Free form vs. Fix form

First things first, dated back to punch card days, first six characters are reserved for label, and important not to exeed column 72

Hello world

      PROGRAM CIRCLE_OF_LIFE
      PRINT *, 'CIRCLE OF LIFE'
      END

Subroutine instead of function in R

Function cannot be called from R, instead subroutine can be called by .Fortran, this mechanism is motivated purely for taking advantage of Fortran numerical computation power. Safe data types are integer, double precision, and logical arguments.


C monte carlo estimate Pi
C R always use double precision 
C23456789
      subroutine mc_pi_r(pi)
        double precision pi
        k = 0
        do 10 i=1,1000000
          x = rand()
          y = rand()
          if(x**2 + y**2 .lt. 1.0) k=k+1
 10     continue
        pi = real(k) / 1000000 * 4
      end subroutine mc_pi_r

Then compile it with R CMD SHLIB -c mc_pi_r.f

running it in R

  dyn.unload('mc_pi_r.so')
  dyn.load('mc_pi_r.so')
  .Fortran("mc_pi_r", pi=as.double(0))
  .C("mc_pi_r_", pi=as.double(0))

Some resources

F77 tutorial
make
sing