Next: Indexed Assignment Optimization, Up: Indexing Objects [Contents][Index]

Objects can be indexed with parentheses, either like

or like `a` (`idx`)

, or even
like `a` {`idx`}

. However, it is up to the user
to decide what this indexing actually means. In the case of our polynomial
class `a` (`idx`).`field`

might mean either the coefficient of the
`p` (`n`)`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.

- Built-in Function:
**subsref***(*`val`,`idx`) 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)`

.If

`idx`is an empty structure array with fields ‘`type`’ and ‘`subs`’, return`val`.

For example we might decide that indexing with `"()"`

evaluates the
polynomial and indexing with `"{}"`

returns the `n`-th coefficient
(of `n`-th power). In this case the `subsref`

method of our
polynomial class might look like

function b = subsref (a, s) if (isempty (s)) error ("polynomial: missing index"); endif switch (s(1).type) case "()" ind = s(1).subs; if (numel (ind) != 1) error ("polynomial: need exactly one index"); else b = polyval (fliplr (a.poly), ind{1}); endif case "{}" ind = s(1).subs; if (numel (ind) != 1) error ("polynomial: need exactly one index"); else if (isnumeric (ind{1})) b = a.poly(ind{1}+1); else b = a.poly(ind{1}); endif endif case "." fld = s.subs; if (strcmp (fld, "poly")) b = a.poly; else error ("@polynomial/subsref: invalid property \"%s\"", fld); endif otherwise error ("invalid subscript type"); endswitch if (numel (s) > 1) b = subsref (b, s(2:end)); endif endfunction

The equivalent functionality for subscripted assignments uses the
`subsasgn`

method.

- Built-in Function:
**subsasgn***(*`val`,`idx`,`rhs`) 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`

.If

`idx`is an empty structure array with fields ‘`type`’ and ‘`subs`’, return`rhs`.

- Built-in Function:
`val`=**optimize_subsasgn_calls***()* - Built-in Function:
`old_val`=**optimize_subsasgn_calls***(*`new_val`) - Built-in Function:
**optimize_subsasgn_calls***(*`new_val`, "local") Query or set the internal flag for subsasgn method call optimizations. If true, Octave will attempt to eliminate the redundant copying when calling subsasgn method of a user-defined class.

When called from inside a function with the

`"local"`

option, the variable is changed locally for the function and any subroutines it calls. The original variable value is restored when exiting the function.

Note that the `subsref`

and `subsasgn`

methods always receive the
whole index chain, while they usually handle only the first element. It is the
responsibility of these methods to handle the rest of the chain (if needed),
usually by forwarding it again to `subsref`

or `subsasgn`

.

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) if (num_indices != 1) error ("polynomial object may only have one index") endif r = length (obj.poly) - 1; 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 as
follows:

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.

- Function File:
`idx`=**subsindex***(*`a`) 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 that`subsindex`

must return a zero-based real integer vector of the class`"double"`

. For example, if the class constructorfunction b = myclass (a) b = class (struct ("a", a), "myclass"); endfunction

then the

`subsindex`

functionfunction idx = subsindex (a) idx = double (a.a) - 1.0; endfunction

can then be used as follows

a = myclass (1:4); b = 1:10; b(a) ⇒ 1 2 3 4

Finally, objects can equally be used like ranges, using the `colon`

method

- Function File:
`r`=**colon***(*`a`,`b`) - Function File:
`r`=**colon***(*`a`,`b`,`c`) Method of a class to construct a range with the

`:`

operator. For example:a = myclass (…); b = myclass (…); c = a : b

Next: Indexed Assignment Optimization, Up: Indexing Objects [Contents][Index]