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]]

It does not currently support the following:

  • Slicing one dask.array with another x[x > 0]
  • Slicing with lists in multiple axes x[[1, 2, 3], [3, 2, 1]]

Both of these are straightforward to add though. If you have a use case then raise an issue.

Efficiency

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

In the example below we create a trillion element Dask array in million element 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 still slightly wasteful on those blocks where we need only partial results. 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.