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I'm gonna try to keep this short b/c it seems I get into trouble when I "yammer on".
In re: the non-integer values for x and y. The full explanation is mathematically forbidding but has to do with the nature of the detecting instrument: coordinate systems are actually transformed somewhat during data processing.
But to cut it short: in my first version of the program I converted these values to integers anyway (a sanctioned move - I didn't just decide to do that on my own). I muddied the issue here by posting the full precision values in this post.
In re: what I am trying to do. Essentially I am trying to find places on the detectors where 'hits' represented by the pixel values seem to "bunch up." These 'hits' represent places on the detectors where photons have struck. A number of "hits" in x or y that goes over a predetermined value _may_ indicate something that needs to be looked at more closely (e.g. by human eyes).
The first version of the program took a list of observation sessions, represented by numbers, as input. For each observation session, it did a database call to find out which of the seven detectors/CCDs were involved.
THEN, for each detector in that observation, it did a database call to pull in the data for the "hits", populating an array for the x axis and one for the y axis of the detector.
Then it iterated over those built-up arrays for x and y, kind of doing a histogram in memory (repeat for each detector, then move on to the next observation) ...
I must emphasize: this approach worked. But it's apparently inefficient, especially in terms of time (total run time: 19 minutes) spent doing db calls. So I figured out how to pull all the data in first. This takes only 2 minutes.
All the lines of the lump are like this:
$observation, $detector, $x_coord, $y_coord
Now I keep getting stuck trying to get the big lump to do what I want:
... to give me an array of the x values and an array of the y values for a SINGLE detector in a SINGLE observation. And so on, through the lump, until I am done. I need to examine the DISTRIBUTION of values in x and y axes of each detector, in each observation, individually.
Maybe I should be satisfied with my 19 minute runtime, and leave the data munging / structures alone until I am more experienced ... ? I don't know.
Do I need a data structure? I don't know that either. It feels like I do, because without one I don't know how to "address" subsets of the lump of data.
I hope that's clearer, anyway. I don't know why I am so stuck, and I am sorry I am.
In reply to Re^2: structuring data: aka walk first, grok later