Perl Cd Bookshelf [Electronic resources] نسخه متنی

1.7.1. Problem

The reverse function is two different functions in
one. Called in scalar context, it joins together its arguments and
returns that string in reverse order. Called in list context, it
returns its arguments in the opposite order. When using
reverse for its character-flipping behavior, use
scalar to force scalar context unless it's
entirely obvious.

$gnirts = reverse($string); # reverse letters in $string
@sdrow = reverse(@words); # reverse elements in @words
$confused = reverse(@words); # reverse letters in join(", @words)

Here's an example of reversing words in a string. Using a single
space, " ", as the pattern to
split is a special case. It causes
split to use contiguous whitespace as the
separator and also discard leading null fields, just like
awk. Normally, split discards
only trailing null fields.

# reverse word order
$string = 'Yoda said, "can you see this?"';
@allwords = split(" ", $string);
$revwords = join(" ", reverse @allwords);
print $revwords, "\n";
this?" see you "can said, Yoda

We could remove the temporary array @allwords and
do it on one line:

$revwords = join(" ", reverse split(" ", $string));

Multiple whitespace in $string becomes a single
space in $revwords. If you want to preserve
whitespace, use this:

$revwords = join(", reverse split(/(\s+)/, $string));

One use of reverse is to test whether a word is a
palindrome (a word that reads the same backward or forward):

$word = "reviver";
$is_palindrome = ($word eq reverse($word));

We can turn this into a one-liner that finds big palindromes in
/usr/dict/words:

% perl -nle 'print if $_ eq reverse && length > 5' /usr/dict/words
deedeed
degged
deified
denned
hallah
kakkak
murdrum
redder
repaper
retter
reviver
rotator
sooloos
tebbet
terret
tut-tut