df_filter_rows_string_pattern Function

    function df_filter_rows_string_pattern(df, col_index, pattern) result(filtered_df)
        type(data_frame), intent(in) :: df
        integer, intent(in) :: col_index
        character(len=*), intent(in) :: pattern
        type(data_frame) :: filtered_df

        integer, dimension(:), allocatable :: selected_rows
        integer :: i, count, idx
        character(len=:), allocatable :: val
        type(column) :: col

        col = df % get_data_col(col_index)

        if (col % get_type() /= CHARACTER_NUM) error stop "column is not character type"

        ! Count matching rows
        count = 0
        do i = 1, df % nrows()
            val = col % getch(i)
            if (index(val, pattern) > 0) count = count + 1
        end do

        ! Collect matching row indices
        allocate (selected_rows(count))
        idx = 0
        do i = 1, df % nrows()
            val = col % getch(i)
            if (index(val, pattern) > 0) then
                idx = idx + 1
                selected_rows(idx) = i
            end if
        end do

        ! Create filtered dataframe
        call filtered_df % new(df % get_max_char_len())
        do i = 1, df % ncols()
            call copy_filtered_column(df, filtered_df, i, selected_rows)
        end do

        deallocate (selected_rows)
    end function df_filter_rows_string_pattern