herons_method.f95

program simple
  implicit none
  real a
  real b
  write(*,fmt='(a)',advance='no') 'Set a to: '
  read*, a
  write(*,fmt='(a)',advance='no') 'Set b to: '
  read*, b
  write(*,*) 'sq(a)=',sq(a),'sq(b)=',sq(b)
  write(*,*) 'average(a,b)=',average(a,b)
  write(*,*) 'improve(a,b)=',improve(a,b)
  write(*,*) 'absolute(-2.5)=',absolute(-2.5)
  write(*,*) 'good_enough(25.0,5.000001)=',good_enough(25.0,5.00001)
  write(*,*) 'heron(a,b)=',heron(sq(a) + sq(b),1.)
  contains
    real function sq(n) result(x)
      real n
      x = n * n
    end function sq 
    real function average(a,b) result(x)
      real a,b
      x = (a + b) / 2.
    end function average
    real function improve(n,g) result(x)
      real n,g
      x = average(g,(n/g))
    end function improve
    real function absolute(n) result(x)
      real n
      x = n
      if ( n .lt. 0. ) then
        x = n * (-1.)
      end if
    end function absolute
    integer function  good_enough(n,g) result(x)
      real n,g
      x = 0
      if ( absolute(sq(g) - n) .le. 0.001 ) then
        x = 1
      end if
    end function good_enough
    real recursive function heron(n,g) result(x)
      real n,g
      x = g
      if ( good_enough(n,g) .eq. 0 ) then
        x = heron(n,improve(n,g))
      end if
    end function heron
end program simple