my @array = 1 .. 1_000_000; 
## Note the above doubles mem. consumption.
## Half of which is immediately availble to the rest of the program.

# Ripple the good values through the array
my( $s, $d );
for( $s = $d = 0; $s < @array; $d++, $s++ ) { 
 	$array[ $s ] %2 and $s++;;
      $array[ $d ] = $array[ $s ]; 
}; 
# and truncate it.
$#array = --$d;

##</code><code>##

#! perl -slw
use strict;

our $SIZE ||= 1_000_000;

# Don't build the test array by assigning a list 
# cos that doubles the memory consumption which
# then serves to conseal the extra consumed by the grep

my @array;
push @array, $_ for 1 .. $SIZE;

printf 'Check ram before ripple'; <STDIN>;

#@array = grep not ($_ % 2), @array;

my( $s, $d );
for( $s = $d = 0; $s < @array; $d++, $s++ ) { 
 	$array[ $s ] %2 and $s++, $array[ $d ] = $array[ $s ]; 
}; 
$#array = --$d;

printf 'Check ram after ripple'; <STDIN>;

die 'M_S: Mismatch:', scalar @array, " @array[ 0, -1 ]"
	unless @array == $SIZE / 2 and "@array[ 0, -1 ]" eq "2 $SIZE";

__END__
P:\test>371457-mem
Check ram before grep 21844 kb
Check ram after grep  35856 kb

P:\test>371457-mem
Check ram before ripple 21,844 kb
Check ram after ripple  21,860 kb

##</code><code>##

#! perl -slw
use strict;
use Benchmark qw[ cmpthese ];

our $SIZE ||= 1000;
our $DEBUG;

cmpthese( 5, {
	simple_grep => q[
		my @array = 1 .. $SIZE;

		@array = grep not ($_ % 2), @array;

		die 'S_G: Mismatch:', scalar @array, " @array[ 0, -1 ]"
			unless @array == $SIZE / 2 and "@array[ 0, -1 ]" eq "2 $SIZE";
	],
	splice_for => q[
		my @array = 1 .. $SIZE;

		$array[ $_ ] % 2 and splice @array, $_, 1 for reverse 0 .. $#array;

		die 'S_F: Mismatch:', scalar @array, " @array[ 0, -1 ]"
			unless @array == $SIZE / 2 and "@array[ 0, -1 ]" eq "2 $SIZE";
	],
	move_splice => q[
		my @array = 1 .. $SIZE;

		my( $s, $d );
		for( $s = $d = 0; $s < @array; $d++, $s++ ) { 
			$array[ $s ] %2 and $s++, $array[ $d ] = $array[ $s ]; 
		}; 
		$#array = --$d;

		die 'M_S: Mismatch:', scalar @array, " @array[ 0, -1 ]"
			unless @array == $SIZE / 2 and "@array[ 0, -1 ]" eq "2 $SIZE";
	],
});

__END__
P:\test>371457 -SIZE=100000
            (warning: too few iterations for a reliable count)

               Rate  splice_for move_splice simple_grep
splice_for  0.798/s          --        -94%        -94%
move_splice  12.3/s       1440%          --         -8%
simple_grep  13.3/s       1571%          9%          --

P:\test>371457 -SIZE=2000000  ## splice_for removed from test.
            s/iter move_splice simple_grep
move_splice   1.66          --         -8%
simple_grep   1.52          9%          --