How do I find the index of the maximum value in a list in Perl 6?

Question

It's easy enough to find the maximum value in a list in Perl 6:

> my @list = 1,4,9,7,3;
> say @list.max;
9

But if I want to find the index of the maximum entry, there doesn't seem to be an elegant way to do this.

> say (^@list).sort({ -@list[$_] })[0];
2
> say @list.pairs.sort(*.value).tail.key;
2
> say @list.first(@list.max, :k);
2

Those all work, but they're hardly elegant, let alone efficient.

Is there a better way to do this?

It'd be nice if max had :k, :v and :kv options, like for instance first has. Of course, there might not be a unique index (for instance, in the case of (1,4,9,7,9).max, but then again, there might not be a unique value either:

> dd (1, 2.0, 2.0e0, 2).max;
2.0
> say <the quick brown fox>.max(*.chars);
quick

max already retrieves the first maximum value, so it would be perfectly reasonable to return the first index with :k (or :kv).

Answer 1

You can use

@list.maxpairs

to get a list of all pairings of indices and maximal values or

@list.pairs.max(*.value).key

to get just a single index.

As far as I can see, both maxpairs and the ability to provide a transformation to max are still undocumented.