Sequences, hyperslabs and selections

[James Gallagher]

Joe,

Thanks!

James

On May 29, 2006, at 10:09 PM, Joe Sirott wrote:

> Hi Roberto,
>
> The constraint takes precedence over the hyperslab operator. This  
> topic came up a few months ago in a private e-mail exchange between  
> James Gallagher, Ethan Davis and me a few months ago; I've attached  
> James' explanation below:
>
> >
> >>
> >> Another spec issue -- what's the precedence of the array operator?
> >> For instance, if I request:
> >>
> >> location[0:9].latitude?latitude>20
> >>
> >> do I get the first 10 values where the latitude > 20? Or just the
> >> subset of latitude values in the first 10 values that are > 20?
> >
>
> The current behavior has the CE's evaluation broken into two phases:
> First the CE is parsed and, as part of the parse, the projection
> information is recorded. Then, as the data are sent, the selection
> clauses are evaluated. In the current servers (and given NASA ESE SPG
> 'rules' that's what I must document in the spec) this means that a CE
> like
>
> values[0:99]&values.v1>200
>
> will return the first 100 values where v1 is greater than 200, not
> those values where v1 is > 200 found in the first 100 values.
>
> Programatically what happens is that the rows are read from the
> sequence starting at zero and proceeding to 99. A row is counted as
> having been read if the relational constraint (selection) holds true.
>
> Suppose these values:
>
> v1
> 210
> 199
> 201
> 202
> 203
> 198
> 204
>
> CE: values[0:2]&values.v1>200
>
> 210
> 201
> 202
>
> CE: values[0:2]&values.v1<200
>
> 199
> 198
>
> Note that there are not three values that satisfy the second CE.
>
> Roberto De Almeida wrote:
>> Hi, all. A quick question:
>>
>> If a client applies both a hyperslab and a selection on a sequence,
>> what is the order of precedence? Eg, if I have the following dataset:
>>
>>  Dataset {
>>    Sequence {
>>      Float64 lat;
>>      Float46 lon;
>>    } coastline;
>>  } coastline.db;
>>
>> And a client requests the following URL:
>>
>>  http://example.com/coastline.db.dods?coastline[0:1:9] 
>> &coastline.lat>80
>>
>> Am I right to assume that the hyperslab should be applied *after*
>> (return the first 10 items were latitude is greater than 80) the
>> selection? This is not clear on the DAP 2 draft.
>>
>> Thanks,
>> Roberto
>

--
James Gallagher                jgallagher at opendap.org
OPeNDAP, Inc                   406.723.8663

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