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How do I create individual identifiers numbered from 1 upwards?

Case 1. I want to create variable id containing 1, 2, 3, ...

Type

        . gen id = _n

_n is the Stata way of referring to the observation number.

In a 10-observation dataset, _n takes on the values 1, 2, ..., 10.

Case 2. I already have an id variable, and I have multiple observations per id, but I want a new id variable containing 1 for the first id, 2 for the second, and so on.

Such questions often arise with panel data and in other circumstances. Perhaps the identifier variable is a string — id "numbers" 1A038, 2B217, ... — and you need numeric identifiers — 1, 2, ... — because some Stata commands require them. Perhaps the original id is numeric — of the form 102938, 149384, 150394, ... — but you want to draw a graph using the identifier as one of the axes and want the data points equally spaced.

Answer 1.
To create a new variable newid from the existing variable oldid, whether oldid is string or numeric, type

        . egen newid = group(oldid)

The new variable newid will contain 1 for the first value of oldid, 2 for the second value, and so on.

Answer 2.
To create a new variable newid from the existing variable oldid, whether oldid is string or numeric, type

        . sort oldid
        . by oldid: gen newid = 1 if _n==1
        . replace newid = sum(newid)
        . replace newid = . if missing(oldid)

Both answers yield the same results: the four lines of answer 2 amount to what egen does. It is, however, worth understanding answer 2.

We start with existing identifier ID, which may be either a numeric variable or a string variable.

        . sort oldid

This command puts the observations in the order of oldid.

        . by oldid: gen newid = 1 if _n == 1 

This command creates a new variable newid that is 1 for the first observation for each individual and missing otherwise. _n is the Stata way of referring to the observation number; in a 10-observation dataset, _n takes on the values 1, 2, ..., 10. When _n is combined with by, however, _n is the observation number within by-group, in this case, within oldid. If there were three oldid==1 observations followed by two oldid==2 observations in the dataset, _n would take on the values 1, 2, 3, 1, 2. Thus, by ...: ... if _n==1 is a way to refer to the first observation in each by-group. See the sections of [U] indexed under by varlist: prefix.

by oldid: gen newid=1 if _n==1 sets newid to 1 in the first observation of each oldid.

        . replace newid = sum(newid)

This command replaces newid by its cumulative or running sum.

        . replace newid = . if missing(oldid)

This command puts missing value into newid, where oldid contained missing value. This step is probably unnecessary because if oldid really is an ID variable, it should never contain missing anyway.

Let us see how that works for a simple dataset. Missing values (.) make no difference to a cumulative sum. In that context, they are treated as numerically equal to 0.

        oldid     newid (as created)   newid (as replaced) 
        1             1                    1 
        1             .                    1
        1             .                    1 
        1             .                    1
        22            1                    2 
        22            .                    2 
        22            .                    2
        33            1                    3
        33            .                    3

We have said that both answers are the same. But there is an advantage to the first. Using the label option

        . egen newid = group(oldid), label

will ensure that the values of the existing variable oldid (or their value labels if they exist) are copied across as value labels for the new variable newid. That way, you get the best of both worlds, tidy identifiers with values 1 and up and labels that preserve information from your existing dataset.