Generating sequence numbers from cluster values [VLOOKUP to the rescue]

Last night I got an email from Joshua, one of our readers with the subject – Hard Excel problem. Hard?!?, at this stage of summer, the hard problems seem to be (in no particular order),

1. Lack of good quality mangoes to eat
2. Intense heat and humidity
3. Lack of good quality mangoes to eat

Yes, I like mangoes.

Any how, back to Joshua’s email, So I got curious and read it. He is facing an interestingproblem.

I have a very difficult inquiry I am hoping you might be able to solve…

Is there a formula (i.e., without using VBA) that will look at another columns values and provide a new sequential number (i.e., reordered) when the value changes; however, keep the same sequential number for the duplicates?

Below is a table with two columns. […] I now need to rank order those cluster groups. Since cluster 12 appears first it would get a value of ‘1’ and all of the cluster 12’s should now be a ‘1’. Since cluster 4 appears next it would get a rank of 2, etc…

Well, it is an interesting problem for sure. But hard problem, it isn’t. For really hard problems, refer to my list above.

So how to generate the sequence numbers?

Logic: If a value is already listed, we fetch corresponding sequence number. Else, we generate a new sequence number.

Implementation: Simple, we use VLOOKUP.

Assuming the cluster values are in column B, from B4 onwards, in C4, write

=IFERROR(VLOOKUP(B4,$B$3:C3,2,FALSE), SUM(MAX($C$3:C3),1))

Let’s examine the formula.

VLOOKUP(B4,$B$3:C3,2,FALSE) portion: This one looks value in column B and tries to find corresponding sequence value in column C.

SUM(MAX($C$3:C3),1) portion: Gives us next sequence number

IFERROR(VLOOKUP(…), SUM(…)) portion: This does the magic of choosing either existing sequence number or generating a new one.

For more, read about VLOOKUP and IFERROR formulas.

Sequence number generation – Example spreadsheet

Play with the sequence number generation spreadsheet embedded below or Click here to grab a copy of the file.

How would you generate the sequence numbers?

Its your turn to take a crack at the hard problem. How would you solve it? Go ahead and share your answers in the comments.

More hard problems – solved:

Hard problems are not new at Chandoo.org. We take lob vlookups and sumproducts regularly to crack them. Here are few examples:

• Generating unique invoice numbers in Excel
• Extracting unique values list and sort them by occurrence
• Finding unique, duplicate and missing values in a list
• More hard problems in Excel