MATLAB: Inputparser mistakes option for value

functioninputparserMATLAB

Hello, I have created a method to draw some plots.

I have used inputParser to assign optional parameters (e.g. the y-axis label or the legend).

Here is the code:

function plot1signal(signal,titleName,varargin)
%   Function to plot one signal per figure
% defaults
default_legendName = '';
default_yLabel = 'default';
p = inputParser;
validSignal = @(x) (isa(x,'timeseries') || isstruct(x));
addRequired(p,'signal',validSignal);
addRequired(p,'titleName',@ischar);
addOptional(p,'legendName',default_legendName,@ischar);
addOptional(p,'yLabel',default_yLabel,@ischar);
parse(p,signal,titleName,varargin{:});
...
end

As you can see there are 2 required inputs (a signal and the title), and two optional (y-label and the legend string).

I call this function with:

plot1signal(the_signal,'Title','yLabel','kN')

The expected behavior of the method would be having the default value for legendName and 'kN' for yLabel. Unfortunately inputParser seems to intepret 'yLabel' not only as an option but also as the input for 'legendName':

p.Results
ans = 
  struct with fields:
    legendName: 'yLabel'
        signal: [1×1 struct]
     titleName: 'Title'
        yLabel: 'kN'

Am I missing something?

Best Answer

Hi,

Adding to what ARK has said, You can see the difference between functions like addRequired , addOptional and addParameter in this example.

Hope this helps.

Related Solutions

MATLAB: Possible bug in Parse function input

This does seem to be a limitation in the implementation of inputParser. Following code show a case where the parser will fail. In this code, the second optional input is a char array.

function a = findArea(width, varargin)
   defaultHeight = 1;
   defaultUnits = 'inches';
   defaultShape = 'rectangle';
   expectedShapes = {'square','rectangle','parallelogram'};
   p = inputParser;
   validScalarPosNum = @(x) isnumeric(x) && isscalar(x) && (x > 0);
   validScalarPosNum2 = @(x) ischar(x); % <---- the optional parameter is char array
   addRequired(p,'width',validScalarPosNum);
   addOptional(p,'height',defaultHeight,validScalarPosNum2);
   addParameter(p,'units',defaultUnits,@isstring);
   addParameter(p,'shape',defaultShape,...
                 @(x) any(validatestring(x,expectedShapes)));
   parse(p,width,varargin{:});
   
   a = p.Results; 
end

now call it like this

>> findArea(7, 'shape', 'square')
Error using findArea (line 14)
No value was given for 'square'. Name-value pair arguments require a name
followed by a value.

I skipped the optional parameter height, but the parser failed to note that.

I guess this is done intentionally by MATLAB to avoid some kind of ambiguity in parsing the inputs. For example, in this case, it is easy to tell since we are using a single optional input. But consider a function that takes several optional inputs and parameters. In that case, it will become difficult for the parser to guess which values are optional inputs and which are parameters.

Note that the parser will only fail if you are taking character array as inputs for optional parameters. I guess the MATLAB expects that the character arrays should be input using parameters.

MATLAB: Undefined function or variable ‘inputParser’ while using Matlab Function in Stateflow

It seems that the problem might be happening because the inputParser does not support code generation. You may try coder.extrinsic

coder.extrinsic('inputParser', 'addOptional', 'parse', 'get')

add this line at beginning of function definition

function x = OK(varargin)
    coder.extrinsic('inputParser', 'addOptional', 'parse', 'get')
    Input_mux = [1:100]; %Would normally be stateflow input, added here for ease.
    
    default = 1:50; 
    p = inputParser; 
    addOptional(p,'i',default);
    parse(p,varargin{:});
    if all(Input_mux(p.Results.i)== 5)
         x = true;
    else
         x = false;
    end
end

'get' is needed as extrensic to extract data from p.Results as shown in comment: https://www.mathworks.com/matlabcentral/answers/542015-undefined-function-or-variable-inputparser-while-using-matlab-function-in-stateflow#comment_884843

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