| Re: structures? [message #86047 is a reply to message #86043] |
Thu, 26 September 2013 07:42   |
spluque
Messages: 33
Registered: September 2013
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On Wednesday, September 25, 2013 4:57:48 PM UTC-5, spl...@gmail.com wrote:
> On Sunday, September 8, 2013 8:37:16 AM UTC-5, Matěj Peterka wrote:
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>> Hi Seb,
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>> it seems to me that you might need some dynamic data structure, because there might be a different number of rows needed for each day. Also, a structure array is represented internally as just one "IDL Variable" and it needs to be stored in one solid block of computer memory. This could be quite resource-unfriendly if it's going to be very large (say, hundreds of MB). Especially, if you decide to enlarge the array, a new solid block of memory has to be allocated, the contents copied there and eventually the original memory can be freed.
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>> You could use pointer array instead, but I recommend using the new dynamic data types introduced in IDL 8, HASH() and LIST(). I guess they work internally through pointers, so each part of the large data structure can be at different place in the memory, which is certainly more resource-friendly. However, the way you work with HASH() or LIST() is in many aspect similar to using normal arrays, which is also quite user-friendly (unlike using pointers).
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>> Try looking to the IDL Help at these two data types and see if it suits to you.
>
> Thank you Matej, hashes did turn out to be a great option for this. Their flexibility is impressive. I am using them to create vectors corresponding to fields in a CSV file. I eventually need to write the data into a new CSV file. I see that the WRITE_CSV procedure can do this, and can take a structure as input. The toStruct method for hashes comes in handy. However, the order of the tags is completely arbitrary. Someone has made available a rather long script ( http://code.google.com/p/sdssidl/source/browse/trunk/pro/str uct/reorder_tags.pro?r=72) to re-order tags in a structure, but was wondering whether there is a simpler/better way to do this.
>
Suppose we have three vectors of data of the same length, all of which are in a hash. We want to create CSV files with these vectors, but each file would contain a subset of each vector. This is how I am doing this:
keys=['a', 'b', 'c']
n=20L
n_group=5L
ts=hash(keys, list(indgen(n), findgen(n), sindgen(n)))
FOR begi=0L, n - 1, n_group DO BEGIN
endi=(begi + n_group - 1)
ts_group=create_struct(keys[0], ts[keys[0], begi:endi])
FOREACH fld, keys[1:*] DO BEGIN
ts_group=create_struct(ts_group, keys[where(keys EQ fld)], $
ts[fld, begi:endi])
ENDFOREACH
write_csv, 'test.csv', ts_group
ENDFOR
In this example, we the full hash has three vectors, each with 20 elements, and we want to create 4 files with the same three vectors, but each containing 5 elemeents of the original. We want to keep the original order of the keys. It seems rather contrived to have two loops here. Is there a better way to accomplish this?
Thanks,
Seb
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