Integer :: a

a = 5
print *, a, "fat(a) = ", fat(a)
print *, a, "fat(a) = ", fatR(a)

Contains

function fat(a) result (f)
Integer, Intent(In) :: a
Integer :: i
f = 1

Do i=1,a
        f = f*i
EndDo

end function

recursive function fatR(a) result (f)
Integer, Intent(In) :: a
Integer :: i

if (a .le. 1) then
        f = 1
else
        f = a*fatR(a-1)
endif

end function
end