head

    head( Int $n=6 )

If $n ≥ 0, returns a new "Data::Frame" with the first $n rows of the "Data::Frame".

If $n < 0, returns a new "Data::Frame" with all but the last -$n rows of the "Data::Frame".

See also: R's head <https://stat.ethz.ch/R-manual/R-devel/library/utils/html/head.html> function.

tail

    tail( Int $n=6 )

If $n ≥ 0, returns a new "Data::Frame" with the last $n rows of the "Data::Frame".

If $n < 0, returns a new "Data::Frame" with all but the first -$n rows of the "Data::Frame".

See also: R's tail <https://stat.ethz.ch/R-manual/R-devel/library/utils/html/head.html> function.

subset

    subset( CodeRef $select )

"subset" is a helper method used to take the result of a the $select coderef and use the return value as an argument to "Data::Frame#select_rows" in "select_rows">.

The argument $select is a CodeRef that is passed the Data::Frame
$select->( $df ); # $df->subset( $select ); and returns a PDL. Within the scope of the $select CodeRef, $_ is set to a "Data::Frame::Column::Helper" for the Data::Frame $df.

    use Data::Frame::Rlike;
    use PDL;
    my $N  = 5;
    my $df = dataframe( x => sequence($N), y => 3 * sequence($N) );
    say $df->subset( sub {
                           ( $_->('x') > 1 )
                         & ( $_->('y') < 10 ) });
    # ---------
    #     x  y
    # ---------
    #  2  2  6
    #  3  3  9
    # ---------

See also: R's subset <https://stat.ethz.ch/R-manual/R-devel/library/base/html/subset.html> function