Question

Say I have the following setup:

x=np.array(range(0,100))
y=x**0.5
z=x**2

How could I get all the values of y between 4 and 8, and create a new array that had a row for those y values, but also a row for the corresponding z values?

i.e. 25 in x has a y value of 5 which is between 4 and 8, I’d like this value, but I’d also like the z value of 625 in the row below.

The math equations are just an example, the forumals could be anything including bessel functions.

Thanks

Answer 1

>>> np.vstack((y, z))[:,(y >= 4) & (y <= 8)]
array([[  4.00000000e+00,   4.12310563e+00,   4.24264069e+00,
          4.35889894e+00,   4.47213595e+00,   4.58257569e+00,
          4.69041576e+00,   4.79583152e+00,   4.89897949e+00,
          ...
          7.81024968e+00,   7.87400787e+00,   7.93725393e+00,
          8.00000000e+00],
       [  2.56000000e+02,   2.89000000e+02,   3.24000000e+02,
          3.61000000e+02,   4.00000000e+02,   4.41000000e+02,
          ...
          3.72100000e+03,   3.84400000e+03,   3.96900000e+03,
          4.09600000e+03]])

Here, vstack() creates a matrix with two rows, one containing y and one containing z. The [...] then eliminates those columns that don’t satisfy the criterion.