How can I create a trailing count for binary data in Stata?

Question

In Stata, I currently have a data set that looks like:

I am trying to create a "trailing counter" in column B so that it looks like:

Here, the counter starts at 1 and for every time a "1" appears in A, B adds on a value.

This seems to be very simple, but I am not sure how to do this exactly. Here is what I have done so far:

Assuming the column A is called "A" in Stata,

I use:

gen B = A + A[_n - 1]

But, this gives me something off. I am not sure how to proceed, would anyone have any tips?

Answer 1

Here's one way:

clear all
set more off

*----- example data -----

input ///
var1
0
0
0
0
1
0
0
1
0
0
0
end

list, sep(0)

*----- what you want -----

gen counter = sum(var1) + 1

list, sep(0)

The sum() function will give you a cumulative sum. See help sum(). This is a very basic Stata function. A search sum would have gotten you there quickly.

Your approach fails because you are only adding up, for each observation, the "current" value of A with the previous value of itself. That might sound like a cumulative sum, but think about it and you will see that it isn't.

With your code and my data, the result would be:

     +----------------+
     | var1   counter |
     |----------------|
  1. |    0         . |
  2. |    0         0 |
  3. |    0         0 |
  4. |    0         0 |
  5. |    1         1 |
  6. |    0         1 |
  7. |    0         0 |
  8. |    1         1 |
  9. |    0         1 |
 10. |    0         0 |
 11. |    0         0 |
     +----------------+

The first observation for counter is missing (.). That is because there's no previous value for the first observation of var1, so Stata does something like var1[1] + var1[0] = 0 + . = ..

The second observation for counter is var1[2] + var1[1] = 0 + 0 = 0.

The fifth observation for counter is var1[5] + var1[4] = 1 + 0 = 1.

The seventh observation for counter is var1[7] + var1[6] = 0 + 0 = 0. And so on.