Slicing

Dask Array supports most of the NumPy slicing syntax. In particular, it supports the following:

  • Slicing by integers and slices: x[0, :5]
  • Slicing by lists/arrays of integers: x[[1, 2, 4]]
  • Slicing by lists/arrays of booleans: x[[False, True, True, False, True]]
  • Slicing one Array with an Array of bools: x[x > 0]
  • Slicing one Array with a zero or one-dimensional Array of ints: a[b.argtopk(5)]

However, it does not currently support the following:

  • Slicing with lists in multiple axes: x[[1, 2, 3], [3, 2, 1]]

    This is straightforward to add though. If you have a use case then raise an issue. Also, users interested in this should take a look at vindex.

  • Slicing one Array with a multi-dimensional Array of ints

Efficiency

The normal Dask schedulers are smart enough to compute only those blocks that are necessary to achieve the desired slicing. Hence, large operations may be cheap if only a small output is desired.

In the example below, we create a Dask array with a trillion elements with million element sized blocks. We then operate on the entire array and finally slice out only a portion of the output:

>>> # Trillion element array of ones, in 1000 by 1000 blocks
>>> x = da.ones((1000000, 1000000), chunks=(1000, 1000))

>>> da.exp(x)[:1500, :1500]
...

This only needs to compute the top-left four blocks to achieve the result. We are slightly wasteful on those blocks where we need only partial results. Moreover, we are also a bit wasteful in that we still need to manipulate the Dask graph with a million or so tasks in it. This can cause an interactive overhead of a second or two.

But generally, slicing works well.