links

< home >
< unix stuff >
< code >
< writing >
< archive >
< music >
< photos >
< recipes >
< site map >
< about badcomputer >

The sieve of eratosthenes

Fortran

! Sieve of Eratosthenes in Fortran
! Written by Rob Hoelz

      function strlen(str)
        character str(*)
        integer i

        i = 0
10      if (str(i+1) .ne. ' ') then
          i = i + 1
          goto 10
        end if
        strlen = i
        return
      end

      function atoi (str)
        character str(*)
        integer s, i, length

        length = strlen(str)
        s = 0

        do 20 i = 1, length
          s = s * 10
          s = s + (iachar(str(i)) - iachar('0'))
20      continue
        atoi = s
        return
      end

      subroutine generatePrimes(primes, limit)
        logical primes(*)
        integer limit, currPrime, i

        primes(1) = .false.
        do 30 i = 2, limit
            primes(i) = .true.
30      continue

        currPrime = 2
40      if ((currPrime * currPrime) <= limit) then
          call removeMultiples(primes, currPrime, limit)
          call nextPrime(primes, currPrime, limit)
          goto 40
        end if
        return
      end

      subroutine nextPrime(primes, currPrime, limit)
        logical primes(*)
        integer currPrime, limit

        do 50 currPrime = currPrime + 1, limit
            if (primes(currPrime)) then
                return
            end if
50      continue

      return
      end subroutine nextPrime

      subroutine removeMultiples(primes, base, limit)
        logical primes(*)
        integer base, limit, i

        do 60 i = base * 2, limit, base
            primes(i) = .false.
60      continue
        return
      end

      subroutine printPrimes(primes, limit)
        logical primes(*)
        integer limit, i
        character(10) :: fmt

        fmt = "(I0 A $)"
        do 70 i = 2, limit
            if(primes(i)) then
                write(*,fmt) i, ' '
            end if
70      continue
        write(*,*)
        return
      end

      program sieve
        logical, dimension(:), allocatable :: primes
        character(32) limitString
        integer limit


        if (IARGC() > 0) then
          call GETARG(1, limitString)
          limit = atoi(limitString)
          allocate(primes(limit))
          call generatePrimes(primes, limit)
          call printPrimes(primes, limit)
          deallocate(primes)
        end if

        stop
      end

back to Eratosthenes page

stats

It is Friday August 29, 2008 12:50 am
This page served 868 times
This page last modified: April 14, 2008 11:28 am
Your IP address is: 38.103.63.61
You are browsing using: CCBot/1.0 (+http://www.commoncrawl.org/bot.html)
You are browsing from: United States.
badcomputer.org's uptime: 00:50:55 up 29 days, 15:57, 2 users, load average: 0.16, 0.06, 0.02