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Table and related classes for Octave

Tablicious Design and Justifications

This document describes the design choices I made in Tablicious, and some justifications for them and why I think they should be allowed into core Octave, or at least an Octave Forge package.



Copy constructors

Each data object has a copy constructor calling form. The Matlab trick of doing this:

function this = Foo(x)
  if isa (x, 'Foo')
    this = x;

doesn’t seem to be supported in Octave; or at least it seemed to lead to some instability when I tried it. Besides, it introduces the problem that the output of the constructor might be a subclass instead of an object of the exact type of the constructor.

So I wrote out copy-constructor forms for most of the classes. The rest should be filled in, too; their absence is a bug.

Planar-organized classes

The data classes like datetime are implemented as planar-organized classes for efficiency. This lets them represent their arrays as arrays of primitives on which vectorized operations can be performed.

“Planar-organized” is opposed to “struct-organized”, which is the default organization for Octave/Matlab classes. In struct-organized classes, arrays are represented by arrays of struct-y objects, each of which contains a single element/record’s data in its fields.

Planar-organized classes are the only way to go fast with objects in Octave/Matlab.

For example, an M-by-N datetime array is a single object wrapper around an M-by-N double array in the dnums property. Instead of it being an M-by-N array of blessed structs, each of which holds a scalar dnums.

The downside is that implementing a planar-organized class requires you to write a lot of boilerplate code. I went ahead and did that, though, because the performance gain is worth it.


The +internal namespace

Any namespace with +internal as one of its components is considered to be an internal, private, implementation detail of a library itself. The language doesn’t enforce any access rules on this; it’s just a naming convention. But I think it’s become a reasonably well-established convention in other languages: Java supports it, and I think Matlab does too. I think Octave would do well to adopt it.

Tablicious’ doco generation tools support +internal by excluding any +internal namespace from the generation of the API Reference section of the manual. So code in there is hidden from the user view at the documentation level.

The +octave/+examples namespace

I’m sticking example code in the octave.examples namespace. This makes it readily available to anyone with the package loaded (as opposed to requiring them to cd into a specific examples/ directory or manually add it to the path), but keeps them out of the global namespace and the Octave and Tablicious public APIs.

The +examples namespace is a suitable place to throw code that might be useful for users, but we don’t want in the actual package API for some reason. This includes:

The stuff in +examples can be lower quality than main code, in the sense that:


datetime constructor for numeric inputs

The single-arg datetime(x) constructor, when called on numeric inputs, treats them as a datevec instead of as datenums. I think that’s a design mistake; datenums are closer to the “canonical” representation for dates in pre-datetime Matlab/Octave. And datenums seem to me to be what you‘d want the implicit conversion behavior to do in the case of mixed-type arithmetic, concatenation, or other combining operations.

But datevecs is what Matlab picked, so we’re stuck with that, for compatibility.

I’m thinking about adding a global todatetime(x) function that will treat its numeric inputs as datenums, for convenience.

Example Datasets


The octave.datasets class provides functions for listing and loading all the example datasets this library provides.

The octave.dataset class provides a list of all the individual examples, so you can use tab-completion on them (or at least you’ll be able to, once Octave supports tab completion for static methods on classes inside namespaces). This class is also how the Texinfo doco for the datasets finds its way into the generated user manual.

This is done as two separate classes to keep the “list of data sets” separate from the “API for accessing datasets”. I think that makes it easier for users to find what they’re looking for; otherwise the generic methods in octave.datasets would get lost in the big list of individual example datasets.

You run octave.internal.generate_datasets_list to generate the octave.dataset file from the individual example classes. This only needs to be done at code authoring time, not by the user.


The Example code sections in the description.texi for the example datasets are stored separately as scripts so that you can view them as M-code in an editor (with syntax highlighting, indentation assistance, and so on) and conveniently run them to test them (instead of having to copy-and-paste them from a view of the helptext, which would require you to go through the doco building cycle each time you made a change and wanted to run the code to test it).

I’m thinking about breaking the load step out into separate load_raw and munge steps, to make it easier for users to view the example dataset code and see how the data-packaging classes like table are used.


Why does table display differently than in Matlab?

In Matlab, doing disp() on a table will result in its printing the entire table contents in tabular form, as it does with primitives and cells. I think that, for any more than a few rows, this spams the console, and is not useful output. In interactive use, it’s much nicer to get a short summary of the table as its default display: tables have enough structure that a short summary is still useful, and it won’t spam your scrollback.

I’ve spent years working with table-style objects that operate each way, and found the summary style to be much more usable.

I think this is fine because disp() is mostly used for interactive use, and is not frequently used to produce actual program output, so this shouldn’t be a real compatibility issue.

Justify all these new top-level functions

Tablicious defines several new top-level (global, not in namespaces or private/ directories) functions. Many of them are ports of Matlab functions, which should be self-justifying. But some of them are not. Here are reasons for the new ones I made up.

I call the top-level namespace that’s not in any explicit namespace the “global” namespace. I don’t know if that’s actually the correct term.

Justify colvecfun

Could easily go in the +octave namespace; there’s no real reason for it to be global, aside from easy discovery and analogy with the global cellfun, arrayfun, and so on.

Justify dispstr and dispstrs

Many modern programming languages provide a generic, polymorphic way of converting an arbitrary input to a string for user/developer presentation. E.g. Java has toString, Ruby has to_str, and Python has str and repr. Octave could really use one of those.

Needs to be global so user-defined classes can override it with object methods.

Because of Octave’s pervasive array nature, we need both a variant for “one string describing the entire array” (dispstr) and “N strings describing each of the array’s elements” (dispstrs).

This dispstr stuff is all based on the Dispstr API I created for use with Matlab.

Justify eqn

This is a straightforward extension if isequaln behavior to the element-wise case.

Needs to be global so user-defined classes can override it with object methods.

Justify isnanny

For some reason, Matlab chose to name the NaN values for datetime objects “NaT” instead, and define a new isnat() function to work with them. So you can’t write generic polymorphic code that works on both numerics and datetimes when testing for NaN-ness. Sigh.

The new ismissing() function doesn’t fit the bill, because it’s too broad: it also considers empty strings and blank characters to be missing, and you may not want that broadness in a NaN-ness test. isnanny() smooths over the NaN/NaT difference without becoming as broad as ismissing().

UPDATE 2021-03-14: I’ve changed my mind. I now think that ismissing probably is a fine generalization of isnan/isnat, and we should just use that, and isnanny should go away.

I made this a global function so that user-defined classes could override it with object methods. Without methods(klass) working for classdef objects, it’d be hard to detect whether a user-defined object supports isnan() and/or isnat(), so classes might want to override isnanny itself. This is not ideal; I’m still thinking about whether this is the best approach.

Justify these extra mustBeXxx functions

Many of the mustBeXxx() functions are just ports of Matlab’s existing validator functions. But I added a few new ones:

These all see a lot of use in my experience, so they’d be useful to provide. And I think their semantics and implementation are obvious enough that we don’t need to be trepidatious about adding them to the public API.

Justify pp

If you end up working with tables or other complex structures that support prettyprint, you end up calling prettyprint a lot, so it’s nice to have a shortcut for it, especially one with the char-interpreting command style. Needs to be have a super short name so users can quickly bang it out at the command line. Namespace would make this inconvenient.

However, pp is only ever called directly by end users; it doesn’t need to be available to other code in the library. So this could just be provided as an example function, and require users to copy it into their default path (like at ~/Documents/octave) to have it take effect.

Justify proxykeysForMatrixes

This one is ugly to have hanging around in the global namespace, but it’s necessary in order to allow user-defined classes to override it with object methods.

Justify scalarexpand

This one is really useful. I use it in the input handling for a lot of functions, and I think it should exist.

This one could probably go in the +octave namespace, though. It does not need to be overridden by anything.

Justify size2str

I use this one a lot in generating error and progress messages. It gets used often enough that I think it could belong in core Octave. It’s trivial to implement. But it’s useful enough that I think it deserves a home in a library or core Octave. This would make it both convenient for users to use, and easier for code readers to recognize and understand if there were a single core, conventional name for this operation.

Its behavior is simple enough that its interface is unlikely to change. Though the format could change: maybe you’d want to do “AxBxC” instead of “A-by-B-by-C”, or something like that. In which case that’s another reason to prefer an established, central function to do this.

Justify tableOuterFillValue

Having this be global is necessary in order to allow user-defined classes to override it with object methods.

This ugliness could probably be hacked around by not having a global function, and just having table.outerjoin test its variable values to see if they are objects which define a tableOuterFillValue, and only call that if they are defined. But this will be hard because Octave currently doesn’t support calling methods on user-defined classes.