Stata: Create id by group

When doing your data analysis, sometimes you will encounter the following situation: in your dataset, everyone has an unique id. However, their IDs are long and each participant has multiple record (or the dataset is in a long format).

To visualize your data, you need to create a new ID for each individual regardless of how many records each person has. For example, the first person has three records, and we would like to assign a new ID 1 for the first person, and the second person would be 2.

Though it sounds difficult and tedious, it is not difficult to do so.  

egen id = group(oldid)

Just one line and your problem will be solved.

Reference: http://www.stata.com/support/faqs/data/group.html