| Re: basic array-structure understanding question [message #63674] |
Wed, 19 November 2008 07:49  |
julia.walterspiel
Messages: 35
Registered: July 2008
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Member |
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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.
juls
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| Re: basic array-structure understanding question [message #63675 is a reply to message #63674] |
Wed, 19 November 2008 07:41  |
m_schellens
Messages: 31
Registered: February 2005
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Member |
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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
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| 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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| Re: basic array-structure understanding question [message #63813 is a reply to message #63674] |
Wed, 19 November 2008 08:47 |
Jean H.
Messages: 472
Registered: July 2006
|
Senior Member |
|
|
> 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.
> juls
As Marc pointed it out, the link is very easy to conceptualize...
select the index on array A and select the corresponding data on array B!
A = [0,1,1,0,1] ;a mask
B = [1,2,3,4,5] ;some data
goodIndex = where(A eq 1)
print, B[goodIndex]
==> 2,3,5
Now, be careful when you do a subscription using where. Your code will
miserably fail in case there is no valid subscript!
ex:
A = [0,1,1,0,1] ;a mask
B = [1,2,3,4,5] ;some data
badIndex = where(A eq 3)
print, badIndex
==> -1
print, B[badIndex]
==> ERROR!
so, unless you are 100% sure that there is ALWAYS a valid index, you
should first search for the index, make sure there is at least 1 valid
one, and then do the subscription!
Jean
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