Python/numpy: Selecting values by multiple indices
As I mentioned in my previous post I’ve been playing around with numpy and I wanted to get the values of a collection of different indices in a 2D array.
If we had a 2D array that looked like this:
>>> x = arange(20).reshape(4,5) >>> x array([[ 0, 1, 2, 3, 4], [ 5, 6, 7, 8, 9], [10, 11, 12, 13, 14], [15, 16, 17, 18, 19]])
I knew that it was possible to retrieve the first 3 rows by using the following code:
>>> x[0:3] array([[ 0, 1, 2, 3, 4], [ 5, 6, 7, 8, 9], [10, 11, 12, 13, 14]])
What I wanted to do, however, was retrieve the 1st, 3rd and 4th rows which we can do by passing a collection to the array lookup function:
>>> x[[0,2,3]] array([[ 0, 1, 2, 3, 4], [10, 11, 12, 13, 14], [15, 16, 17, 18, 19]])
My collection of indices was actually in a tuple so I needed to use the list function to convert it to the appropriate data structure first:
>>> x[list((0,2,3))] array([[ 0, 1, 2, 3, 4], [10, 11, 12, 13, 14], [15, 16, 17, 18, 19]])
Pretty neat!
