A few minor points that caught my attention:
- $a and $b are not the best variable names, as they are special
- You could use the replacement capability of substr to eliminate some temp vars
- In general, it is a good idea to unload args to a function as early as possible, so the shift statement would better become the first line
- It is a good idea to limit the scope of your vars as much as possible, so my ($a, $b) would be closer to home inside the for loop
use strict;
use warnings;
my @TEST_STRINGS
= ( q(), qw(
1 12 123 1234 12345
)
);
for my $t (@TEST_STRINGS) {
printf qq(%-7s => "%s"\n), qq("$t"), swap( $t );
}
exit( 0 );
sub swap {
my ($str) = @_;
die 'No input?' unless defined $str;
## 0 to half-length minus one
for my $i (0 .. (length( $str ) >> 1) - 1) {
my $j = $i << 1;
substr( $str, $j, 2, reverse substr( $str, $j, 2 ) );
}
return $str;
}
prints:
"" => ""
"1" => "1"
"12" => "21"
"123" => "213"
"1234" => "2143"
"12345" => "21435"
Cheers.
Update: Took sub swap {...} out of readmore