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pythonnumpymultidimensional-array
earnric
earnric

Reputation: 533

python data structure with multidimensional numpy arrays

So I have some irregular, multi-dimensional data that I'd like to be able to index by the 'age' and 'Z' value.

For each 'age', and 'Z' I have an array of 100 wavelengths and assoc'd fluxes (ex data):

age    = np.array([10,20,30,40,50])
Z      = np.array([7,8])
waveln = np.array([np.array([a for a in arange(100)]) for b in arange(2*5)])
flux   = np.array([np.array([a*10 for a in arange(100)]) for b in arange(2*5)])

SO in this example, waveln[0] (an array of 100 entries) and flux[0] would get assoc'd with 

myData['age' = 10, 'Z' = 7]['waveln'] # which I want to return the waveln array

and something like 

myData['age' = 10, 'Z' = 7]['flux'] # which I want to return the flux array

how should I set this up?? The problem is, age and Z are both floats...

Thx,

Upvotes: 1

Views: 456

Answers (1)

hpaulj
hpaulj

Reputation: 231738

Do you realize the waveln is a 10x100 2d array, not an array of arrays? You could construct the same with

np.repeat(np.arange(100)[None,:],10,axis=0)

If you really want waveln to be a 1d array containing 10 arrays, you'll have to use an alternative object dtype construction.

As defined flux=waveln*10, though I suspect that is just illustrative values.

But let's define waveln so it is more interesting - so each row is different

In [983]: waveln=np.arange(10)[:,None]+np.arange(100)[None,:]

I can construct an indexing tuple with np.ix_ from your Z and age arrays:

In [984]: np.ix_(Z,age)
Out[984]: 
(array([[7],
        [8]]), array([[10, 20, 30, 40, 50]]))

In [985]: waveln[np.ix_(Z,age)]
Out[985]: 
array([[17, 27, 37, 47, 57],
       [18, 28, 38, 48, 58]])

So this has selected 2 rows, and 5 columns from that.

To do myData['age' = 10, 'Z' = 7]['waveln'], I'd create a class with a __getitem__ method. Python converts expressions in [] to a tuple which is passed to this method. But it would choke on that = syntax. You can't use keyword arguments in an indexing expression. Correct dictionary syntax is {'age':17, 'Z':7} or dict(age=16, Z=12).

Study the /numpy/lib/index_tricks.py file where ix_ is defined to get ideas on how to construct a custom class.

myData[age = 10, Z = 7, var = 'waveln') lets you use straight function definitions.

Upvotes: 2

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