PROGRAM grow
!---------------------------------------------------
! Program to calculate the nubmer of growing
! degree days experienced on a particular
! day based on the high and low temperatures
! (fahrenheit) entered by the user.
!
! Identifiers used are:
! maxT  : Maximum daily recorded temperature (F)
! minT  : Minimum daily recorded temperature (F)
! aveT  : average daily temperature (F)
!         calculated from maxT and minT
! baseT : 50 degrees F
! gdd   : number of growing degree days
!
! Input   : Maximum and minimum recorded 
!           temperatures (F) for the day
! Constant: Base temperature (F)
!---------------------------------------------------

  IMPLICIT NONE
  REAL :: maxT, minT
  real :: gdd

  !Obtain values for maxT and minT
  PRINT *, "Enter Maximum Temperature and" // &
       " Minimum Temperature (degrees F)"
  PRINT *, "Separated by a comma or space."
  READ *, maxT, minT

  !Call function to calculate the number of growing degree days.
  gdd = grow_days(maxT,minT)

  !Display growing degree days
  PRINT *, "Number of growing degree days &
       is", gdd

CONTAINS

function grow_days(max_temp,min_temp)

  real :: grow_days, aveT
  real, intent(in) :: max_temp, min_temp
  real :: max_calc, min_calc 
  real, parameter :: baseT = 50

  max_calc = max_temp
  min_calc = min_temp

 !Set value of max_temp between 50F and 86F
  IF (max_calc > 86) THEN
     max_calc = 86
  ELSE IF (max_calc < 50) THEN
     max_calc = 50
  ENDIF

  !Set value of min_temp between 50F and 86F
  IF (min_calc < 50) THEN
     min_calc = 50
  ELSE IF (min_calc > 86) THEN
     min_calc = 86
  ENDIF

  !Calculate daily average temperature
  aveT = calc_average(max_calc,min_calc)

  !Calculate growing degree days
  grow_days = aveT - baseT

  !Make sure grow_days is non-negative
  IF (grow_days < 0) THEN
     grow_days = 0
  ENDIF

end function grow_days

function calc_average(Tmax, Tmin)

real:: calc_average, Tmax, Tmin

calc_average = (Tmax + Tmin) / 2.0

end function calc_average

END PROGRAM grow