Interpolating a 2D data grid in python

Question

I have a 2D grid with radioactive beta-decay rates. Each vale corresponds to a rate on a specific pair of temperature and density (both on logarithmic scale). What I would like to do, is when I have a temperature and density data pair (after getting their logarithms), to find the matching values in the table. I tried using the scipy interpolate interpn function , but I got a little confused, I would be grateful for the help.

What I have so far:

pointsx = np.array([7+0.2*i for i in range(0,16)]) #temperature range
pointsy = np.array([i for i in range(0,11) ]) #rho_el range
data = numpy.loadtxt(filename) #getting data from file
logT = np.log10(T) #wanted temperature logarithmic
logrho = np.log10(rho) #wanted rho logarithmic

The interpn function has the following arguments: points, values, xi, method='linear', bounds_error=True, fill_value=nan . I figure that the points will be the pointsx and pointsy I have, the data is quite obvious, and xi will be the (T,rho) I'm looking for. But I'm not sure, what dimensions they should have? The points is the same size, as the data ? So I have to make an array of the corresponding pairs of T and rho , which will be the points part, and then have a (T, rho) pair as xi ?

Answer 1

When you aren't certain about how a function works, it's always a good idea to open up a REPL and test it yourself. In this case, the function works exactly as expected, given your understanding of the documentation.

>>> points = [[1, 2, 3, 4], [1, 2, 3, 4]]  # Input values for each grid dimension
>>> values = [[1, 2, 3, 4], [2, 3, 4, 5], [3, 4, 5, 6], [4, 5, 6, 7]]  # The grid itself
>>> xi = (1, 1.5)
>>> scipy.interpolate.interpn(points, values, xi)
array([ 1.5])
>>> xi = [[1, 1.5], [2, 1.5], [2, 2.5], [3, 2.5], [3, 3.5], [4, 3.5]]
>>> scipy.interpolate.interpn(points, values, xi)
array([ 1.5,  2.5,  3.5,  4.5,  5.5,  6.5])

The only thing you missed was that points is supposed to be a tuple. But as you can see from the above, it works even if points ins't a tuple.