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

I have some questions about the following code: 
#!/usr/bin/perl -w
use strict;

my @fr = ([], [], [], [], []);
my $low_count;

for my $x (10 .. 99) {
  for my $y (101 .. 999) {
    my $result = $x * $y;
    if ($result < 10000) {
      $low_count++;
      next;
    }
    $result =~ m/(\d)?(\d)?(\d)?(\d)?(\d)?/;
    $fr[0][$5]++;
    $fr[1][$4]++;
    $fr[2][$3]++;
    $fr[3][$2]++;
    $fr[4][$1]++;
  }
}

$fr[4][0] = 0; #This line 

print "$low_count\n\n";

for my $digit (0 .. 9) {
  print "$digit\t";
  for my $exp (reverse 0 .. 4) {
    print "$fr[$exp][$digit]\t";
  }
  print "\n";
}

[download]
1. Is there a better way to write it, that would enable it to be scaled up to multiplcations of any size?

2. Is there a way to prevent a 'use of uninitialised value' error other than explicitly setting the variable like I have done in the line marked '#This line' (which I think is kludgy)?

3. Can you tell from the code what my editor of choice is? :-)
  • Comment on Frequency analysis of the digits of the result of a multiplication
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Re: Frequency analysis of the digits of the result of a multiplication
by Masem (Monsignor) on Nov 17, 2001 at 23:54 UTC
    There's definitely an easier way to build up your arrays: 
    foreach my $x ( 10..99 ) {
   foreach my $y ( 10..999 ) {
       @result = reverse split //, $x*$y;
       $fr[ $_ ][ $result[$_] ]++ foreach (0..$#result);
   }
}

    [download]
    And to remove that uninitialized value, you can do something like: 
    my @fr;
$fr[$_] = [ (0)x10 ] foreach (0..$maxdigits);

    [download]
    where $maxdigits is the largest number of digits you expect in your results.

    -----------------------------------------------------
    Dr. Michael K. Neylon - mneylon-pm@masemware.com || "You've left the lens cap of your mind on again, Pinky" - The Brain
    "I can see my house from here!"
    It's not what you know, but knowing how to find it if you don't know that's important

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[d/l]
[select]
      Thanks for that.

      I found that after getting @result, I had to add: 
      while (scalar @result < $maxdigits) {
  push @result, 0;
}

      [download]
      to cope with when the result is less than $maxdigits in length. (M-x mpuz, which is behind why I wrote this code in the first place, throws away results less than 5 digits in length anyway.)
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[d/l]
Re: Frequency analysis of the digits of the result of a multiplication
by demerphq (Chancellor) on Nov 18, 2001 at 01:07 UTC
    I came up with pretty much the same as Masem and Grinder but my table generater looked like this: 
    for my $digit (0 .. 9) {
    print "$digit\t";
    print map { ($_->[$digit]||=0)."\t" } reverse @fh;
    print "\n";
}

    [download]
    This seemed like a good time to use map.

    :-)

    Yves / DeMerphq
    --
    Have you registered your Name Space?

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[d/l]
Re: Frequency analysis of the digits of the result of a multiplication
by grinder (Bishop) on Nov 17, 2001 at 23:56 UTC

    re: question 2, try, a few lines below your kluge, the following fix: 

      $fr[$exp][$digit] ||= 0;
  print "$fr[$exp][$digit]\t";

    [download]

    That is, if you never actually saw any digits at a certain power of ten, then set it to zero before printing it out.

    Cool idea. I love applying computers to existential mathematical questions.

    --
    g r i n d e r
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[d/l]
Re: Frequency analysis of the digits of the result of a multiplication
by hopes (Friar) on Nov 18, 2001 at 01:19 UTC
    This is my code to make the program independent of the number of digits.
    I donīt know how to interprete 999 and 99, maybe 10^3-1 and 10^2-1, but I don't know how would be if digits is "6".
    999 and 999?
    99 and 9999?

    Regards
    Hopes 
    #!/usr/bin/perl -w
use strict;

my $fr;
my $low_count;
my $digits=5;

for my $x (10 .. 99) {
  for my $y (101 .. 999) {
    my $result = $x * $y;
    if ($result < 10**($digits-1)) {
      $low_count++;
      next;
    }
    my $index;
    $fr->[5-$index++][$&]++ while ($result =~ /\d/g);
  }
}

$fr->[$digits-1][0] = 0; #This line 

print "$low_count\n\n";

for my $digit (0 .. 9) {
  print "$digit\t";
  for my $exp (reverse 0 .. 4) {
    print "$fr->[$exp][$digit]\t";
  }
  print "\n";
}

    [download]
[reply]
[d/l]
Re: Frequency analysis of the digits of the result of a multiplication
by pjf (Curate) on Nov 18, 2001 at 01:57 UTC
    If you want to be able to work with integers of an arbitary size, you might want to check out the Math::BigInt package.

    Cheers,
    Paul

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