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Re: [ddlm-group] LOOP versus LIST

It makes sense to distinguish a scalar from a loop of size=1 because it 
is easier in a programming context for loop items to always be stored in 
an array. However, handling data in a programming context requires a 
DDL, which can define how the data is stored. If the distinction between 
a scalar and single-row loop is not made at the CIF syntax level, the 
DDL should not be able to dictate the use of loops. For example, CIF 
defines ordering of items as not significant, and DDL cannot override 
this. It makes sense for a DDL to suggest a preferred ordering, but it 
is only a suggestion, unless CIF format rules change.

If there is a desire for DDL to mandate loop and non-loop items, then 
CIF2 should make an explicit distinction.

To avoid similar conflicts with list items, CIF2 should state that loop 
and list items are not interchangeable, so that the following two pairs 
are not equivalent:



item.name [data1 data2 data3]


item.name [data1]

item.name data1

Of course, these are implementation details that can be worked out after 
the lexing syntax is finalized.


Herbert J. Bernstein wrote:
> A list with lists nested to arbitrary depth can be a single data 
> value either in a loop or just for a single tag.
> DDL2 make no distinction between a one-element loop and the same unlooped
> tag with the same value.  DDL1 (see _list) and DDLm (see 
> _definition.class) try to make a distinctions among things that are 
> and are not permitted to be looped.
> I do not understand why it is desirable to make such a distinction for
> a single row table, following the DDL2 approach of allowing it
> to be handled as either
>    _xxx.aaa data1
>    _xxx.bbb data2
>    _xxx.ccc data3
> or
> loop_
>    _xxx.aaa
>    _xxx.bbb
>    _xxx.ccc
>    data1  data2  data3
> seem harmless to me, but DDL1 and DDLm make the distinction and a proper
> parser should note violations of what was specified for the category.
> An index key is not a name, but a string, so I think it reasonable to
> accept the empty string as a table index value.
> Case sensitivity is an interesting question.  I would prefer case sensitive
> table indices, but I suppose that matter should be discussed.
> At 12:41 PM -0500 1/5/10, Joe Krahn wrote:
>> I assume that a list of items defined via a loop is distinct from a list
>> of items defined by a list. Is that correct?
>> Likewise, is a list of one item distinct from a scalar value?
>> Currently, CIF files don't differentiate between a one-element loop and
>> a scalar. For example, RCSB components.cif does not use loops for atom
>> data when there is only one atom. Is this stated anywhere?
>> Also, is an empty string a valid TABLE index? Other CIF names require at
>> least one character, but my understanding is that a TABLE index is any
>> valid string, which includes an empty string. Strings are also
>> case-sensitive, so I assume that TABLE indices are also case-sensitive.
>> Thanks,
>> Joe Krahn
>> _______________________________________________
>> ddlm-group mailing list
>> ddlm-group@iucr.org
>> http://scripts.iucr.org/mailman/listinfo/ddlm-group

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