problem 003

ソース・プログラム

   program PEuler3
      implicit none
      integer, parameter :: ki = selected_int_kind(12)
      integer(ki), parameter :: nx = 600851475143 !13195
      integer, allocatable :: iptab(:)
      iptab = ieratos( int(sqrt(real(nx))) ) ! prime numbers up to possible max factor
      !print '(a, *(i8))', 'factors ', pack(iptab, mask = mod(nx, iptab) == 0)
      print '(a,   i8 )', 'max factor =', maxval(iptab, mask = mod(nx, iptab) == 0)
      stop
    contains
      function ieratos(n) ! thieve of Eratostenes
        integer, intent(in) :: n
        integer, allocatable :: ieratos(:)
        logical :: tab(n)
        integer :: i
        tab = .true.
        tab(1) = .false.
        do i = 2, int( sqrt(real(n)) ) 
          if ( tab(i) ) tab(i * i:n:i) = .false.
        end do
        ieratos = pack( [ (i, i = 1, n) ], tab )
        return
      end function ieratos
    end program PEuler3

実行結果

max factor = 6857
続行するには何かキーを押してください . . .