How to deal with name/value pairs of function argu

Question

I have a function that takes optional arguments as name/value pairs.

function example(varargin)
% Lots of set up stuff
vargs = varargin;
nargs = length(vargs);
names = vargs(1:2:nargs);
values = vargs(2:2:nargs);

validnames = {'foo', 'bar', 'baz'};    
for name = names
   validatestring(name{:}, validnames);
end

% Do something ...
foo = strmatch('foo', names);
disp(values(foo))
end

example('foo', 1:10, 'bar', 'qwerty')

It seems that there is a lot of effort involved in extracting the appropriate values (and it still isn't particularly robust again badly specified inputs). Is there a better way of handling these name/value pairs? Are there any helper functions that come with MATLAB to assist?

Answer 1

Here's the solution I'm trialling, based upon Jonas' idea.

function argStruct = NameValuePairToStruct(defaults, varargin)
%NAMEVALUEPAIRTOSTRUCT Converts name/value pairs to a struct.
% 
% ARGSTRUCT = NAMEVALUEPAIRTOSTRUCT(DEFAULTS, VARARGIN) converts
% name/value pairs to a struct, with defaults.  The function expects an
% even number of arguments to VARARGIN, alternating NAME then VALUE.
% (Each NAME should be a valid variable name.)
% 
% Examples: 
% 
% No defaults
% NameValuePairToStruct(struct, ...
%    'foo', 123, ...
%    'bar', 'qwerty', ...
%    'baz', magic(3))
% 
% With defaults
% NameValuePairToStruct( ...
%    struct('bar', 'dvorak', 'quux', eye(3)), ...
%    'foo', 123, ...
%    'bar', 'qwerty', ...
%    'baz', magic(3))
% 
% See also: inputParser

nArgs = length(varargin);
if rem(nArgs, 2) ~= 0
   error('NameValuePairToStruct:NotNameValuePairs', ...
      'Inputs were not name/value pairs');
end

argStruct = defaults;
for i = 1:2:nArgs
   name = varargin{i};
   if ~isvarname(name)
      error('NameValuePairToStruct:InvalidName', ...
         'A variable name was not valid');
   end
   argStruct = setfield(argStruct, name, varargin{i + 1});  %#ok<SFLD>
end

end

Answer 2

Read Loren's informative post on this issue. Don't forget to read the comments section... - You will see that there are quite a few different approaches to this topic. They all work, so selecting a prefered method is really a matter of personal taste and maintainability.

Answer 3

I'm a bigger fan of home-grown boiler plate code like this:

function TestExample(req1, req2, varargin)
for i = 1:2:length(varargin)
    if strcmpi(varargin{i}, 'alphabet')
        ALPHA = varargin{i+1};

    elseif strcmpi(varargin{i}, 'cutoff')
        CUTOFF = varargin{i+1};
        %we need to remove these so seqlogo doesn't get confused
        rm_inds = [rm_inds i, i+1]; %#ok<*AGROW>

    elseif strcmpi(varargin{i}, 'colors')
        colors = varargin{i+1};
        rm_inds = [rm_inds i, i+1]; 
    elseif strcmpi(varargin{i}, 'axes_handle')
        handle = varargin{i+1};
        rm_inds = [rm_inds i, i+1]; 
    elseif strcmpi(varargin{i}, 'top-n')
        TOPN = varargin{i+1};
        rm_inds = [rm_inds i, i+1];
    elseif strcmpi(varargin{i}, 'inds')
        npos = varargin{i+1};
        rm_inds = [rm_inds i, i+1];
    elseif strcmpi(varargin{i}, 'letterfile')
        LETTERFILE = varargin{i+1};
        rm_inds = [rm_inds i, i+1];
    elseif strcmpi(varargin{i}, 'letterstruct')
        lo = varargin{i+1};
        rm_inds = [rm_inds i, i+1];
    end
end

This way I can simulate the 'option', value pair that's nearly identical to how most Matlab functions take their arguments.

Hope that helps,

Will

Answer 4

I ended up writing this today, and then found these mentions. Mine uses struct's and struct 'overlays' for options. It essentially mirrors the functionality of setstructfields() except that new parameters can not be added. It also has an option for recursing, whereas setstructfields() does it automatically. It can take in a cell array of paired values by calling struct(args{:}).

% Overlay default fields with input fields
% Good for option management
% Arguments
%   $opts - Default options
%   $optsIn - Input options
%       Can be struct(), cell of {name, value, ...}, or empty []
%   $recurseStructs - Applies optOverlay to any existing structs, given new
%   value is a struct too and both are 1x1 structs
% Output
%   $opts - Outputs with optsIn values overlayed
function [opts] = optOverlay(opts, optsIn, recurseStructs)
    if nargin < 3
        recurseStructs = false;
    end
    isValid = @(o) isstruct(o) && length(o) == 1;
    assert(isValid(opts), 'Existing options cannot be cell array');
    assert(isValid(optsIn), 'Input options cannot be cell array');
    if ~isempty(optsIn)
        if iscell(optsIn)
            optsIn = struct(optsIn{:});
        end
        assert(isstruct(optsIn));
        fields = fieldnames(optsIn);
        for i = 1:length(fields)
            field = fields{i};
            assert(isfield(opts, field), 'Field does not exist: %s', field);
            newValue = optsIn.(field);
            % Apply recursion
            if recurseStructs
                curValue = opts.(field);
                % Both values must be proper option structs
                if isValid(curValue) && isValid(newValue) 
                    newValue = optOverlay(curValue, newValue, true);
                end
            end
            opts.(field) = newValue;
        end
    end
end

I'd say that using the naming convention 'defaults' and 'new' would probably be better :P

Answer 5

Since ages I am using process_options.m. It is stable, easy to use and has been included in various matlab frameworks. Don't know anything about performance though – might be that there are faster implementations.

Feature I like most with process_options is the unused_args return value, that can be used to split input args in groups of args for, e.g., subprocesses.

And you can easily define default values.

Most importantly: using process_options.m usually results in readable and maintainable option definitions.

Example code:

function y = func(x, y, varargin)

    [u, v] = process_options(varargin,
                             'u', 0,
                             'v', 1);

Answer 6

function argtest(varargin)

a = 1;

for ii=1:length(varargin)/2
    [~] = evalc([varargin{2*ii-1} '=''' num2str(varargin{2*ii}) '''']);
end;

disp(a);
who

This does of course not check for correct assignments, but it's simple and any useless variable will be ignored anyway. It also only works for numerics, strings and arrays, but not for matrices, cells or structures.