Get unique Week number for a particular Month in Excel - excel

Question

I have a source table data with the below date, week, month and year information.
CALENDAR_DATE WEEK MONTH YEAR
15-Jun-15 25 2015 / 06 2015
16-Jun-15 25 2015 / 06 2015
17-Jun-15 25 2015 / 06 2015
18-Jun-15 25 2015 / 06 2015
19-Jun-15 25 2015 / 06 2015
20-Jun-15 25 2015 / 06 2015
21-Jun-15 26 2015 / 06 2015
22-Jun-15 26 2015 / 06 2015
23-Jun-15 26 2015 / 06 2015
24-Jun-15 26 2015 / 06 2015
25-Jun-15 26 2015 / 06 2015
26-Jun-15 26 2015 / 06 2015
27-Jun-15 26 2015 / 06 2015
28-Jun-15 27 2015 / 06 2015
29-Jun-15 27 2015 / 06 2015
30-Jun-15 27 2015 / 06 2015
I am building a dependent data validation list where i need to extract all unique week numbers for each month as below
2016 2016 / 01 2016 / 02
2016 / 01 1 5
2016 / 02 2 6
2016 / 03 3 7
2016 / 04 4 8
2016 / 05
2016 / 06
2016 / 07
2016 / 08
2016 / 09
2016 / 10
2016 / 11
2016 / 12
Is there a formula with a combination of index, countif and/or vlookup that would serve the purpose. Any guidance would be super helpful

Answer 1

You can do it with an array formula. If you have the dates and week numbers starting in A2 and B2, and the required month number in D1,E1 etc. the formula is:-
=IFERROR(INDEX($B$2:$B$1000,MATCH(1,(COUNTIF(D$2:D2,$B$2:$B$1000)=0)*(MONTH($A$2:$A$1000)=D$1)*($A$2:$A$1000<>""),0)),"")
entered in D3 using CtrlShiftEnter
Note: there must be another approach to this which doesn't need all the week numbers to be listed - just start at the beginning of each month and step forward by 7 days each time, with a bit of logic for the end of the month.
Starting with the first of the month in G3, I came up with this formula to give the 8th, 15th etc. up to the last week of the month. Then you just need to get the WEEKNUM of each of those dates.
=IF((G3+7)>EOMONTH(G3,0),IF(WEEKNUM(EOMONTH(G3,0))>WEEKNUM(G3),EOMONTH(G3,0),""),G3+7)