C
C ideal-gas.f -- program to compute values from the ideal gas law
C
C Rich Thomson, May 4th, 1994
C CS101 - FORTRAN

C menu -- returns an integer indicating which type of computation we want
C to perform using the ideal gas law PV = nRT, or if the user wants to exit.
C
C The return values indicate:
C 1. Compute pressure given volume, moles of gas and temperature
C 2. Compute volume given pressure, moles of gas and temperature
C 3. Compute moles of gas given pressure, volume and temperature
C 4. Compute temperature given pressure, volume and moles of gas
C 5. Exit the program.
C
      integer function menu()
      implicit none
C-- choice will hold the user's input
      integer choice

C-- initialize choice to an invalid selection, so the loop will be
C-- entered the first time.
      choice = 0

C-- keep asking for input until we get a valid choice
      do while ((choice .lt. 1) .or. (choice .gt. 5))

C-- print out the menu and get the user's choice
         print *, 'Please select which quantity you would like to'
         print *, 'compute from the ideal gas law PV=nRT:'
         print *
         print *, '1. Compute pressure'
         print *, '2. Compute volume'
         print *, '3. Compute moles of gas'
         print *, '4. Compute temperature'
         print *
         print *, '5. Exit the program.'
         print *
         print *, 'Your choice?'
         read *, choice

C-- warn them they've been naughty!
         if ((choice .lt. 1) .or. (choice .gt. 5)) then
            print *, 'Please enter a number from 1 to 5, inclusive.'
            print *
         endif
      enddo

C-- return the choice to the caller
      menu = choice
      return
      end

C input -- gets the appropriate input values from the user, given the
C  quantity being computed.
C
      subroutine input(quanty, pressr, volume, moles, temp)
      implicit none
      
C-- IN: the quantity to be computed (one of the menu choices from menu)
      integer quanty

C-- OUT: the appropriate values for the other values in the gas law
      real pressr, volume, moles, temp

C-- for some choice we don't understand, just return
      if ((quanty .lt. 1) .or. (quanty .gt. 4)) return

C-- For each menu choice (i.e. 1 means compute pressure), we need to
C-- prompt for the other quantities in the gas law.  This means that
C-- when the choice *isn't* to compute the pressure, we must ask for
C-- the pressure, and when the choice *isn't* to compute the volume,
C-- we need to ask for the volume, etc.

C-- first take care of getting the pressure
      if (quanty .ne. 1) then
         print *,
     +'Please enter the pressure of the gas, in atmospheres:'
         read *, pressr
      endif

C-- next take care of getting the volume
      if (quanty .ne. 2) then
         print *, 'Please enter the volume of the gas, in liters:'
         read *, volume
      endif

C-- next, the moles of gas
      if (quanty .ne. 3) then
         print *, 'Please enter the number of moles of the gas:'
         read *, moles
      endif

C-- and finally, the temperature
      if (quanty .ne. 4) then
         print *, 'Please enter the temperature of the gas,'
         print *, 'in degrees kelvin:'
         read *, temp
      endif
      
      return
      end


C main -- The part that brings it all together; as long as the user
C wants to keep computing values, prompts for the input data
C and prints out the desired quantity.
C 
      program main
      implicit none

C-- holds the users choice as returned by the menu
      integer choice

C-- function that returns a menu result
      integer menu

C-- various quantities from the ideal gas law
      real pressr, volume, moles, temp, gasconst

C-- the ideal gas constant R, is 8.31 J/(g-mole-K)
      parameter (gasconst = 8.31)

C-- keep doing computations until the user selects choice 5 to exit
      choice = 0
      do while (choice .ne. 5)
         choice = menu()

C-- get the needed quantities for the gas law
         call input(choice, pressr, volume, moles, temp)

C-- compute the desired quantity, based on the menu choice and output it
         if (choice .eq. 1) then
            pressr = moles*gasconst*temp/volume
            print *, 'The pressure is', pressr, ' atmospheres.'
            print *
         else if (choice .eq. 2) then
            volume = moles*gasconst*temp/pressr
            print *, 'The volume is', volume, ' liters.'
            print *
         else if (choice .eq. 3) then
            moles = pressr*volume / (gasconst*temp)
            print *, 'The number of moles is', moles
            print *
         else if (choice .eq. 4) then
            temp = pressr*volume / (moles*gasconst)
            print *, 'The temperature is', temp, ' degrees kelvin'
            print *
         endif

C-- go do the next computation
      enddo

      stop
      end