CSV数据生成降序

Question

I have a CSV file to read and the sample data is below:

SEQ     NAME    GROUP
 1      MARK      A
 2      KEVIN     A
 3      JOE       A
 1      MARY      B
 2      JANE      B
 3      ANN       B
 4      MAY       B

I can't find an easy way to do it in Java that will read the above CSV file and add another column which is just a descending sequence like the REV column:

SEQ     REV     NAME   GROUP
1       3       MARK    A
2       2       KEVIN   A
3       1       JOE     A
1       4       MARY    B
2       3       JANE    B
3       2       ANN     B
4       1       MAY     B

I know how to use a buffered reader or a scanner but don't know how to create a reverse seq based on group column since you have to know the maximum length for each group.

Any ideas please?

Update: Sorry but I have to use plain java without any library as we are working with a legacy system.

Answer 1

You can try using supercsv , which will give you map of column value map(check for method CsvMapReader) then you can iterate and group it according to your requirement.

Hope this helps

Answer 2

If you are reading one line, you don't know the group length for the current group. So split it off into two passes.

Pass 1 is only to determine the group length for each group. You can store it in a Map<String, Integer> with the group name as key and the (so far) maximum group length. For each line you have to look, if there is already an entry for that group. If yes, increment the value. If not store 1 for this group.

Then in Pass 2 you read the file line by line a second time. But with the above map, you already know the maximum group length for the current group.

If the file is too big to store all groups in memory then you should store them in a file, eg with a RandomAccessFile . But it is easier to increase the memory.