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

Objects can be indexed with parentheses or braces, either like

or like `obj`(`idx`)

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

. However, it is up to the
programmer to decide what this indexing actually means. In the case of the
polynomial class `obj`(`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.

- :
**subsref***(*`val`,`idx`) Perform the subscripted element selection operation on

`val`according to the subscript specified by`idx`.The subscript

`idx`must 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 first two 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, this class uses the convention that indexing with `"()"`

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

returns the
`n`-th coefficient (of the `n`-th power). The code for the
`subsref`

method looks like

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

The equivalent functionality for subscripted assignments uses the
`subsasgn`

method.

- :
**subsasgn***(*`val`,`idx`,`rhs`) Perform the subscripted assignment operation according to the subscript specified by

`idx`.The subscript

`idx`must 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`.**See also:**subsref, substruct, optimize_subsasgn_calls.

- :
`val`=**optimize_subsasgn_calls***()* - :
`old_val`=**optimize_subsasgn_calls***(*`new_val`) - :
**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 the

`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.**See also:**subsasgn.

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 there must be an `end`

method defined. For example, the
`end`

method for the 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. An example using the polynomial
class is then

p = polynomial ([1,2,3,4]); p{end-1} ⇒ 3

Objects can also be used themselves as the index in a subscripted expression
and this is controlled by the `subsindex`

function.

- :
`idx`=**subsindex***(*`obj`) Convert an object to an index vector.

When

`obj`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 constructor werefunction obj = myclass (a) obj = class (struct ("a", a), "myclass"); endfunction

then the

`subsindex`

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

could be used as follows

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

Finally, objects can be used like ranges by providing a `colon`

method.

- :
`r`=**colon***(*`base`,`limit`) - :
`r`=**colon***(*`base`,`increment`,`limit`) Return the result of the colon expression corresponding to

`base`,`limit`, and optionally,`increment`.This function is equivalent to the operator syntax

or`base`:`limit`

.`base`:`increment`:`limit`**See also:**linspace.

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