Project Euler 8: Greatest Product of Consecutive Digits in String
This problem is one of the easiest ones at Project Euler. You can even solve it manually in less than a minute; just highlight the nines in your browser, and you will see the potential candidates. You’ll have problems proving it that way, though, but you can just try the solution and check if it works.
Here is a nice Perl script that checks every product. Being the lazy perl-programmer I am, I let Perl format the numbers instead of doing it by hand.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 | # put the numbers into an array @n = split //, join '', qw/ 73167176531330624919225119674426574742355349194934 96983520312774506326239578318016984801869478851843 85861560789112949495459501737958331952853208805511 12540698747158523863050715693290963295227443043557 66896648950445244523161731856403098711121722383113 62229893423380308135336276614282806444486645238749 30358907296290491560440772390713810515859307960866 70172427121883998797908792274921901699720888093776 65727333001053367881220235421809751254540594752243 52584907711670556013604839586446706324415722155397 53697817977846174064955149290862569321978468622482 83972241375657056057490261407972968652414535100474 82166370484403199890008895243450658541227588666881 16427171479924442928230863465674813919123162824586 17866458359124566529476545682848912883142607690042 24219022671055626321111109370544217506941658960408 07198403850962455444362981230987879927244284909188 84580156166097919133875499200524063689912560717606 05886116467109405077541002256983155200055935729725 71636269561882670428252483600823257530420752963450 /; my $m; # this is the greatest product yet found foreach my $i ( 0..(@n-5) ) { my $p = $n[$i]*$n[$i+1]*$n[$i+2]*$n[$i+3]*$n[$i+4]; # find current product $m = $p if $p > $m; # update $m if this product is greater } print $m; # print the result |
There is really nothing interesting here, except perhaps the second line. Using the qw// operator, Perl first makes a list where every element is a row of the long number. join joins the rows together to one long string, and split splits them such that each element contains one number.
I thought about writing a more elaborate script that would move through the list, dividing by one element and multiplying by another. However, I decided that the existence of zeroes would make it too difficult to be worth the trouble.
Warning: The code looks ugly with all those pink numbers if you’re using a weird colour hilighting.
Använder FireStats
A slightly less scary version.
#! -*- perl -*-
use strict;
use warnings;
my($final,$current) = (0,0);
my @n = split //, join ”, qw/
73167176531330624919225119674426574742355349194934
96983520312774506326239578318016984801869478851843
85861560789112949495459501737958331952853208805511
12540698747158523863050715693290963295227443043557
66896648950445244523161731856403098711121722383113
62229893423380308135336276614282806444486645238749
30358907296290491560440772390713810515859307960866
70172427121883998797908792274921901699720888093776
65727333001053367881220235421809751254540594752243
52584907711670556013604839586446706324415722155397
53697817977846174064955149290862569321978468622482
83972241375657056057490261407972968652414535100474
82166370484403199890008895243450658541227588666881
16427171479924442928230863465674813919123162824586
17866458359124566529476545682848912883142607690042
24219022671055626321111109370544217506941658960408
07198403850962455444362981230987879927244284909188
84580156166097919133875499200524063689912560717606
05886116467109405077541002256983155200055935729725
71636269561882670428252483600823257530420752963450
/;
sub product
{
my $x = 1;
$x *= $_ for @_;
$x
}
while(defined (my $point = shift(@n)) && scalar(@n) >= 4)
{
$current = product($point,@n[0..3]);
$final = $current if $current > $final;
}
print $final;
Comment by alpha — August 19, 2007 @ 2:27 pm
…………… and still I wasn’t too pleased with this code. So here’s one more version:
#! -*- perl -*-
use strict;
use warnings;
my($final,$current) = (0,0);
my $str = join ”, qw/
73167176531330624919225119674426574742355349194934
96983520312774506326239578318016984801869478851843
85861560789112949495459501737958331952853208805511
12540698747158523863050715693290963295227443043557
66896648950445244523161731856403098711121722383113
62229893423380308135336276614282806444486645238749
30358907296290491560440772390713810515859307960866
70172427121883998797908792274921901699720888093776
65727333001053367881220235421809751254540594752243
52584907711670556013604839586446706324415722155397
53697817977846174064955149290862569321978468622482
83972241375657056057490261407972968652414535100474
82166370484403199890008895243450658541227588666881
16427171479924442928230863465674813919123162824586
17866458359124566529476545682848912883142607690042
24219022671055626321111109370544217506941658960408
07198403850962455444362981230987879927244284909188
84580156166097919133875499200524063689912560717606
05886116467109405077541002256983155200055935729725
71636269561882670428252483600823257530420752963450
/;
sub product
{
my $x = 1;
$x *= $_ for @_;
$x
}
while ($str =~ m#(\G^\d{5}|\d{4}\G\d{1})#go)
{
$current = product split ”, $1;
$final = $current if $current > $final;
}
print $final;
Comment by alpha — August 19, 2007 @ 4:13 pm