program triangle

  implicit none

  real :: x,y
  ! the two known sides of the triangle

  real :: alpha
  ! the angle enclosed by the two known sides of the triangle

  real :: z,area
  ! the third side and the area of the triangle (these are functions)

  real :: pi
  ! constant

  integer :: again
  ! whether to perform another operation

  pi=3.1415926

  print *,'' ! skip a line
  print *,'Given the lengths x and y of two sides of a triangle,'
  print *,'and the angle alpha contained by these two sides'
  print *,'this program calculates the length of the third side z'
  print *,'and the area of the triangle'
  print *,''

  do

     do

        ! Ask user for values:
        print *,'Enter the value of x'
        read *,x
        print *,'Enter the value of y'
        read *,y
        print *,'Enter the value of alpha in degrees'
        read *,alpha

        ! Check that they are sensible:
        if(x>0. .and. y>0. .and. alpha>0. .and. alpha<180.) exit
        
        ! If they are not print a warning:
        print *,'WARNING: incorrect values entered, please re-enter the values'

     end do
        
     ! Convert alpha to radians
     alpha=alpha*pi/180.

     print *,''
     print '("The length of the third side is ",F10.5)',z(x,y,alpha)
     print '("The area of the triangle is     ",F10.5)',area(x,y,alpha)
     print *,''

     ! Check whether the user wishes to try again
     do
        print *,'Do you wish to try again? enter 1 for yes and 0 for no'
        read *,again
        ! Check if the value is correct
        if(again==1 .or. again==0) exit
        ! Otherwise print warning
        print *,'WARNING: please enter 1 or 0'
     end do
     print *,''

     ! If again==0 then end the program
     if(again==0) exit

  end do

  print *,'Program ended - Bye!'
  print *,''

end program triangle

real function z(side1,side2,angle)

  implicit none

  real,intent(in) :: side1,side2,angle
  ! the two sides and the enclosed angle

  ! calculate z:
  z=sqrt(side1**2.+side2**2.-2.*side1*side2*cos(angle))
     
end function z

real function area(side1,side2,angle)

  implicit none
  
  real,intent(in) :: side1,side2,angle
  ! the two sides and the enclosed angle

  ! calculate the area:
  area=0.5*side1*side2*sin(angle)

end function area