set_header_at_index Subroutine

    subroutine set_header_at_index(this, index, header)
        class(data_frame), intent(inout) :: this
        integer, intent(in) :: index
        character(len=*), intent(in) :: header

        if (.not. this % with_headers) error stop "data frame has no headers"
        if (index < 1 .or. index > this % num_cols) error stop "header index out of range"

        this % headers(index) = header
    end subroutine set_header_at_index