Best way to alternate a variable value in loop? (Perl) - perl

Question

my #arr = (1,2,3,4,5,6,7,8,9);
my $counter = 0;
foreach my $a (#arr) {
my $str;
if ($counter % 2 == 0) {
$str = 'hi';
} else {
$str = 'bye';
}
print $str . "\n";
$counter++;
}
What is the best way to alternate between two different values for each iteration of a while loop? Simple example above, is there a better way than keeping a counter and modding to find even values?

Answer 1

my #arr = qw( a b c );
for my $i (0..$#arr) {
print "$arr[$i] ", $i % 2 ? 'bye' : 'hi', "\n";
}

Answer 2

Good name.
my /*boolean*/ $even = 0;
foreach ...
next if ... # skipping line
$even = !$even;
... # work
print $even ? 'hi' : 'bye';

Answer 3

I'd do like this:
...
my #arr = (1..9);
for (my $i=0; $i<scalar(#arr); $i++) {
my $str = ($i % 2 == 0) ? 'hi' : 'bye';
print "$str\n";
}

Answer 4

my ($foo, $bar) = qw(foo bar);
for (0..10) {
print "$foo\n";
($foo, $bar) = ($bar, $foo);
}

Answer 5

A weird one, but no counters and no modding to find even values:
use strict;
sub foo {
shift || return and bar(#_);
print "doing something with Value2 \n";
}
sub bar {
shift || return and foo(#_);
print "doing something with Value1 \n";
}
foo(1..100);
Do whatever you want by calling foo() or bar() and chosing an even or odd values insted of 100. Maybe you would like to use $#arr insted of hardcoded values and so on.
But please don't write code like this.

Answer 6

Couldn't resist:
my #arr = (1,2,3,4,5,6,7,8,9);
my #alt = qw( hi bye );
my $counter = 0;
foreach my $a (#arr) {
print $alt[($counter++ % 2)] . "\n";
}
Or per dolmen's comment using "&" which should be more efficient compared to modulo (%)
my #arr = (1,2,3,4,5,6,7,8,9);
my #alt = qw( hi bye );
my $counter = 0;
foreach my $a (#arr) {
print $alt[($counter++ & 1)] . "\n";
}