Perl makes this easy with its autovivification feature: 
my $DATA_X;
my $DATA_Y;
while (<>) {
  my ($obs, $det, $x, $y) = split(' ', $_);
  push(@{$DATA_X->{$obs}->{$det}}, $x);
  push(@{$DATA_Y->{$obs}->{$det}}, $y);
}

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This uses two HoHoA - one for the X coordinate data and another for the Y coordinate data. Or you could store the coordinates in a data structure: 
my $DATA;
while (<>) {
  my ($obs, $det, $x, $y) = split(' ', $_);
  push(@{$DATA->{$obs}->{$det}}, {x => $x, y => $y});
}

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And here's how to access the data (using the second approach): 
print $DATA->{21}->{'DET-2'}->[0]->{x}; # prints 896.657564735788
print $DATA->{47}->{'DET-7'}->[1]->{y}; # prints 519.649226713148

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Whether or not this is the best data structure for your data depends on what you want to do with it.

In reply to Re: structuring data: aka walk first, grok later by pc88mxer
in thread structuring data: aka walk first, grok later by chexmix

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