Julia Inconsistent bounds check for slices

Question

I was writing some code that makes use of array views and slices, but encountered some inconsistencies.

Why does the following not cause an exception:

a = [1,2]
@show a[3:end]
@show a[4:end]

The above all return empty arrays as expected

But this causes a BoundsError

a = [1,2]
@show a[2:3]

Why is the first index of the slice allowed to be larger than the size of the array itself, but Julia seems to have a problem with the last index being larger than the size?

Julia version: 1.3.1

David Varela David Varela · Accepted Answer · 2020-01-07T02:52:45

x[c:end] is syntax for getindex(x, UnitRange(c, lastindex(x))).

Any range a:b with a > b is empty. Indexing an array with an empty range will result in an empty array by definition of getindex.

You index an array with an empty range in your first set of examples. In your second set of examples, you index with an out-of-bounds range, which errors as expected.

Julia Inconsistent bounds check for slices

1 Answers