| Re: basic array-structure understanding question [message #63788 is a reply to message #63674] |
Thu, 20 November 2008 00:42   |
R.Bauer
Messages: 1424
Registered: November 1998
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Senior Member |
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julia.walterspiel@gmail.com schrieb:
> On 19 Nov., 16:41, "m_schell...@hotmail.com" <mschell...@gmail.com>
> wrote:
>> I am sure David has an appropiate link for this issue,
>> but for now:
>>
>> On 19 Nov., 15:21, julia.waltersp...@gmail.com wrote:
>>
>>
>>
>>
>>
>>> hi everybody
>>> This is what does not get into my head:
>>> I have one array containing my data.
>>> I have a second array containing the times when the data was
>>> collected.
>>> They have the same lenght.
>>> I put them into one structure because we all like structures and I
>>> learned about their advantages.
>>> now:
>>> IN SHORT are those arrays linked somehow? in other words: when working
>>> with a structure, does IDL know which value matches the corresponding
>>> date, given the data and the date array have the same length. Are
>>> those arrays somehow connected or are they completely independent from
>>> each other?
>> As you described, they are semantically linked.
>> I. e. structure.data[ i] corresponds to structure.time[ i] for all i.
>> For IDL there is no such association.
>>
>>> IN LONG: if I want to get e.g. all the data from all Januarys from
>>> 2000 to 2007, can I do it somehow like
>>> all_jan = structure.data (where(structure.time EQ 200?01??????) )..
>>> then it would automatically "select" only the right values. (and then
>>> I would have to make a separate array of the corresponding dates, make
>>> those two new arrays a structure so that I can plot data vs. date all
>>> in one?)
>>> or do I have to do it via some indexing, like:
>>> all_jan = where (structure.time EQ 200?01??????) )
>>> and then apply the index to my array of data?
>> Both ways are fine. In the first case the index array (output of
>> WHERE) is a temporary variable,
>> in the second case you make it a (normal) variable named all_jan.
>>
>> In the first case your selected data is in all_jan.
>> In the second case the indices are in all_jan and you have to create
>> the selected data by using
>> structure.data[ all_jan]
>>
>> It is quite simple, no magic.
>> What might seem like magic is the
>> EQ (or any other logical) operator working on arrays.
>> WHERE just returns the inidices where the elements of the
>> expression (structure.time EQ 200?01??????) (which is a byte array)
>> are 'true'
>> (depends on the data type. For integers including byte 'true' is NE
>> 0).
>> (you are aware that 'structure.time EQ 200?01??????' is pseudo code)
>>
>> To understand this better, do:
>>
>> IDL> eqArr = structure.time EQ 200?01??????
>> IDL> whereArr = where(eqArr)
>> IDL> print,eqArr
>> IDL> print,whereArr
>>
>> You should however use the second case and check the result for -1
>> (IF all_jan[0] EQ -1 THEN ...)
>> as this is the result of WHERE if no elements are 'true'.
>>
>> HTH,
>> Marc- Zitierten Text ausblenden -
>>
>> - Zitierten Text anzeigen -
>
> thanks Marc! the knot is coming undone..
> it's a pity that IDL does not associate the two arrays. But I guess
> nobody's perfect :) (or is there a way to TELL IDL to associate them?)
> cheers for the explanation. I'll go hunt around for some more of
> David's articles on this topic.
Hi Julia
depends on what you do.
if you create the array on the structure then the two tags are associated.
e.g.
test = replicate(create_struct('time', 1.d, 'H2O', 10),10)
help, test
TEST STRUCT = -> <Anonymous> Array[10]
if you want to reform your struct to something like this you may be
interested in.
http://www.fz-juelich.de/icg/icg-1/idl_icglib/idl_source/idl _html/dbase/reform_struct_dbase.pro.html
eamples:
http://www.fz-juelich.de/icg/icg-1/idl_icglib/idl_source/idl _html/examples/reform_struct_example.pro.html
cheers
Reimar
> juls
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