How do I set default values for functions parameters in Matlab

Question

Is it possible to have default arguments in Matlab   For instance  here   function wave a  b  n  k  T  f  flag  fTrue inline  0      I would like to have the true solution be an optional argument to the wave function   If it is possible  can anyone demonstrate the proper way to do this   Currently  I am trying what I posted above and I get       Error  File  wave m Line  1 Column  37 The expression to the left of the equals sign is not a valid target for an assignment

User · Answer

I am confused nobody has pointed out this blog post by Loren, one of Matlab's developers. The approach is based on varargin and avoids all those endless and painfull if-then-else or switch cases with convoluted conditions. When there are a few default values, the effect is dramatic. Here's an example from the linked blog:

function y = somefun2Alt(a,b,varargin)
% Some function that requires 2 inputs and has some optional inputs.

% only want 3 optional inputs at most
numvarargs = length(varargin);
if numvarargs > 3
    error('myfuns:somefun2Alt:TooManyInputs', ...
        'requires at most 3 optional inputs');
end

% set defaults for optional inputs
optargs = {eps 17 @magic};

% now put these defaults into the valuesToUse cell array, 
% and overwrite the ones specified in varargin.
optargs(1:numvarargs) = varargin;
% or ...
% [optargs{1:numvarargs}] = varargin{:};

% Place optional args in memorable variable names
[tol, mynum, func] = optargs{:};

If you still don't get it, then try reading the entire blog post by Loren. I have written a follow up blog post which deals with missing positional default values. I mean that you could write something like:

somefun2Alt(a, b, '', 42)

and still have the default eps value for the tol parameter (and @magic callback for func of course). Loren's code allows this with a slight but tricky modification.

Finally, just a few advantages of this approach:

Even with a lot of defaults the boilerplate code doesn't get huge (as opposed to the family of if-then-else approaches, which get longer with each new default value)
All the defaults are in one place. If any of those need to change, you have just one place to look at.

Trooth be told, there is a disadvantage too. When you type the function in Matlab shell and forget its parameters, you will see an unhelpful varargin as a hint. To deal with that, you're advised to write a meaningful usage clause.

User · Answer

Yes  it might be really nice to have the capability to do as you have written  But it is not possible in MATLAB  Many of my utilities that allow defaults for the arguments tend to be written with explicit checks in the beginning like this   if  nargin lt 3  or isempty myParameterName    MyParameterName   defaultValue  elseif       tests for non-validity of the value actually provided        error  The sky is falling    end   Ok  so I would generally apply a better  more descriptive error message  See that the check for an empty variable allows the user to pass in an empty pair of brackets      as a placeholder for a variable that will take on its default value  The author must still supply the code to replace that empty argument with its default value though   My utilities that are more sophisticated  with MANY parameters  all of which have default arguments  will often use a property value pair interface for default arguments  This basic paradigm is seen in the handle graphics tools in matlab  as well as in optimset  odeset  etc    As a means to work with these property value pairs  you will need to learn about varargin  as a way of inputing a fully variable number of arguments to a function  I wrote  and posted  a utility to work with such property value pairs  parse pv pairs m  It helps you to convert property value pairs into a matlab structure  It also enables you to supply default values for each parameter  Converting an unwieldy list of parameters into a structure is a VERY nice way to pass them around in MATLAB

User · Answer

This is my simple way to set default values to a function  using  try    function z   myfun  a varargin      Default values b   1  c   1  d   1  e   1   try      b   varargin 1       c   varargin 2       d   varargin 3       e   varargin 4   end     Calculation z   a   b   c   d   e   end   Regards

User · Answer

I ve found that the parseArgs function can be very helpful

User · Answer

This is more or less lifted from the Matlab manual  I ve only got passing experience     function my output   wave   a  b  n  k  T  f  flag  varargin     optargin   numel varargin     fTrue   inline  0      if optargin  gt  0     fTrue   varargin 1     end     code     end

User · Answer

if you would use octave you could do it like this - but sadly matlab does not support this possibility  function hello  who    World     printf   Hello   s  n   who   endfunction    taken from the doc

User · Answer

There isn t a direct way to do this like you ve attempted   The usual approach is to use  varargs  and check against the number of arguments  Something like   function f arg1  arg2  arg3     if nargin  lt  3     arg3      some default    end  end   There are a few fancier things you can do with isempty  etc   and you might want to look at Matlab central for some packages that bundle these sorts of things   You might have a look at varargin  nargchk  etc  They re useful functions for this sort of thing   varargs allow you to leave a variable number of final arguments  but this doesn t get you around the problem of default values for some all of them

User · Answer

Matlab doesn t provide a mechanism for this  but you can construct one in userland code that s terser than inputParser or  if nargin  lt  1     sequences   function varargout   getargs args  defaults   GETARGS Parse function arguments  with defaults     args is varargin from the caller  By convention  a    means  use default     defaults  optional  is a cell vector of corresponding default values  if nargin  lt  2   defaults       end  varargout   cell 1  nargout   for i   1 nargout     if numel args   gt   i  amp  amp   isequal args i               varargout i    args i       elseif numel defaults   gt   i         varargout i    defaults i       end end   Then you can call it in your functions like this   function y   foo varargin   FOO      y   foo a  b  c  d  e  f  g    a  b   c        d  e  f  g    getargs varargin      1  14   dfltc       The formatting is a convention that lets you read down from parameter names to their default values  You can extend your getargs   with optional parameter type specifications  for error detection or implicit conversion  and argument count ranges   There are two drawbacks to this approach  First  it s slow  so you don t want to use it for functions that are called in loops  Second  Matlab s function help - the autocompletion hints on the command line - don t work for varargin functions  But it is pretty convenient

User · Answer

Another slightly less hacky way is  function output   fun input     if  exist  input   var    input  BlahBlahBlah   end        end

User · Answer

I believe I found quite a nifty way to deal with this issue  taking up only three lines of code  barring line wraps   The following is lifted directly from a function I am writing  and it seems to work as desired   defaults    50 6 3 true false  375 20 50 0     set all defaults defaults 1 nargin-numberForcedParameters    varargin   overload with function input  sigma shifts applyDifference loop weights             defaults      unfold the cell struct   Just thought I d share it

User · Answer

After becoming aware of ASSIGNIN  thanks to this answer by b3  and EVALIN I wrote two functions to finally obtain a very simple calling structure   setParameterDefault  fTrue   inline  0       Here s the listing   function setParameterDefault pname  defval    setParameterDefault pname  defval    Author  Tobias Kienzler  https   stackoverflow com users 321973    sets the parameter NAMED pname to the value defval if it is undefined or   empty  if  isParameterDefined  pname       error  paramDef noPname    No parameter name defined     elseif  isvarname pname      error  paramDef pnameNotChar    pname is not a valid varname     elseif  isParameterDefined  defval       error  paramDef noDefval     No default value for   pname   defined      end     isParameterNotDefined copy amp pasted since evalin can t handle caller s   caller    if  evalin  caller      exist     pname        var     amp  amp   isempty   pname           callername   evalin  caller    mfilename        warnMsg     Setting   pname   to default value        if isscalar defval     ischar defval     isvector defval          warnMsg    warnMsg      num2str defval            end      warnMsg    warnMsg           warning  callername   paramDef assigning    warnMsg       assignin  caller   pname  defval   end   and  function b   isParameterDefined pname    b   isParameterDefined pname    Author  Tobias Kienzler  https   stackoverflow com users 321973    returns true if a parameter NAMED pname exists in the caller s workspace   and if it is not empty  b   evalin  caller      exist     pname        var     amp  amp   isempty   pname

User · Answer

you might want to use the parseparams command in matlab  the usage would look like   function output   wave varargin     comments  etc  reg  props    parseparams varargin   ctrls   cell2struct props 2 2 end  props 1 2 end  2     yes this is ugly  a   reg 1   b   reg 2    etc fTrue   ctrl fTrue

User · Answer

There is also a  hack  that can be used although it might be removed from matlab at some point  Function eval actually accepts two arguments of which the second is run if an error occurred with the first   Thus we can use  function output   fun input     eval  input     input   1            end   to use value 1 as default for the argument

User · Answer

I like to do this in a somewhat more object oriented way  Before calling wave   save some of your arguments within a struct  eg  one called parameters   parameters flag  42  parameters fTrue  1  wave a b n k T f parameters     Within the wave function then check  whether the struct parameters contains a field called  flag  and if so  if its value is non empty  Then assign it eighter a default value you define before  or the value given as an argument in the parameters struct   function output   wave a b n k T f parameters    flagDefault 18    fTrueDefault 0    if  isfield parameters  flag      0    isempty parameters flag   flag flagDefault else flag parameters flag  end   if  isfield parameter  fTrue      0    isempty parameters fTrue   fTrue fTrueDefault else fTrue parameters fTrue  end       end   This makes it easier to handle large numbers of arguments  because it does not depend on the order of the given arguments  That said  it is also helpful if you have to add more arguments later  because you don t have to change the functions signature to do so

User · Answer

function f arg1  arg2  varargin   arg3   default3  arg4   default4    etc   for ii   1 length varargin  2   if  exist varargin 2 ii-1       error   unknown parameter    varargin 2 ii-1       end    eval  varargin 2 ii-1      varargin 2 ii     end    e g  f 2 4  c  3  causes the parameter c to be 3

User · Answer

I ve used the inputParser object to deal with setting default options  Matlab won t accept the python-like format you specified in the question  but you should be able to call the function like this   wave a b n k T f flag  fTrue  inline  0      After you define the wave function like this   function wave a b n k T f flag varargin   i p   inputParser  i p FunctionName    WAVE    i p addRequired  a   isnumeric   i p addRequired  b   isnumeric   i p addRequired  n   isnumeric   i p addRequired  k   isnumeric   i p addRequired  T   isnumeric   i p addRequired  f   isnumeric   i p addRequired  flag   isnumeric    i p addOptional  ftrue  inline  0   1        i p parse a b n k T f flag varargin        Now the values passed into the function are available through i p Results  Also  I wasn t sure how to validate that the parameter passed in for ftrue was actually an inline function so left the validator blank

[matlab] How do I set default values for functions parameters in Matlab?

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