!*********************************************
!*     Elementary operations on fractions    *
!* ----------------------------------------- *
!* This program allows user to perform       *
!* operations on fractions such as:          *
!*         3/4 x 8/9 - 4/5                   *
!*                                           *
!* Sample run:                               *
!*                                           *
!* 3 4                                       *
!* '*'                                       *
!* 8 9                                       *
!* '+'                                       *
!* -4 5                                      *
!* '='                                       *
!*  Result is    -2 /    15                  *
!* 'q'                                       *
!* ----------------------------------------- *
!* Ref.: "Mathématiques par l'informatique   *
!*        individuelle (1) par H. Lehning    *
!*        et D. Jakubowicz, Masson, Paris,   *
!*        1982".                             *
!*                                           *
!*        F90 Version By J-P Moreau, Paris.  *
!*               (www.jpmoreau.fr)           *
!*********************************************
PROGRAM Fractions

implicit none
character cs
integer b,g,h,p,q,r,s,t,u
print *,''
!*** enter first fraction p/q (p=0 to exit) ***
10 read *, p,q ; if (p.eq.0.or.q.eq.0) stop
!*** enter operator:
!    = to view current result p/q
!    s to save current result
!    q exit program
!    +,-,*,/ arithmetic operator
!***
20 read *, cs
if (cs.eq.'=') then
  print *, ' Result is ',p,'/',q; goto 20
end if
if (cs.eq.'s') then 
  g=p; h=q; goto 10
end if
if (cs.eq.'q') stop
!*** enter next fraction r/s (if s=0, take current stored result) ***
read *, r,s ; if (r.eq.0) stop
if (s.eq.0) then 
  r=g; s=h
end if
!computing result
if (cs.eq.'+') then
  p=p*s+q*r
  q=q*s  !common denominator for all operations
  goto 50
end if
if (cs.eq.'-') then 
  r=-r
  p=p*s+q*r
  q=q*s  !common denominator for all operations
  goto 50
end if
if (cs.eq.'*') then 
  p=p*r
  q=q*s  !common denominator for all operations
  goto 50
end if
if (cs.eq.'/') then
  b=r
  r=s; s=b
  p=p*r
  q=q*s  !common denominator for all operations
end if
!simplification section
50 t=p
u=q
100 b=t-u*int(t/u)
if (b.eq.0) goto 110
t=u; u=b
goto 100
110 p=p/u; q=q/u
goto 20
end

!end of file fraction.f90