How can I get the last occurrence of a repeated item in Numbers

Question

Hi everyone, I'm hoping this question has a straightforward answer.
I have a simple 2-column table (Table 1). Column A is a list of names. Column B is a list of dates where those names have had appointments. (Most names in Column A are repeated because they have had multiple appointments).
I'd like to create an automated table based Table 1 that lists each name from Column A only once, with the last appointment that each person has had.
Any ideas, I'd be grateful.
Thanks!

Answer 1

Hi Paul,
Late to the party, as I was called to dinner shortly after starting on a solution and wasn't able to get back to it for a few hours.
This was worked out in Numbers '09 (v2.3), but the formulas should work as well in Numbers 3. The solution requires the data on the Data table to be entered (or sorted) in ascending order of dates. Dulicate dates, though not included in the example, should not create an issue.
The added column (Name index—column C) may be hidden. It is needed to mark the first occurrence of each name, and required only if you want the name list in column A of the summary table (Most recent appointment) filled automatically.
Formulas:
Data::C2, and filled down to the end of column C: =IF(COUNTIF($A$2:A2,A)=1,MAX($C$1:C1)+1,"")
COUNTIF($A$2:A2,A) counts the number of occurences of the name in the same row as the formula from row 2 of column A to the row containing the formula. IF that count is 1, the number of the row is placed in the cell, otherwise the cell receives a null string ( "" ).
Most recent appointment table:
A2, and filled down to end of column A:
=IF(ROW()-1>MAX(Data :: C),"",OFFSET(Data :: $A$1,MATCH(ROW()-1,Data :: $C,0)-1,0))
If the row number minus 1 is greater than the number of distinct names in the index column, a null string is placed in the cell. Otherwise, the OFFSET function uses MATCH to determine the row in which to find the next name (in order of first appearance) and places that name in its cell.
B2 and filled down: =IF(LEN(A)>0,LOOKUP(A,Data :: $A,Data :: $B),"")
IF here uses LEN() to determine if there is data (a name) in column A. If so, then LOOKUP looks up the name in column A of the Data table and returns the date from the same row of column B. If there's no data (name) in column A, IF places a null string in the cell.
This formula takes advantage of the fact that LOOKUP searches from the bottom of the column, and returns the result from the first match it finds.
Regards,
Barry