Start an increment count based on changes in data values - how?

[justchecking]

Okay - I realize this is probably extremely easy to do in SAS but I for the life cannot figure it out - everything I've tried has not worked the way I need it to and am at my wits end.  Any help would be greatly appreciated.

Let's say I have the following dataset in my work direcotry that consist of the first name of a customer and their date of purchase:

First_Name	Date_of_Purchase
John	01/01/2010
Jill	01/01/2010
Beth	01/01/2010
Will	02/01/2010
Jen	02/01/2010
Lori	03/01/2010
Tom	03/01/2010
Heidi	04/01/2010

What I need to do is an incremental count based on the change in date_of_purchase starting with the value of 1, results of which is shown below.  I've tried various do-loop statements but cannot seem to populate the variable count with the value  when the date remains unchanged (for example, for 01/01/2010, I've no problem populating first.date_of_purchase with 1, but subsequent cells under Count tied to 01/01/2010 remain empty.  I've about 3 years worth of purchase data, with over thousand of customers.  Thanks!  If any clarification is needed, don't hestitate to let me know. 

First_Name	Date_of_Purchase	Count
John	01/01/2010	1
Jill	01/01/2010	1
Beth	01/01/2010	1
Will	02/01/2010	2
Jen	02/01/2010	2
Lori	03/01/2010	3
Tom	03/01/2010	3
Heidi	04/01/2010	4

[art297]

Are you trying to increment based on number of purchases (which aren't shown in your sample data) or simply by number of months since 1JAN2010?

[Howles]

data want ;

set have ;

by Date_of_Purchase ;

Count + first.Date_of_Purchase ;

run ;

[Ksharp]

If the order were the same with the order at your post.

That will be easy.

data want;

set have;

if date_of _purchase ne lag(date_of_purchase) then count+1;

run;

Ksharp