construct a lookup hash mapping sentences to chapters

The keys of this hash turn out to be the consecutive numbers from one up to the total number of sentences. The lookup could just as well be an array.

my @chapters          = ( 15, 24, 23, 110, 30, 58, 3, 22, 79, 6 );
my $chapter = 1;
my @sentenceToChapter = (
    undef, # start counting at 1
    map { ($chapter++) x $_ } @chapters,
);

print qq{Sentence $_ is in chapter $sentenceToChapter[ $_ ]\n}
   for qw{ 250 15 16 98 };

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In reply to A lookup array would be simpler: Re^2: chapters,sentences, and arrays by Narveson
in thread chapters,sentences, and arrays by Anonymous Monk

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