After a long time, I checked the list of tasks not implemented in Perl on RosettaCode. One of them was " Sum to 100", kind of similar to mjd's Simple but difficult arithmetic puzzle:
In the string 123456789, you can prepend + or - before any digit to form an expression. You should
- list all the possible expressions that evaluate to 100
- show the number that is a result of the maximal number of expressions
- show the lowest positive number that can't be expressed
- show the ten highest numbers that can be expressed
Here's my solution: #!/usr/bin/perl
use warnings;
use strict;
use feature qw{ say };
my $string = '123456789';
my $length = length $string;
my @possible_ops = ("" , '+', '-');
{
my @ops;
sub Next {
return @ops = (0) x ($length) unless @ops;
my $i = 0;
while ($i < $length) {
if ($ops[$i]++ > $#possible_ops - 1) {
$ops[$i++] = 0;
next
}
# + before the first number
next if 0 == $i && '+' eq $possible_ops[ $ops[0] ];
return @ops
}
return
}
}
sub evaluate {
my ($expression) = @_;
my $sum;
$sum += $_ for $expression =~ /([-+]?[0-9]+)/g;
return $sum
}
my %count = ( my $max_count = 0 => 0 );
say 'Show all solutions that sum to 100';
while (my @ops = Next()) {
my $expression = "";
for my $i (0 .. $length - 1) {
$expression .= $possible_ops[ $ops[$i] ];
$expression .= substr $string, $i, 1;
}
my $sum = evaluate($expression);
++$count{$sum};
$max_count = $sum if $count{$sum} > $count{$max_count};
say $expression if 100 == $sum;
}
say 'Show the sum that has the maximum number of solutions';
say "sum: $max_count; solutions: $count{$max_count}";
my $n = 1;
++$n until ! exists $count{$n};
say "Show the lowest positive sum that can't be expressed";
say $n;
say 'Show the ten highest numbers that can be expressed';
say for (sort { $b <=> $a } keys %count)[0 .. 9];
I tried to avoid eval to evaluate the expressions, at the same time, I didn't want to implement the traditional full math expression parser as there were only two operations of the same precedence in use.
$sum += $_ for $expression =~ /([-+]?[0-9]+)/g;
Feel free to comment on perlishness, effectiveness, golfness, or beauty of the solution, or propose your own.
Note: Those interested in Perl 6 can read the solution just below mine.
($q=q:Sq=~/;[c](.)(.)/;chr(-||-|5+lengthSq)`"S|oS2"`map{chr |+ord
}map{substrSq`S_+|`|}3E|-|`7**2-3:)=~y+S|`+$1,++print+eval$q,q,a,
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