Re: struct arrays and cs-lists

[Benjamin Abbott]

On Nov 13, 2012, at 10:12 AM, Ben Abbott <address@hidden> wrote:

> 
> On Nov 13, 2012, at 9:42 AM, Jordi Gutiérrez Hermoso wrote:
> 
>> On 13 November 2012 09:32, Ben Abbott <address@hidden> wrote:
>>> 
>>> On Nov 13, 2012, at 8:06 AM, Jordi Gutiérrez Hermoso wrote:
>>> 
>>>> On 13 November 2012 08:05, Jordi Gutiérrez Hermoso <address@hidden> wrote:
>>>>> On 13 November 2012 00:02, John W. Eaton <address@hidden> wrote:
>>>>>> On 12-Nov-2012, Jordi Gutiérrez Hermoso wrote:
>>>>>> 
>>>>>> | I don't really understand why if s is a struct array with field
>>>>>> | foo, then "s.foo" is a cs-list. What's the purpose of this? How
>>>>>> | can you get something like a cell array instead without
>>>>>> | something as roundabout as resize(size(s), {s.foo})? Does it
>>>>>> | have to be this way, is there something else we can do instead?
>>>>>> 
>>>>>> I don't see that we can do anything else if we want to preserve
>>>>>> Matlab compatibility.
>>>>> 
>>>>> Sure, we can't change it, but how is it supposed to be used? Why
>>>>> is it like this at all? There must *some* method to the Madlab.
>>>> 
>>>> By way of contrast, for cell arrays you can avoid cs-lists and get
>>>> subcells if you index with () instead of {}. Why is there nothing
>>>> equivalent for struct arrays?
>>> 
>>> For cells indexing with () preserves the cell class just as it does
>>> for any data type (including structures). That's equivalent correct?
>>> ... So, its not clear to me what you want to do.
>> 
>> Consider the following:
>> 
>>   s = struct("foo", {1, "lol"}, "bar", {2, {2}})
>> 
>> then s.foo is a cs-list with two different types, as is s.bar.
>> 
>>> Are you looking to create a cell-array from a structure while
>>> maintaining the original cell-array's size, or something else.
>> 
>> Precisely this, I want the equivalent of
>> 
>>   reshape({s.foo}, size(s))
>> 
>> which isn't such a mouthful. I would expect something like s("foo") to
>> do this, indexing with () and strings, or s.("foo"), but the latter
>> syntax is already a synonym for s.foo.
>> 
>> The point is that I think struct arrays are best interpreted as cell
>> arrays where one dimension is indexed with strings instead of
>> positive integers. Indeed, the struct2cell and cell2struct functions
>> perform the conversion this way, by changing the way that one
>> dimension is indexed, whether with strings or positive integers. The
>> problem is that there seems to be no way to index struct arrays along
>> their string dimension in the same way that you can index cell arrays.
>> 
>> So, if the language doesn't have what I want, then what's the point of
>> s.foo returning cs-lists?
> 
> I'm pretty sure the cs-list behavior for structures predates the introduction 
> of cell-arrays in Matlab 5.   Meaning I think the language evolved as a 
> reaction to a need, rather than a plan.
> 
>> How is this supposed to be used? It seems to
>> me like the most natural use of struct arrays is to do something like
>> a CSV file where the column headers turn into struct array field
>> names, but if indexing by those field names gives you cs-lists, then a
>> weird indirection appears in the middle.
>> 
>> So how are struct arrays meant to be used?
> 
> Would supporting an empty index to produce an equivalent cell array be useful 
> ...
> 
>    cellarray = s.()
> 
> Where the leading dimensions of the cellarray would be equal size(foo) and an 
> extra dimension would be added for the fields?
> 
> Ben

Maybe s.(:) is a better choice

Ben