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Fix partitions(0)
, closes #143
#192
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Could we not instead use
xs = Int[]
and doisempty(xs)
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I don't think so, and that's exactly the point.
collect(partitions(0))
must returnVector{Int}[[]]
.This means that on the first call
iterate(partitions(0))
must return(Int[], Int[])
and on the second calliterate(partitions(0), Int[])
must returnnothing
. Therefore the default value ofxs
(which is used in the first call) cannot beInt[]
(which is used in the second call), because the behavior must be different.Of course we could swap the role of
nothing
andInt[]
and haveiterate(partitions(0))
return(Int[], nothing)
anditerate(partitions(0), nothing)
returnnothing
, but this doesn't really work around the necessity of having an extra sentinel value which is different fromInt[]
.Leaving the code as it currently is preserves the "symmetry" that
iterate
always returns(xs, xs)
. Swappingnothing
andInt[]
instead has the benefit that the sentinel valuenothing
is used only forpartitions(0)
and notpartitions(n)
withn>0
.Uh oh!
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Just for reference, the "swapped" code would look something like this:
In this code it is maybe clearer that the
nothing
in the two lines marked with# sentinel value
can be replaced by anything else we like as long as it is different fromInt[]
. For instance, we could use[0]
, which makes the code more type stable. I tried benchmarking this and it is slightly faster forcollect(permutations(5))
(thanks to type stability), at the expenses ofcollect(permutations(0))
which is slower becauseInt[] === nothing
is faster thanInt[] == [0]
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Thanks for the clear explanation. I would be more fan of the latter approach. @inkydragon What do you think?