Palindrome Question

[mwesolow]

Hello,

I'm having trouble getting my code to work. It seems that my function isPalindrome works correctly when I just test it out on some particular products of 3-digit integers, but when I use it in my full program, it fails to do its job.

Here's my code:

for ($j = "100"; $j <= "110"; $j += 1){
 for($i = "100"; $i <= $j; $i += 1){
  if(isPalindrome($i*$j)){
   print STDOUT ($i*$j)." Is a product of two three-digit integers and a palindrome.";
  }
 }
}

sub isPalindrome {
 my($Qstring) = @_;
 for($i = 0; $i <= length($Qstring)-1; $i++){
  if(substr($Qstring,$i,1) == substr($Qstring,length($Qstring)-1-$i,1)){}
  else{return 0;}
 }
 return 1;
}

Thanks for any help!

P.S. Yes, this is for one of the early Project Euler problems..

[nithinpes]

If you had used use strict; and use warnings;, you would have caught the problem. The problem is with the use of global variable inside the subroutine. The code below works (make use of local variables):

for (my $j = "100"; $j <= "110"; $j += 1){ 
 for(my $i = "100"; $i <= $j; $i += 1){ 
  if(isPalindrome($i*$j)){ 
   print STDOUT ($i*$j)." Is a product of two three-digit integers and a palindrome.\n"; 
  } 
 } 
} 

  
sub isPalindrome { 
 my($Qstring) = @_ ; 
 for(my $i = 0; $i <= length($Qstring)-1; $i++){ 
  if(substr($Qstring,$i,1) == substr($Qstring,length($Qstring)-1-$i,1)){} 
  else{return 0;} 
 } 
 return 1
}

[mwesolow]

Thanks! That worked perfectly. Now I have to figure out why the largest one it gives is 580085 = 583 * 995, when the solution is apparently 906609

[mwesolow]

nevermind, got it :D

for ($j = "100"; $j <= "999"; $j += 1){
 for($i = "100"; $i <= $j; $i += 1){
  if(isPalindrome($i*$j) and $i*$j > $l*$p){
    $l=$i;
    $p=$j;
  }
 }
}
print STDOUT ($l*$p)." Is a product of two three-digit integers and a palindrome.\n";
print STDOUT "($l,$p)";

sub isPalindrome {
 my($Qstring) = @_;
 for($k = 0; $k <= length($Qstring)-1; $k++){
  if(substr($Qstring,$k,1) == substr($Qstring,length($Qstring)-1-$k,1)){}
  else{return 0;}
 }
 return 1;
}

Thanks again!

[toolic]

sub isPalindrome { 
    my ($Qstring) = @_;
    return ($Qstring eq reverse($Qstring))
}

[Chendil]

sub isPalindrome { 
    my ($Qstring) = @_;
    return ($Qstring eq reverse($Qstring))
}

This solution is simple. Why reinvent the wheel.

Use the reverse function in the perl and compare the value with the original string.

[Jyoti Ballabh]

or could be done in the following manner - convert the whole sequence into an alphanumeric code and then verify every digit in ASCII reference. At the end , set up a counter for the Competitive match score. it should settle it.