[X,Y,Z]=dealOneByOne(getPosition());
I'm not aware of a simpler way to do it.
Let's define a test function as follows:
function x = test()
x = [1 2 3];
end
Given the function above, this is what I would normally perform in order to split the output array into many distinct variables:
out = num2cell(test());
[a,b,c] = deal(out{:});
A wrapper function can be defined in order to avoid spreading the above assignment into multiple lines:
[a,b,c] = vout_num(test());
function varargout = vout_num(x)
C = num2cell(x);
varargout = C(:).';
end
In your example, the wrapper function would be used as follows:
[X,Y,Z] = vout_num(getPosition());
I have a class called foo with only property M,
classdef foo
properties
M
end
methods
function obj = foo(arg1)
M=arg1;
end
end
end
and I create the object:
F=foo(1)
In C++ it is possible, that when I call F, that I directly get the value of one chosen variable. In my case I would like to get F.M when I just write F. For example this would allow to write
size(F)
instead of writing
size(F.M)
or defining a method size for my class. Is there a way in MATLAB to achieve this?
Edit: In other words, whenever I write F, I want that MATLAB reads it as F.M.
Edit: I will clarify what I want to acchieve.
My class represents a matrix whose values are multivariate infinite sequences with compact support. Thus I have to do a lot of bookkeeping not to mix up all the indices. Things I want to do with my matrix:
Matrix multiplication.
Convolution.
Upsampling, downsampling.
Everything else which is possible in a vector/matrix space.
I thought this is easier to acchieve when I encapsulate all the meta-data (this are the indices of the 0^th entries of the sequence) together with the data.
Because my class is actually a matrix, most operations work on that class in the same way as on ordinary matrices. Therefore I do not want to code it all over again (Because of code-maintenance, less bugs, code-readability).
In C++ I could do it that way. It seems, my approach to that problem is the wrong one in MATLAB.
You have to overload the function size as I mentioned in my comment.
classdef foo
properties
M
end
methods
function obj = foo(arg)
obj.M = arg;
end
function varargout = size(obj, varargin)
if nargin == 1
varargout = size(obj.M);
elseif nargin == 2 && nargout == 1
varargout = size(obj.M, varargin{1})
else
error('Wrong number of input or output arguments.')
end
end
function ret = mtimes(varargin)
try
ret = varargin{1}.M * varargin{2};
catch
ret = varargin{1} * varargin{2}.M;
end
end
end
end
Console Output:
>> F=foo(0:3);
>> size(F)
ans =
1 4
>> size(F, 2)
ans =
4
>> F*(4:7).'
ans =
38
>> (4:7).'*F
ans =
0 4 8 12
0 5 10 15
0 6 12 18
0 7 14 21
I could continue but preparing the console output for StackOverflow is horrible...
I am trying to write a function transform(A) which when given a matrix A returns a new matrix. The new matrix should be obtained according to following:
if A has more than one row then interchange the first and second row. After this square the elements in the first row.
So far thats what I have written:
function[Anew] = transform(A)
dimension = size(A);
if dimension(1) > 1 %if there is more than 1 row
A([1 2],:)=A([2 1],:);
end
A(1,:,:) = A(1,:,:).^2 %squares elements in the first row
end
I tested my function by invoking it in matlab.
I noticed that because i dont have a semi colon next to A(1,:,:) = A(1,:,:).^2
I still obtain the desired result but not as output of the function.
I obtain A =
instead of Anew =
If i put a semi colon next to A(1,:,:) = A(1,:,:).^2; then I dont get an output at all.
Could you tell me what is wrong and what should I change in my program to obtain output as Anew?
Thank you
To return a value from a function in Matlab, you must directly assign to it.
In your case, you need to assign to Anew at the end of the operation (you could also technically just use that variable all-together).
function [Output1, Output2] = SomeFunction( ... )
% Do Some Work
% Store Output
Output1 = Result1(:,2) %some output
Output2 = Result2(3:end, :) %some other result
end
function[Anew] = transform(A)
dimension = size(A);
Anew = A;
if dimension(1) > 1 %if there is more than 1 row
Anew ([1 2],:)=Anew([2 1],:);
end
Anew(1,:,:) = Anew(1,:,:).^2; %squares elements in the first row
end
I have a 500-by-1 matrix of strings S and a 500-by-1 matrix of numbers N. I want to see them in the variable editor together like this:
S(1) N(1)
S(2) N(2)
...
S(500) N(500)
Is this possible?
The following should allow you to look at the variables together in the Command Window:
disp([char(S) blanks(numel(N))' num2str(N)]);
The array S (which I presume is a cell array) is converted to a character array using the function CHAR. It's then concatenated with a column vector of blanks (made using the function BLANKS) and then a string representation of the numeric array N (made using the function NUM2STR). This is then displayed using the function DISP.
Speaking narrowly to your question, just convert the numbers to cells. You'll have a single variable that the array editor can handle.
X = [ S num2cell(N) ];
More broadly, here's an array-oriented variant of sprintf that's useful for displaying records constructed from parallel arrays. You'd call it like this. I use something like this for displaying tabular data a lot.
sprintf2('%-*s %8g', max(cellfun('prodofsize',S)), S, N)
Here's the function.
function out = sprintf2(fmt, varargin)
%SPRINTF2 Quasi-"vectorized" sprintf
%
% out = sprintf2(fmt, varargin)
%
% Like sprintf, but takes arrays of arguments and returns cellstr. This
% lets you do formatted output on nonscalar arrays.
%
% Example:
% food = {'wine','cheese','fancy bread'};
% price = [10 6.38 8.5];
% sprintf2('%-12s %6.2f', food, price)
% % Fancier formatting with width detection
% sprintf2('%-*s %6.2f', max(cellfun('prodofsize',food)), food, price)
[args,n] = promote(varargin);
out = cell(n,1);
for i = 1:n
argsi = grab(args, i);
out{i} = sprintf(fmt, argsi{:});
end
% Convenience HACK for display to command line
if nargout == 0
disp(char(out));
clear out;
end
function [args,n] = promote(args)
%PROMOTE Munge inputs to get cellstrs
for i = 1:numel(args)
if ischar(args{i})
args{i} = cellstr(args{i});
end
end
n = cellfun('prodofsize', args);
if numel(unique(n(n > 1))) > 1
error('Inconsistent lengths in nonscalar inputs');
end
n = max(n);
function out = grab(args, k)
%GRAB Get the kth element of each arg, popping out cells
for i = 1:numel(args)
if isscalar(args{i})
% "Scalar expansion" case
if iscell(args{i})
out{i} = args{i}{1};
else
out{i} = args{i};
end
else
% General case - kth element of array
if iscell(args{i})
out{i} = args{i}{k};
else
out{i} = args{i}(k);
end
end
end