Mercurial > coderloop

     1       program collatz

     2       implicit none

     3       character (len = 100) :: file_name  

     4       integer, parameter :: big = selected_int_kind(11)

     5       integer  :: n

     6       call getarg( 1, file_name )  

     7       OPEN(unit = 7, file = file_name)

     8       read (7,*) n

     9       write (unit = *, fmt = "(I0)") search_collatz(n)

    10 

    11       contains

    12 

    13       function collatz_next (n) result (r)

    14       implicit none

    15       integer (kind = big)  :: r,n

    16       if (0 == mod(n,2)) then

    17          r = n/2

    18       else

    19          r = 1 + 3*n

    20       end if 

    21       end function collatz_next

    22       

    23       function collatz_length (n) result (r)

    24       implicit none

    25       integer (kind = big)  :: t

    26       integer :: n,r

    27       r = 1

    28       t = n

    29       do 

    30          if (1 == t) then

    31          exit

    32          end if

    33          r = r + 1

    34          t = collatz_next(t)

    35       end do

    36       end function collatz_length

    37       

    38       function search_collatz (n) result (r)

    39       implicit none

    40       integer n,r,i,max,max_val,temp_val

    41       integer (kind = big) :: t

    42       t = n

    43       do i = 1,n-1

    44          temp_val = collatz_length(i)

    45          if (temp_val > max_val) then

    46          max = i

    47          max_val = temp_val

    48       end if

    49       end do

    50       r = max

    51       end function search_collatz

    52       end program collatz
author	Robert McIntyre <rlm@mit.edu>
date	Tue, 18 Oct 2011 01:17:49 -0700
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