`numArgumentsFromSubscript`

Method a class can overload to say how many outputs an indexing expression returns.

Syntax

n = numArgumentsFromSubscript(obj, s, indexingContext)

Returns the number of values produced when obj is indexed with the chain s in the given context.

Description

When a class overloads subsref or subsasgn, MATLAB must know how many values an expression such as obj.list{:} yields before it can call the overloaded method with the right nargout. The default rule counts the elements an ordinary index would select. A class overrides numArgumentsFromSubscript to apply its own rule, for instance to return 1 for any index into a container.

The language normally calls this method itself, not user code.

Input arguments

obj: The object being indexed.
s: Indexing structure array with fields type and subs, as built by substruct.
indexingContext: A member of matlab.indexing.IndexingContext: Statement, Expression or Assignment.

Output arguments

n: Number of outputs (or inputs, for an assignment) the expression represents.

Example

A class that indexes into its own list

writelines(["classdef Bag", "  properties", "    Items = {}", "  end", "  methods", "    function n = numArgumentsFromSubscript(obj, s, ctx)", "      n = 1;", "    end", "    function out = subsref(obj, s)", "      out = obj.Items{s(1).subs{1}};", "    end", "  end", "end"], 'Bag.m');
b = Bag;
b.Items = {10, 20, 30};
b(2)

Output

ans = 20

b(2) calls the overloaded subsref; numArgumentsFromSubscript tells the language it returns exactly one value.

Notes

MatLivre does not provide the matlab.indexing.IndexingContext enumeration, so calling numArgumentsFromSubscript directly always reports an error. A class can still define the method, and indexing of such classes works.

numArgumentsFromSubscript