Choose m evenly spaced elements from a sequence of length n

Question

I have a vector/array of n elements. I want to choose m elements.

The choices must be fair / deterministic -- equally many from each subsection.

With m=10, n

Answer 1

This will always select the first and last elements:

which_idxs = lambda m, n: np.rint( np.linspace( 1, n, min(m,n) ) - 1 ).astype(int)

evenly_spaced = np.array( your_list )[which_idxs(m,n)]

This will only select a maximum of n elements, in case m is higher than n.

If you truly want it equally spread throughout the array, even at the ends, then it would be this instead:

which_idxs = lambda m, n: [idx for idx in np.rint( np.linspace( 1-n/(2*min(m,n)), n+n/(2*min(m,n)), min(m,n)+2 ) - 1 ).astype(int) if idx in range(n)]

evenly_spaced = np.array( your_list )[which_idxs(m,n)]

Which gives you something like this:

>>> np.array( [1, 2, 3, 'a', 'b', 'c'] )[which_idxs(m,n)]
Out: array(['2', 'b'])