http://www.perlmonks.org?node_id=80910

marvell has asked for the wisdom of the Perl Monks concerning the following question:

As an excersise in learning "tie", I knocked up this series generator. I know this is probably not the ideal solution to the problem of generating series, but it was "tie" I was interested in.

Series.pm

```package Series;

use strict;

sub TIEARRAY {

my \$pkg = shift;

my \$rsub = shift; # sub for formula

my @vals = @_;

bless { 'values' => [ @vals ],
'next' => \$rsub
}, \$pkg;

}

sub FETCHSIZE {

# rather meaningless in context, but gets called a lot
# presume it's to check that any values at all exist

return 1; # for now
}

sub FETCH {

die "can't get subscripted value of series";

}

sub SHIFT {

my (\$obj) = @_;

my \$ra = \$obj->{'values'};

push(@\$ra, \$obj->{'next'}->(@\$ra));

return shift @\$ra;
}

1;

series.pl

```#!/usr/bin/perl -w

use strict;
use Series;

print "Arithmetic Series: 1,4,7, ... N_{i-1}+3 ...\n";

tie my @arith, 'Series', sub { return 3 + shift }, 1;

print shift(@arith)." " for (1..15);
print "\n\n";

print "Geometric Series: 1,2,4, ... N_{i-1}*2 ...\n";

tie my @geo, 'Series', sub { return 2 * shift }, 1;

print shift(@geo)." " for (1..15);
print "\n\n";

print "Combination Series: 2,7,17, ... N_{i-1}*2+3 ...\n";

tie my @combo, 'Series', sub { return 3 + 2 * shift }, 2;

print shift(@combo)." " for (1..15);
print "\n\n";

print "Fibonacci Numbers: 0,1,1,2,3, ... N_{i-1}+N_{i-2} ...\n";

tie my @fibo, 'Series', sub { my @n = @_; return \$n[0]+\$n[1] }, 0,1;

print shift(@fibo)." " for (1..15);
print "\n";

And here is the output:

```Arithmetic Series: 1,4,7, ... N_{i-1}+3 ...
1 4 7 10 13 16 19 22 25 28 31 34 37 40 43

Geometric Series: 1,2,4, ... N_{i-1}*2 ...
1 2 4 8 16 32 64 128 256 512 1024 2048 4096 8192 16384

Combination Series: 2,7,17, ... N_{i-1}*2+3 ...
2 7 17 37 77 157 317 637 1277 2557 5117 10237 20477 40957 81917

Fibonacci Numbers: 0,1,1,2,3, ... N_{i-1}+N_{i-2} ...
0 1 1 2 3 5 8 13 21 34 55 89 144 233 377