Palindrome Program without Reverse function

Anonymous Monk has asked for the wisdom of the Perl Monks concerning the following question:

Replies are listed 'Best First'.
Re: Palindrome Program without Reverse function by Ratazong (Monsignor) on Oct 30, 2011 at 19:09 UTC
Hi The examples you find look probably like `my $back = reverse $forw; if ($back eq $forw) { # it is a palindrome` [download] In order to avoid the `reverse` function you could try to code it yourself: loop through the string `$forw` character by character, and construct `$back` by appending the characters at the correct place. Hope that idea helps you! Rata	[reply] [d/l] [select]
Re: Palindrome Program without Reverse function by moritz (Cardinal) on Oct 30, 2011 at 20:00 UTC
First of all you should only read one string from `<>`. If you were given a long word on paper, and were to check if it is a palindrome, how would you do it? Just write a program that takes the same approach as you would. Perl 6 - second systems done right	[reply] [d/l]
Re: Palindrome Program without Reverse function by ikegami (Patriarch) on Oct 30, 2011 at 21:54 UTC
It's a palindrome if `str[0] eq str[str_length-1] and str[1] eq str[str_length-2] and str[2] eq str[str_length-3] and ...` [download] Or, it's not a palindrome if `str[0] ne str[str_length-1] or str[1] ne str[str_length-2] or str[2] ne str[str_length-3] or ...` [download]	[reply] [d/l] [select]
Re: Palindrome Program without Reverse function by ww (Archbishop) on Oct 31, 2011 at 02:39 UTC
`perldoc -f shift` and following this bit of advice therein... `See also "unshift", "push", and "pop". "shift" and "unshift" do the same thing to the left end of an array that "pop" and "push" do to the right end.` and understanding how to `split` on character boundaries... and you have it knocked!.	[reply] [d/l] [select]
Re: Palindrome Program without Reverse function by Anonymous Monk on Oct 31, 2011 at 04:37 UTC
`my $palindrome_re = qr{ \b (?<pal> # A palindrome is a sandwich... (?<char>\w) # starting with white bread... (?: # filled with either... \w? # peanut butter (maybe)... \| # or... (?&pal) # another sandwich... ) \k<char> # topped off with white bread! ) \b }x; @ARGV = 'words'; /$palindrome_re/ and print while <>;` [download]	[reply] [d/l]
Re^2: Palindrome Program without Reverse function by BrowserUk (Patriarch) on Oct 31, 2011 at 05:02 UTC
Clever, but slow: `sub isPalindrome{ substr( $_[0], $_, 1 ) eq substr( $_[0], -($_+1), 1 ) or return for 0 .. length( $_[0] ) >> 1; 1; }` [download] With the rise and rise of 'Social' network sites: 'Computers are making people easier to use everyday' Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error. "Science is about questioning the status quo. Questioning authority". In the absence of evidence, opinion is indistinguishable from prejudice.	[reply] [d/l]
Re^2: Palindrome Program without Reverse function by ww (Archbishop) on Oct 31, 2011 at 13:49 UTC
...Liked the regex; not so much, lines 15 and 16... so tried 5.012 which didn't like 16 either. So left the regex alone and re-wrote Lines 15-16 (much more verbosely) to work nicely with 5.12: `#!/usr/bin/perl use Modern::Perl; # 934820 my $palindrome_re = qr{ \b (?<pal> # A palindrome is a sandwich... (?<char>\w) # starting with white bread... (?: # filled with either... \w? # peanut butter (maybe)... \| # or... (?&pal) # another sandwich... ) \k<char> # topped off with white bread! ) \b }ix; my @Rev_arr = qw /Research tattarrattat Kanakanak Qaanaaq Foobarblivit +z/; for my $word(@Rev_arr) { if ($word =~ /$palindrome_re/) { say $word; } else { say "$word is NOT a palindrome"; } }` [download] Execution: `C:\>934820.pl Research is NOT a palindrome tattarrattat Kanakanak Qaanaaq Foobarblivitz is NOT a palindrome C:\>` [download]	[reply] [d/l] [select]
Re: Palindrome Program without Reverse function by JavaFan (Canon) on Oct 31, 2011 at 11:03 UTC
A word is a palindrome if it's either the empty string, a string of one character, or if it's a palindrome proceeded and succeeded by the same character. So, I present you the following, untested, subroutine: `my is_palindrome {length($_[0])<2\|\|substr($_[0],0,1) eq substr($_[0],- +1,1)&&is_palindrome(substr($_[0],1,-1);}` [download]	[reply] [d/l]
Re^2: Palindrome Program without Reverse function by choroba (Cardinal) on Oct 31, 2011 at 12:32 UTC
Tested, fixed: `sub is_palindrome {length($_[0])<2\|\|substr($_[0],0,1) eq substr($_[0],-1,1)&&is_palindrome(substr($_[0],1,-1))}`	[reply] [d/l]
Re: Palindrome Program without Reverse function by kranthi_votary (Initiate) on Jul 16, 2015 at 04:41 UTC
Try this!!!! `#!\\C:\\Perl\\bin\\perl -w my $name=<STDIN>; chomp($name); @arr=split(//,$name); $count=@arr; $num=0; #print $count; for ($i=0;$i<$count;$i++){ if ($arr[$i] eq $arr[$count-1]){ #print "$arr[$i] ===== $arr[$count]\n"; $count--; }else{ $num++; last; } } if ($num == 0){ print "Word is polindrome!!!"; }else{ print "Word is not a Polindrome!!!"; }` [download]	[reply] [d/l]
Re^2: Palindrome Program without Reverse function by choroba (Cardinal) on Jul 16, 2015 at 13:27 UTC
You replied to a 4 years old thread. Moreover, you check half the pairs pointlessly - there's no need to check `$arr[$#arr]` versus `$arr[0]`, when you've already checked `$arr[0]` versus `$arr[$#arr]`. Here's how I'd code your idea: `#!/usr/bin/perl use strict; use warnings; chomp(my $name = <>); my @arr = split //, $name; my $is_palindrome = 1; for my $i (0 .. $#arr / 2) { # warn "$arr[$i] eq $arr[ -$i - 1 ]"; if ($arr[$i] ne $arr[ -$i - 1 ]) { undef $is_palindrome; last } } print 'Word is', $is_palindrome ? q() : ' not', " a palindrome!!!\n";` [download] لսႽ� ᥲᥒ⚪⟊Ⴙᘓᖇ Ꮅᘓᖇ⎱ Ⴙᥲ𝇋ƙᘓᖇ	[reply] [d/l] [select]