How can I print only every third index in Perl or Python?

Question

How can I do a for() or foreach() loop in Python and Perl, respectively, that only prints every third index? I need to move every third index to a new array.

Answer 1

Python:

for x in a[::3]:
   something(x)

Answer 2

Python

print list[::3] # print it
newlist = list[::3] # copy it

Perl

for ($i = 0; $i < @list; $i += 3) {
    print $list[$i]; # print it
    push @y, $list[$i]; # copy it
}

Answer 3

In Perl:

$size = @array; 
for ($i=0; $i<$size; $i+=3)  # or start from $i=2, depends what you mean by "every third index"
{  
        print "$array[$i] ";  
}

Answer 4

Perl 5.10 new state variables comes in very handy here:

my @every_third = grep { state $n = 0; ++$n % 3 == 0 } @list;

Also note you can provide a list of elements to slice:

my @every_third = @list[ 2, 5, 8 ];  # returns 3rd, 5th & 9th items in list

You can dynamically create this slice list using map (see Gugod's excellent answer) or a subroutine:

my @every_third = @list[ loop( start => 2, upto => $#list, by => 3  ) ];

sub loop {
    my ( %p ) = @_;
    my @list;

    for ( my $i = $p{start} || 0; $i <= $p{upto}; $i += $p{by} ) {
        push @list, $i;
    }

    return @list;
}

Update:

Regarding runrig's comment... this is "one way" to make it work within a loop:

my @every_third = sub { grep { state $n = 0; ++$n % 3 == 0 } @list }->();

/I3az/

Answer 5

Perl:

As with draegtun's answer, but using a count var:

my $i;
my @new = grep {not ++$i % 3} @list;

Answer 6

@array = qw(1 2 3 4 5 6 7 8 9);
print @array[(grep { ($_ + 1) % 3 == 0 } (1..$#array))];

Answer 7

Perl:

# The initial array
my @a = (1..100);

# Copy it, every 3rd elements
my @b = @a[ map { 3 * $_ } 0..$#a/3 ];

# Print it. space-delimited
$, = " ";
say @b;

Answer 8

You could do a slice in Perl.

my @in = ( 1..10 );

# need only 1/3 as many indexes.
my @index = 1..(@in/3);

# adjust the indexes.
$_ = 3 * $_ - 1 for @index;
# These would also work
# $_ *= 3, --$_ for @index;
# --($_ *= 3) for @index

my @out = @in[@index];