package GradeSchool;
use strict;
use warnings;
use Exporter qw<import>;
our @EXPORT_OK = qw<roster>;

sub roster {
  my ( $students, $grade ) = @_;
  my @roster;
  my %student_by_grade;
  foreach my $student (@{ $students }) {
      push @{ $student_by_grade{$student->[1]} } => $student->[0];
  }
  foreach my $student_grade (sort keys %student_by_grade) {
      if (not defined $grade or $student_grade == $grade) {
        push @roster => sort @{ $student_by_grade{$student_grade} };
      }
  }
  return \@roster;
}

1;