program prime
c
       implicit      none
c
       character*9   filename
       logical       lflag
       integer       nmax    , icount , n
       real          cputot , etime
c
       real          tdummy (    2)
c
       external      divisor , etime
c
       cputot = etime ( tdummy )
c
       filename = "prime.txt"
       open (10, file=filename, status="unknown")
c
c
       write( 6,2010)
       read ( 5,   *) nmax
       write( 6,2011) nmax, filename
       write(10,2020) nmax
c
c      Loop over all integers .le. nmax, and report all the prime
c numbers.
c
       icount = 0
       do 10 n=2,nmax
         call divisor ( n, lflag )
         if (lflag .eqv. .true.) then
           icount = icount + 1
           write(10,2030) n
         endif
10     continue
       write( 6,2040) icount, nmax
       write(10,2040) icount, nmax
c
       close ( 10 )
c
       cputot = etime ( tdummy ) - cputot
       write( 6,2050) cputot
c
c-----------------------------------------------------------------------
c----------------------- Write format statements -----------------------
c-----------------------------------------------------------------------
c
2010   format("PRIME    : Enter the maximum integer to be checked for "
     1       ,"prime.")
2011   format("PRIME    : All prime numbers less than or equal to ",i4
     1       ," are listed in file ",/
     2       ,'PRIME   : ''',a9,'''.')
2020   format('PRIME    : The following is a list of all prime numbers '
     1       ,'.le. ',i8,'.',/)
2030   format('PRIME    : ',i9)
2040   format('PRIME    :',/
     1       ,'PRIME   : There are ',i8,' prime numbers less than or '
     2       ,'equal to ',i8,'.')
2050   format('PRIME    :',/
     1       ,'PRIME   : Total cpu usage for this run is ',1pg12.5
     2       ,' seconds.')
c
       stop
       end
c
c
       subroutine divisor ( number, lflag )
c
c-----------------------------------------------------------------------
c
       implicit      none
c
       logical       lflag
       integer       number , imax      ,i        , num
       real          rnum    , sqrtnum , small   , qty
c
       data          small    / 0.0001 /
c
c-----------------------------------------------------------------------
c
       lflag = .true.
       if (number .le. 3) return
c
c      The maximum divisor that needs to be checked is the greatest
c integer less than or equal to the square root of the number (‘imax’).
c The lowest divisor is 2. Thus, check all divisors between 2 and
c ‘imax’. As soon as one divisor is found, set ‘lflag’ to .false.,
c (i.e., "number" is not prime) and return to the calling program.
c
       rnum    = real ( number )
       sqrtnum = sqrt ( rnum )
       imax    = int ( sqrtnum + small )
       do 10 i=2,imax
         qty = rnum / real ( i )
         num = i * int ( qty + small )
         if ( num .eq. number ) then
           lflag = .false.
           return
         endif
10     continue
c
       return
       end
c