Filter out every second value of a list

Filter out every second value of a list, starting with the first

my $t = 1; grep { $t^=1 } LIST
my $t = 1; grep { $t = 1-$t } LIST
my $t = 1; grep { ++$t % 2 } LIST
map { $_%2 ? $ARRAY[$_] : () } 0..$#ARRAY

Example:

>perl -le"$t=1; print grep { $t^=1 } qw( a b c d e );" bd
>perl -le"$t=1; print grep { $t = 1-$t } qw( a b c d e );" bd
>perl -le"$t=1; print grep { ++$t % 2 } qw( a b c d e );" bd
>perl -le"@a = qw( a b c d e ); print map { $_%2 ? $a[$_] : () } 0..$# +a;" bd

Filter out every second value of a list, starting with the second

my $t = 0; grep { $t^=1 } LIST
my $t = 0; grep { $t=1-$t } LIST
my $t = 0; grep { ++$t % 2 } LIST
map { $_%2 ? () : $ARRAY[$_] } 0..$#ARRAY

Example:

>perl -le"$t=0; print grep { $t^=1 } qw( a b c d e );" ace
>perl -le"$t=1; print grep { $t = 1-$t } qw( a b c d e );" ace
>perl -le"$t=0; print grep { ++$t % 2 } qw( a b c d e );" bd
>perl -le"@a = qw( a b c d e ); print map { $_%2 ? () : $a[$_] } 0..$# +a;" ace

Transform a list based on odd/even-ness of index

my $t = 1; map { ($t^=1) ? EXPR_FOR_ODD : EXPR_FOR_EVEN } LIST
my $t = 1; map { ($t = 1-$t) ? EXPR_FOR_ODD : EXPR_FOR_EVEN } LIST
my $t = 1; map { (++$t % 2) ? EXPR_FOR_ODD : EXPR_FOR_EVEN } LIST
map { $_%2 ? EXPR_FOR_ODD : EXPR_FOR_EVEN } 0..$#ARRAY

Example:

>perl -le"$t=1; print map { ($t^=1) ? uc : lc } qw( a b c d e );" aBcDe
>perl -le"$t=1; print map { ($t=1-$t) ? uc : lc } qw( a b c d e );" aBcDe
>perl -le"$t=1; print map { (++$t % 2) ? uc : lc } qw( a b c d e );" aBcDe
>perl -le"@a = qw( a b c d e ); print map { $_%2 ? uc($a[$_]) : lc($a[ +$_]) } 0..$#a;" aBcDe

Usage note

None of the above are particularly self-explanatory, so a comment is in order if they their use if not obvious from the context.

Update: Added % solution as an alternative
Update: Added 1- solution as an alternative


In reply to Mini-Tutorial: Working with Odd/Even Elements by ikegami

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