I want to call a function in Matlab using another one, which has the same number of inputs and outputs. In fact, those inputs and outputs have the same name.
Example:
function [a,b] = gettwo(matrix,string,varargin)
[a,b] = getone(matrix,string,varargin{:});
end
This code produces the following error:
Error in getone(line 3)
aux = 'matrix(varargin{:})';
Output argument "b" (and maybe others) not assigned during
call to "C:\Users\baister\Documents\MATLAB\soft\getone.m>getone".
Error in results (line 4)
[a,b] = getone(matrix,string,varargin{:});
How should I wrap getone?
(The definitive function will have more lines than those shown in this post.)
Thanks.
The general wrapping for variable number of outputs should work like this:
function [varargout] = gettwo(matrix,string,varargin)
[varargout{1:nargout}] = getone(matrix,string,varargin{:});
end
You'll get the same error as above though, in case you do
[a,b] = gettwo(...);
and getone returns only 1 argument.
Related
function [ muln, varargout ] = my_mul( varargin )
%MY_MUL This function is used to multiply numbers.
% My_mul function multiplies array of entered numbers, and outputs single
% solution.
% For example: my_mul(12, 2, 3, 5) gives ans = 360
if nargout >=1
disp('Error, wrong number of output arguments');
varargout{1} = 0;
return
end
if nargin <= 1
disp('Error, small number of input argumnets');
return
else
muln = 1;
for i = 1:nargin
muln = muln*varargin{i};
end
end
end
Hi, everyone, I'm just doing my assignment for uni and have a qiuck question.
How can I make this function to give an error if it is called with more than one output.(It meant to give only one) Thanks!
In your function definition, you have defined your function to allow for an unlimited number of outputs. The keyword varargout is a place-holder for a variable number of outputs.
As you have stated in your question, you only want one possible output which in your case looks to be muln. So if you simply remove varargout from your function definition, MATLAB should automatically throw an error if too many outputs are requested
function muln = my_mul(varargin)
If you ever do need to use varargout but want to place constraints on how many outputs are provided for any given scenario, you can check the number of output arguments that were requested using nargout and then throw an error with the error function.
if nargout > 4
error('my_mul:TooManyOutputs', 'Too many outputs requested');
end
My opinion is that if a return value is expected the function needs to throw. Otherwise the caller (function calling this function) will expect everything to be ok. Note that disp('Error') gives information to the developer, but it does not give the program any indication on what happens. More importantly, the information does not give any indication of where the error occurs. This can force the developer to do heavy debugging just to find the error, which is completely unnecessary.
The use of variable output arguments should only be used in case a different number of output arguments should be expected. An example is some customized plot function
function varargout = myplot(varargin)
filename = '';
idx = find(strcmp(varargin,'filename'));
if (~isempty(idx) && length(varargin)<idx+1 && ~ischar(varargin{idx+1}))
error('filename property must be followed by a directory');
elseif(~isempty(idx))
filename = varargin{idx+1};
varargin([idx,idx+1]) = [];
end
h = plot(varargin{:});
varagout{1} = h;
if (~isempty(idx))
save(filename, h);
end
varagout{2} = filename;
This function works as plot except it saves the figure to file in case a filename is specified. In case the developer needs the handle it will be returned and in case the developer wants the save directory it can be returned as well. None of these arguments are necessary though. The developer may want to use this function as a standard plot function and this means that the user may want to call myplot as myplot(x,y);which does not return a value. Further note that even if 'filename' is not specified, the function can still return 2 outputs. The second output may be an empty array of char, but two outputs for the caller will never cause a crash.
Also, note that no further error handling is required. The only unchecked crashes are in plot and save. How this is handled may be different for different users and this means that it only is reasonable to let the user catch the error and handle it (as he would have done if save or plot would have thrown).
Apart from this you may also want to have a check so that the number of output variables are within the correct range (in this case 0,1 or 2 outputs).
I am trying to use this function in my m file but I get an error(mentioned in question). Everything seems correct and a, b and c are defined in my m file. Any thoughts?
Error:
Error in modal2 (line 8)
[v,an]=eig(a);
Output argument "am" (and maybe others) not assigned during call to "C:\Users\Cena\Desktop\Thesis\My Codes\SMC\modal2.m>modal2".
function [r,am,bm,cm] = modal2(a,b,c)
% this function determines the modal representation 2(am,bm,cm)
%given a generic state-space representation (a,b,c)
%and the transformation r to the modal representation
%such that am=inv(r)*a*r, bm=inv(r)*b and cm=c*r
%transformation to complex-diagonal form:
[v,an]=eig(a);
bn=inv(v)*b;
cn=c*v;
%transformation to modal form 2:
i = find(imag(diag(an))');
index = i(1:2:length(i));
j=sqrt(-1);
t = eye(length(an));
if isempty(index)
am=an;bm=bn;cm=cn;
else
for i=index
t(i:i+1,i:i+1)=[j 1;-j 1];
end
%Modal transformation
r=v*t;
end
The problem is likely in
if isempty(index)
am=an;bm=bn;cm=cn;
The assignment to those variables is only being done if the conditional passes. If it doesn't , there is no assignment.
You need to modify your code to assign to those variables under all conditions if they are going to be output arguments.
Function:
My MATLAB function has one output and several input arguments, most of which are optional, i.e.:
output=MyFunction(arg1,arg2,opt1,opt2,...,optN)
What I want to do:
I'd like to give only arg1, arg2 and the last optional input argument optN to the function. I used the tilde operator as follows:
output=MyFunction(str1,str2,~,~,...,true)
Undesired result:
That gives the following error message:
Error: Expression or statement is incorrect--possibly unbalanced (, {, or [.
The error points to the comma after the first tilde, but I don't know what to make of it to be honest.
Problem identification:
I use MATLAB 2013b, which supports the tilde operator.
According to MATLAB's documentation the above function call should work:
You can ignore any number of function inputs, in any position in the argument list. Separate consecutive tildes with a comma...
I guess there are a few workarounds, such as using '' or [] as inputs, but I'd really like to understand how to correctly use '~' because actually leaving inputs out allows me to use exist() when checking the input arguments of a function.
If you need any further info from me, please let me know.
Thank you very much!
The tilde is only for function declaration. Matlab's mlint recommends to replace unused arguments by ~. The result is a function declared like this function output = MyFunction(a, b, ~, c). This is a very bad practice.
Since you have a function where the parameters are optional, you must call the function with empty arguments output=MyFunction(str1,str2,[],[],...,true).
A better way to do it is to declare the function with the varargin argument and prepare your function for the different inputs:
function output = MyFunction(varargin)
if nargin == 1
% Do something for 1 input
elseif nargin == 2
% Do something for 3 inputs
elseif nargin == 3
% Do something for 3 inputs
else
error('incorrect number of input arguments')
end
It is even possible to declare your function as follows:
function output = MyFunction(arg1, arg2, varargin)
The declaration above will tell Matlab that you are expecting at least two parameters.
See the documentation of nargin here.
... and the documentation of varargin here
To have variable number of inputs, use varargin. Use it together with nargin.
Example:
function varlist2(X,Y,varargin)
fprintf('Total number of inputs = %d\n',nargin);
nVarargs = length(varargin);
fprintf('Inputs in varargin(%d):\n',nVarargs)
for k = 1:nVarargs
fprintf(' %d\n', varargin{k})
end
I have a MATLAB file that contains a single top-level function, called sandbox. That function in turn contains two nested functions, mysum and myprod, which are identical in functionality and what parameters they allow except that one uses #sum internally and the other uses #prod internally. My goal is to create a wrapper function to use in both mysum and myprod that takes care of all the validation and input parsing. This function is called applyFunc.
Here's where it gets tricky. mysum and myprod come in two forms:
mysum(v) returns sum(v, 1).
mysum(v, 'imag') returns sum(v, 1) + 1i
Any other combinations of input should throw an error.
I'm having trouble using inputParser to parse these various combinations of input, specifically the optional string input. Here's the code:
function sandbox()
%% Data
v = [1 4; 3 3];
%% Calculations
s = mysum(v);
si = mysum(v, 'imag');
p = myprod(v);
pi = myprod(v, 'imag');
%% Accuracy tests
assert(isequal(s, [4 7]))
assert(isequal(si, [4+1i 7+1i]))
assert(isequal(p, [3 12]))
assert(isequal(pi, [3+1i 12+1i]))
function x = mysum(varargin)
x = applyFunc(#sum, varargin{:});
end
function x = myprod(varargin)
x = applyFunc(#prod, varargin{:});
end
end
function x = applyFunc(func, varargin)
p = inputParser();
p.addRequired('func', #(x) validateattributes(x, {'function_handle'}, {'scalar'}));
p.addRequired('v', #(x) validateattributes(x, {'double'}, {}, 'applyFunc:msg', 'v'));
p.addOptional('imag', '', #(x) validatestring(x, {'imag', ''})); % THIS LINE IS THE PROBLEM
p.parse(func, varargin{:});
f = p.Results.func;
v = p.Results.v;
strflag = p.Results.imag;
x = f(v);
if ~isempty(strflag)
validatestring(strflag, {'imag'});
x = x + 1i;
end
end
The line that's causing the problem is this one (as marked in the code above):
p.addOptional('imag', '', #(x) validatestring(x, {'imag', ''}));
The documentation for inputParser says that:
For optional string inputs, specify a validation function. Without a validation function, the input parser interprets valid string inputs as invalid parameter names and throws an error.
Unfortunately I don't have any idea how to do this. Is there something simple Im missing or what? If the 'imag' argument isn't passed at all (as in the assignment of s and p), the code works fine, but if I do pass it, I get this error:
Error using sandbox>applyFunc (line 32)
The value of 'imag' is invalid. It must satisfy the function:
#(x)validatestring(x,{'imag',''}).
Error in sandbox/mysum (line 18)
x = applyFunc(#sum, varargin{:});
Error in sandbox (line 7)
si = mysum(v, 'imag');
Any help?
The problem is that validatestring returns the matching string from the cell argument ({'imag',''}) rather than a Boolean indicating if it passes validation. Instead, use strcmp and any:
#(x) any(strcmp(x,{'imag', ''}))
Also, with validatestring, if the input string did not match either 'imag' or '' (actually just 'imag' since empty strings only match in R2014a+), it would throw an error rather than returning false so that the inputParser could return the appropriate error.
Another nice way to fix the problem is to change the syntax of applyFunc entirely so that instead of just 'imag' as an optional string input argument, use a Parameter-Value with 'imag' as the parameter and a validated boolean as the input.
The input definition suggested by Amro in the comments:
p.addParameter('imag', false, #(x)validateattributes(x, {'logical'}, {'scalar'}))
The usage:
mysum(x,'imag',true)
mysum(x) % default is equivalent to mysum(x,'imag',false)
This would simplify the rest of the code with p.Result.imag being a logical scalar. I would suggest:
x = f(v) + p.Result.imag*1i;
The problem is not inputParser, I think the issue is with validatestring.
1) First it does not match on empty strings:
>> x = ''
x =
''
>> validatestring(x, {'imag',''})
Expected input to match one of these strings:
imag,
The input did not match any of the valid strings.
Caused by:
Error using validatestring>checkString (line 85)
Expected input to be a row vector.
2) Second, if it successfully matches, it returns the resolved string (from one of the valid choice), instead of true/false. inputParser requires that the validation function either return a boolean, or nothing but throws error on failure.
I am trying to write a minimal function that can be called with a variable number of arguments but that will not throw a wrong number of arguments error if miscalled.
Here is where I start from :
function varargout=fname(varargin)
% FNAME
% Usage: output=fname(input)
% Arguments check
if(nargin~=1 || nargout~=1)
disp('Function fname requires one input argument');
disp('and one output argument');
disp('Try `help fname`');
varargout(1:nargout)={0};
return;
end
input=varargin{1};
output=input;
varargout(1)={output};
end
However this does not work as I would like it to. Is there a way to write a function that :
never throw a "wrong number of arguments" error (so that the rest of the execution can continue)
accepts variable number of input and output arguments and checks them inside the function
(maybe more tricky) if the number of input / output arguments is not correct, does not replace the value of the provided output arguments (so that any misplaced call does not erase the previous value of the output argument)
I am open to any suggestions / other methods.
Thank you for your help.
UPDATE: thanks to #Amro for his answer, I guess what I miss here is either a call by address of reference for Matlab functions or a way to interrupt a function without returning anything and without stopping the rest of the execution.
Here is one way to implement your function:
function varargout = fname(input,varargin)
%# FNAME
%# Usage: output=fname(input)
%%# INPUT
if nargin<1
varargout(1:nargout) = {[]};
warning('Not enough input arguments.'), return
end
if ~isempty(varargin)
warning('Too many input arguments.')
end
%%# YOUR CODE: manipulate input, and compute output
output = input;
%%# OUTPUT
varargout{1} = output;
if nargout>1
warning('Too many output arguments.')
varargout(2:nargout) = {[]};
end
end
Obviously you can customize the warning messages to your liking...
Also, if you want your function to simply print the message instead of issuing warnings, replace all WARNING calls with simple DISP function calls.
Examples of function call:
fname()
fname(1)
fname(1,2)
x = fname()
x = fname(1)
x = fname(1,2)
[x,y] = fname()
[x,y] = fname(1)
[x,y] = fname(1,2)
The above calls execute as expected (showing warning messages when applicable). One caveat though, in the last three calls, if the variable y already existed in the workspace prior to the calls, it would be overwritten by the empty value y=[] in each...
If I understand your question correctly, then the answer is no. If a caller calls a function like this:
[a, b, c] = fname('foo');
then fname is required to return (at least) three outputs. There's no way to tell MATLAB that it should leave b and c alone if fname only returns one output.