Re: Project Euler (a series of challenging mathematical/computer programming problems)

[Anonymous Monk]

in reply to Project Euler (a series of challenging mathematical/computer programming problems)

here's another one i thought was correct: 77.

my $target = 5_001;  # also tried 5_000
my $max = 1_000_000;

my @primes;
my $sieve = '';

# generate list of primes
for (my $try=2; $try <= $max; $try++)
{
  next if vec($sieve, $try, 1);
  push @primes, $try;
  for (my $mults=$try*$try; $mults <= $max; $mults+=$try)
  {
    vec($sieve, $mults, 1) = 1;
  }
}

# number of composites <= n
# http://research.att.com/~njas/sequences/A065855 
#
# equivalent to number of primes between prime(n) and n, so that's wha
+t 
# i'm doing here
for (my $i=0; $i<@primes; $i++)
{
  my $j = 0;
  $j++ while $primes[$j] <= $i+1;

  my $count = 0;
  $j++ && $count++ while $primes[$j] < $primes[$i];

  next if $count < $target;
  printf "%d: %d\n", $i+1, $count;
  last;
}