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program optional_01
implicit none
character(len=1) :: response = "Y"
if (lower(response) /= 'y') error stop
print *, lower(response)
contains
elemental pure function lower(str, begin, end) result (string)
character(*), intent(in) :: str
character(len(str)) :: string
integer, intent(in), optional :: begin, end
integer :: i
integer :: ibegin, iend
string = str
ibegin = 1
if (present(begin)) then
ibegin = max(ibegin, begin)
end if
iend = len_trim(str)
if (present(end)) then
iend = min(iend,end)
end if
do i = ibegin, iend
select case (str(i:i))
case ('A':'Z')
string(i:i) = char(iachar(str(i:i)) + 32)
case default
end select
end do
end function lower
end program
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