Select first/second half of rows to form a subset

[#1]

I have a question about how to select first half or second of rows to form a subset. I don't want to manually type in 1,2,3,4,5,...,10 (if there are 20 rows) because my rows could be different from time to time. I have a equation that define number of rows that will appear in the original table. Is there anything like select row (1:10) or others that make it easier?

[#2]

You can use the following JSL

dt=current data table();
dt<<select where(Row()<=N Rows(dt)/2);

dt2=dt<<subset(selected rows(1), selected columns(0));