In Python and Perl, index -1 counts from the end. In Raku it does not even compile.
Python and Perl count from the end of a list with negative numbers. Raku counts from the end too, but not with negative numbers.
In Python
cities = ["Riga", "Leiden", "Turin"]
print(cities[-1])
print(cities[-2])Turin Leiden
In Perl
my @cities = ("Riga", "Leiden", "Turin");
print $cities[-1], "\n";
print $cities[-2], "\n";Turin Leiden
In Raku
my @cities = 'Riga', 'Leiden', 'Turin'; say @cities[-1];
Unsupported use of a negative -1 subscript to index from the end
The program does not start at all. Raku sees -1 while it compiles the program, before the first line runs, and stops with a message that also tells you what to write instead.
If the negative number comes from a variable, Raku cannot see it in advance, so the program starts and stops when it gets there:
my @cities = 'Riga', 'Leiden', 'Turin'; my $i = -1; say @cities[$i];
Index out of range. Is: -1, should be in 0..^Inf
Why
In Python and Perl, an index that goes below zero by mistake quietly picks an element from the other end of the list. That kind of bug is hard to find, because nothing looks wrong.
Raku separates the two things. A number in [ ] always counts from the start, so a negative number is always an error. To count from the end, you say so with *, which stands for the number of elements in the array. *-1 means “the number of elements, minus one”: the index of the last element.
Say it in Raku
my @cities = 'Riga', 'Leiden', 'Turin'; say @cities[*-1]; say @cities[*-2]; say @cities[*-2 .. *-1];
Turin Leiden (Leiden Turin)
When you only want the last element, or the last few, the methods .tail and .head say it in words:
my @cities = 'Riga', 'Leiden', 'Turin'; say @cities.tail; say @cities.tail(2); say @cities.head;
Turin (Leiden Turin) Riga