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Objects in can be indexed with parenthesises, either like
a (
idx)
or like a {
idx}
, or even
like a (
idx).
field. However, it is up to the user
to decide what this indexing actually means. In the case of our polynomial
class p (
n)
might mean either the coefficient of the
n-th power of the polynomial, or it might be the evaluation of the
polynomial at n. The meaning of this subscripted referencing is
determined by the subsref
method.
Perform the subscripted element selection operation according to the subscript specified by idx.
The subscript idx is expected to be a structure array with fields ‘type’ and ‘subs’. Valid values for ‘type’ are ‘"()"’, ‘"{}"’, and ‘"."’. The ‘subs’ field may be either ‘":"’ or a cell array of index values.
The following example shows how to extract the two first columns of a matrix
val = magic(3) val = [ 8 1 6 3 5 7 4 9 2 ] idx.type = "()"; idx.subs = {":", 1:2}; subsref(val, idx) [ 8 1 3 5 4 9 ]Note that this is the same as writing
val(:,1:2)
.See also: subsasgn, substruct.
For example we might decide that indexing with "()" evaluates the
polynomial and indexing with "{}" returns the n-th coefficient.
In this case the subsref
method of our polynomial class might look like
function b = subsref (a, s) switch s.type case "()" ind = s.subs; b = polyval (fliplr(a.poly), ind{:}); case "{}" ind = s.subs; b = polynomial (a.poly(ind{:})); case "." fld = s.subs; if (strcmp (fld, "poly")) b = a.poly; else error ("@polynomial/subsref: invalid property \"%s\"", fld); endif endswitch endfunction
The equivalent functionality for subscripted asignments uses the
subsasgn
method.
Perform the subscripted assignment operation according to the subscript specified by idx.
The subscript idx is expected to be a structure array with fields ‘type’ and ‘subs’. Valid values for ‘type’ are ‘"()"’, ‘"{}"’, and ‘"."’. The ‘subs’ field may be either ‘":"’ or a cell array of index values.
The following example shows how to set the two first columns of a 3-by-3 matrix to zero.
val = magic(3); idx.type = "()"; idx.subs = {":", 1:2}; subsasgn (val, idx, 0) [ 0 0 6 0 0 7 0 0 2 ]Note that this is the same as writing
val(:,1:2) = 0
.See also: subsref, substruct.
If you wish to use the end
keyword in subscripted expressions
of an object. Then the user needs to define the end
method for
the class.
For example the end
method for our polynomial class might look like
function r = end (obj, index_pos, num_indices) dv = size (obj.x); for i = (num_indices + 1) : length (dv) dv(num_indices) *= dv(i); endfor if (index_pos <= length (dv)) r = dv (index_pos); elseif r = 1; endif endfunction
which is a fairly generic end
method that has a behavior similar to
the end
keyword for Octave Array classes. It can then be used for
example like
p = polynomial([1,2,3,4]); p(end-1) 3
Objects can also be used as the index in a subscripted expression themselves
and this is controlled with the subsindex
function.
Convert an object to an index vector. When a is a class object defined with a class constructor, then
subsindex
is the overloading method that allows the conversion of this class object to a valid indexing vector. It is important to note thatsubsindex
must return a zero-based real integer vector of the class "double". For example, if the class constructorfunction b = myclass (a) b = myclass (struct ("a", a), "myclass"); endfunctionthen the
subsindex
functionfunction idx = subsindex (a) idx = double (a.a) - 1.0; endfunctioncan then be used as follows
a = myclass (1:4); b = 1:10; b(a) 1 2 3 4See also: class, subsref, subsasgn.
Finally, objects can equally be used like ranges, using the colon
method