Perl Array Dereference Problem with DBI::fetchall_arrayref - perl

Question

I'm a Perl newbie and am having issues with dereferencing an array that is a result of fetchall_arrayref in the DBI module:
my $sql = "SELECT DISTINCT home_room FROM $classlist";
my $sth = $dbh->prepare($sql);
$sth->execute;
my $teachers = $sth->fetchall_arrayref;
foreach my $teacher (#{$teachers}) {
print $teacher;
}
Running this will print the reference instead of the values in the array.
However, when I run:
my $arrref = [1,2,4,5];
foreach (#{$arrref}) {
print "$_\n";
}
I get the values of the array.
What am I doing wrong? Thank you for your help!
Jeff

Answer 1

From the doc
The fetchall_arrayref method can be
used to fetch all the data to be
returned from a prepared and executed
statement handle. It returns a
reference to an array that contains
one reference per row.
So in your example, $teacher is an ARRAY ref.
So you will need to loop through this array ref
foreach my $teacher (#{$teachers}) {
foreach my $titem (#$teacher) {
print $titem;
}
}

Answer 2

if you want to extract only the teacher column, you want to use:
my #teachers = #{$dbh->selectcol_arrayref($sql)};

Answer 3

fetchall_arrayref fetches all the results of the query, so what you're actually getting back is a reference to an array of arrays. Each row returned will be an arrayref of the columns. Since your query has only one column, you can say:
my $teachers = $sth->fetchall_arrayref;
foreach my $teacher (#{$teachers}) {
print $teacher->[0];
}
to get what you want.
See more:
Arrays of arrays in Perl.

Answer 4

You have a reference to an array of rows. Each row is a reference to an array of fields.
foreach my $teacher_row (#$teachers) {
my ($home_room) = #$teacher_row;
print $home_room;
}
You would have seen the difference with Data::Dumper.
use Data::Dumper;
print(Dumper($teachers));
print(Dumper($arrref));

Answer 5

$sth->fetchall_arrayref returns a reference to an array that contains one reference per row!
Take a look at DBI docs here.

Answer 6

Per the documentation of DBI's fetchall_arrayref():
The fetchall_arrayref method can be
used to fetch all the data to be
returned from a prepared and executed
statement handle. It returns a
reference to an array that contains
one reference per row.
You're one level of indirection away:
my $sql = "SELECT DISTINCT home_room FROM $classlist";
my $sth = $dbh->prepare($sql);
$sth->execute;
my $teachers = $sth->fetchall_arrayref;
foreach my $teacher (#{$teachers}) {
local $" = ', ';
print "#{$teacher}\n";
}
The data structure might be a little hard to visualize sometimes. When that happens I resort to Data::Dumper so that I can insert lines like this:
print Dumper $teacher;
I've found that sometimes by dumping the datastructure I get an instant map to use as a reference-point when creating code to manipulate the structure. I recently worked through a real nightmare of a structure just by using Dumper once in awhile to straighten my head out.

Answer 7

You can use map to dereference the returned structure:
#teachers = map { #$_->[0] } #$teachers;
Now you have a simple array of teachers.