option nolet  ! don't need let statement
print "************************************************"
print "* math  : factorial 'error catch ',version1    *"
print "* peter.vlasschaert@gmail.com ,16/05/2018      *"
print "************************************************"
declare function factorial
print
print "*******"
print " x < 0 "
print "*******"
x=-5
print " x = ";x
call test_factorial(x)
print "*******"
print " y = 0 "
print "*******"
y=0
print " y = ";y
call test_factorial(y)
print "******"
print " u >0 "
print "******"
u=5
print " u = ";u
call test_factorial(u)
print "*******"
print " v=170 "
print "*******"
v=170
print " v = ";v
call test_factorial(v)
print "*******"
print " w=171 "
print "*******"
w=171
print " w = ";w
call test_factorial(w)
end

sub test_factorial(m)
declare function factorial
if factorial(m) <> 0 then
   print " factorial("& str$(m) & ")= ";factorial(m)
end if
end sub

function factorial(x)
if x < 0 or x <> int(x) then
   a$ = "error :factorial of negative or non-integer." 
   print " factorial("& str$(x) & ")= " & a$
elseif  x > 170 then
  b$ = "error:overflow"
  print " factorial("& str$(x) & ")= " &  b$
  exit function
elseif x >1 and x <= 170 then
  factorial = x*factorial(x-1)
else
  factorial = 1
end if
end function