`VL_ARGPARSE` - Parse list of parameter-value pairs.

OPTS = VL_ARGPARSE(OPTS, ARGS) updates the structure OPTS based on the specified parameter-value pairs ARGS={PAR1, VAL1, ... PARN, VALN}. If a parameter PAR cannot be matched to any of the fields in OPTS, the function generates an error.

Parameters that have a struct value in OPTS are processed recursively, updating the individual subfields. This behaviour can be suppressed by using VL_ARGPARSE(OPTS, ARGS, 'nonrecursive'), in which case the struct value is copied directly (hence deleting any existing subfield existing in OPTS). A direct copy occurrs also if the struct value in OPTS is a structure with no fields. The nonrecursive mode is especially useful when a processing time is a concern.

One or more of the (PAR, VAL) pairs in the argument list can be replaced by a structure; in this case, the fields of the structure are used as paramater names and the field values as parameter values. This behaviour, while orthogonal to structure-valued parameters, is also disabled in the 'nonrecursive' mode.

A shorthand notation for substructs is available: the pair ('field.subfield', value) creates a substruct, i.e. ('field', struct('subfield', value)). It also works recursively (sub-sub-fields).

[OPTS, ARGS] = VL_ARGPARSE(OPTS, ARGS) copies any parameter in ARGS that does not match OPTS back to ARGS instead of producing an error. Options specified as structures are passed back as a list of (PAR, VAL) pairs.

A further option is VL_ARGPARSE(OPTS, ARGS, 'merge'), which merges any parameter in ARGS that do not match OPTS into OPTS. This essentially turns off errors for unknown options, merging them anyway, and is useful if OPTS does not necessarily define all possible default values. The second output argument will always be empty. Another view is that VL_ARGPARSE(OPTS1, OPTS2, 'merge') merges two structs into one.

Example

The function can be used to parse a list of arguments passed to a MATLAB functions:
```
   function myFunction(x,y,z,varargin)
   opts.parameterName = defaultValue ;
   opts = vl_argparse(opts, varargin)
```
If only a subset of the options should be parsed, for example because the other options are interpreted by a subroutine, then use the form
```
   [opts, varargin] = vl_argparse(opts, varargin)
```
that copies back to VARARGIN any unknown parameter.

VL_ARGPARSE - Parse list of parameter-value pairs.

`VL_ARGPARSE` - Parse list of parameter-value pairs.