Finding product palindromes

I am new to perl and want some help in tuning this script if possible cause it takes time if I increased my array. also if I use warnings I get a lot of warnings which I'm unable to handle. I'm using perl (v5.24.0) built for darwin-thread-multi-2level.

Here is an example of what I want to achieve:
The number 886 is like 8*86 = 688, if we reverse it 886 which is the original number.
Also the same for 920781, 9*20781 = 187029, reverse 920781,
etc...

my $start_run = time();
use strict;
foreach (1..1000000000) {
   if ($_==reverse(substr($_, 0, 1) * substr($_, 1)))
      { print $_."\n" ;}
    if ($_==reverse(substr($_, 0, 2) * substr($_, 2)))
    { print $_."\n" ;}
    if ($_==reverse(substr($_, 0, 3) * substr($_, 3)))
    { print $_."\n" ;}
    if ($_==reverse(substr($_, 0, 4) * substr($_, 4)))
    { print $_."\n" ;}
    if ($_==reverse(substr($_, 0, 5) * substr($_, 5)))
    { print $_."\n" ;}
    if ($_==reverse(substr($_, 0, 6) * substr($_, 6)))
    { print $_."\n" ;}
    if ($_==reverse(substr($_, 0, 7) * substr($_, 7)))
    { print $_."\n" ;}
}
my $end_run = time();
my $run_time = $end_run - $start_run;
print "Time taken: ".$run_time." sec\n";

I want some help in tuning this script if possible cause it takes time
if I increased my array

Yes, the script can be tuned. I will discuss optimizing you script at the end of this post.

The warnings pragma


also if I use warnings I get a lot of warnings which I'm unable to
handle

Yes correct, you will get many warnings in case you add the statement use warnings. Using the warnings pragma is highly recommended, see for example:

-
Why use strict and warnings?

-
Why not use strict and warnings in Perl?

In short, the warnings pragma help you find bugs in your code. If you add the use warnings to the top of your program, the first warning you get is

Argument "" isn't numeric in multiplication (*)

This happens because in the first iteration of the for loop, $_ will be equal to 1, and at line 4 of your program where you mulitply the substrings:

substr($_, 0, 1) * substr($_, 1))

the first substring will be "1" and the second will be the empty string "". When you try multiply with the empty string (and warnings are disabled) it will be converted silently to a zero, see perldata. To demonstrate:

$ perl -E 'say 1 + ""'
1

but if you turn on warnings (use the -w flag):

$ perl -wE 'say 1 + ""'
Argument "" isn't numeric in addition (+) at -e line 1.
1

you will get the warning Argument "" isn't numeric.

The next warning you get will be

substr outside of string

this happens because substr returns an undefined value
if the substring is beyond either end of the string. In this case, substr will also produce a warning (if you have enabled warnings with use warnings). The reason why you get the warning is that $_ is equal to "1" and at line 6 of your script you try call substr $_, 2. To demonstrate:

$ perl -wE '$_="1"; my $s  = substr $_, 2'
substr outside of string at -e line 1.

The next warning you will get is:

Use of uninitialized value in multiplication (*)

This happens also at line 6, where you have

substr($_, 0, 2) * substr($_, 2)

Here the first substring will be "1" ( since $_ is equal to "1" in the first iteration of the for loop), whereas the second substring will be undefined, as discussed above. Now if you try multiply 1 * undef you will get 0, and a warning (if you have enabled warnings). To demonstrate:

$ perl -wE 'say 1 * undef'
Use of uninitialized value in multiplication (*) at -e line 1.
0

The problem with the code

From the above discussion, it should be clear that the problem with the code that causes the warnings are that it does not use substr correctly when the numbers ( i.e $_ in the for loop ) are less than 8 digits long. If the numbers are more than 8 digits long another problem arises. In this case, the problem is of logical nature in that the program will fail to check all possible palindromes. For example, for a 9 digit number there is no check

if ($_ == reverse(substr($_, 0, 8) * substr($_, 8))) { ... }

Fortunately, these problems can be solved easily using nested for loops:

for (1 .. $max_integer) {
    for my $length ( 1 .. ((length $_) - 1) ) {
        say if $_ == reverse (substr ($_, 0, $length) * substr ($_, $length) );
    }
}

Here, I have defined a variable $max_integer which should be set to the highest integer to check for. For your case, it should be set to 1 billion:

my $max_integer = 1_000_000_000;

The inner for loop takes care of all the if statements in your original code.

Programming style

In Perl there is more than one way to do it (TMTOWTDI). The language was designed with this idea in mind, in that it "doesn't try to tell the programmer how to program". Still, in my experience (I started programming Perl 3 years ago) there are certain programming style guidelines that have been adopted by the community. I have seen these programming styles in practice reading source code of CPAN modules and looking at answers at the Perl tag at stackoverflow.com. Some of these guidelines are described in perlstyle, in Chapter 21 of Programming Perl and in the book Perl Best Practices.

-
Put all use statements at the top of your program. Since all use statements occur at compile time, it often does not matter for the Perl interpreter where in the source file you place them1. To demonstrate:

$ perl -E 'say getcwd(); use Cwd'

The above code works fine, even if use Cwd comes after its exported function getcwd is called in the code. However, the above code will
likely confuse humans. You can read more about compile time versus run time in Chapter 16 of the book Programming Perl.

Perl pragmas are a subclass of Perl modules that can have a lexical scope. Since these pragmas only stay in effect within the scope in which they are used, their position in the code is important also to the Perl interpreter. However, these pragmas are typically used at the package scope ( to enable them in

StackExchange Code Review Q#133916, answer score: 7