string data type is not a class?

I would expect the following to pull up a list of methods for the string data type, but it does not:

methods("1")

No class 1.

Is there any rationale to this behavior?

[Rik]

s={string([])};cellfun('isempty',s),cellfun(@isempty,s)

ans = logical
   0

ans = logical
   1

This implies the isempty function is overloaded for strings (which is what the cellfun doc warns about for the legacy syntax), but this fact is not reflected in the values methods returns.

Why all these implementation details are hidden is a mystery to me.

Answer 1

[Rik]

help methods

 METHODS Display class method names.
    METHODS CLASSNAME displays the names of the methods for the
    class with the name CLASSNAME.
 
    Use the functional form of METHODS, such as METHODS(S), when
    CLASSNAME is a string scalar.
 
    METHODS(OBJECT) displays the names of the methods for the
    class of OBJECT.
 
    M = METHODS(...) returns the method names in a cell array
    of character vectors.
 
    If CLASSNAME represents a MATLAB or Java class, then only
    public methods are returned, including those inherited 
    from base classes.
 
    METHODS differs from WHAT in that the methods from all method
    directories are reported together, and METHODS removes all
    duplicate method names from the result list.
 
    METHODS CLASSNAME -full  displays a full description of the
    methods in the class, including inheritance information and,
    for MATLAB and Java methods, method attributes and signatures.  
    Duplicate method names with different signatures are not
    removed.
 
    M = METHODS( ..., '-full') returns the full method descriptions
    in a cell array of character vectors.
 
    The word METHODS is also used in a MATLAB class definition to
    denote the start of a methods definition block.
 
    Examples:
 
    %Example 1:
    %Retrieve the names of the public methods of class 'memmapfile'
    %and capture the result in a cell array of character vectors.
    methodnames = methods('memmapfile');
 
    %Example 2:
    %Construct a java.lang.String instance and display the names of
    %the public methods of that instance.
    s = java.lang.String;
    methods(s);
 
    See also METHODSVIEW, PROPERTIES, EVENTS, CLASSDEF, WHAT, WHICH.

    Documentation for methods
       doc methods

    Other functions named methods

       icdevice/methods       imaqdevice/methods
       icgroup/methods        iviconfigurationstore/methods
       icinterface/methods    serial/methods
       imaqchild/methods

This suggests that you can't use the methods(object) syntax for char and string inputs.

methods('1')

No class 1.

methods(class("1"))

Methods for class string:

append          contains        eq              extractAfter    gt              issorted        lt              pad             reverse         startsWith      
cellstr         count           erase           extractBefore   insertAfter     join            matches         plus            sort            strip           
char            double          eraseBetween    extractBetween  insertBefore    le              ne              replace         split           strlength       
compose         endsWith        extract         ge              ismissing       lower           or              replaceBetween  splitlines      upper           

[Matt J]

Yes, it makes sense now that I think about it.

[Rik]

The documentation doesn't explicitly mention this restriction. Have you filled in the feedback form yet? Otherwise I'll do that. I think this restriction should be mentioned somewhere.

[Matt J]

Well, it seems self-evident in hindsight. If the input is a char or string, naturally methods() has to assume that the string contains the name of a class. How could it be expected to differentiate between the usage

methods('double')

and

methods('1')

[Steven Lord]

Yes. We break the potential ambiguity between "is this a char vector or string scalar containing the name of a class" and "is this a char vector or string scalar that tells MATLAB it should return the methods of char or string respectively" by choosing the former. If you want the methods of string:

methods('string')

Methods for class string:

append          contains        eq              extractAfter    gt              issorted        lt              pad             reverse         startsWith      
cellstr         count           erase           extractBefore   insertAfter     join            matches         plus            sort            strip           
char            double          eraseBetween    extractBetween  insertBefore    le              ne              replace         split           strlength       
compose         endsWith        extract         ge              ismissing       lower           or              replaceBetween  splitlines      upper           

methods("string")

Methods for class string:

append          contains        eq              extractAfter    gt              issorted        lt              pad             reverse         startsWith      
cellstr         count           erase           extractBefore   insertAfter     join            matches         plus            sort            strip           
char            double          eraseBetween    extractBetween  insertBefore    le              ne              replace         split           strlength       
compose         endsWith        extract         ge              ismissing       lower           or              replaceBetween  splitlines      upper           

[Walter Roberson]

This change made it to the release notes a while ago, by the way.

[Steven Lord]

There are a couple entries in the Release Notes related to the interaction between string arrays and the ismethod function, but I don't see any about string arrays and the methods function. I could have overlooked one, though, since searching the Release Notes for the word "method" or "methods" finds a lot of hits.

[Rik]

It makes sense to solve the ambiguity this way, but I still think it would merit a remark in the doc.