Content only in every 25th case

[Sue Farb]

I want to create a variable which contains the value of 1 in every 25th case, all other cases should hava a missing value. The dataset has in total 1000 cases.
How can I do this? Any suggestions? Thanks for your help!

[Carlo Lazzaro]

Sue:
welcome to the list.
You may want to try:

set obs 25
forvalues i = 1(1)40 {
generate x`i' =1 in 25
replace x`i' =0 if x`i'==.
}
stack x1-x40, into(A)

[Nick Cox]

No loop is needed

gen foobar = mod(_n, 25) == 0

See also http://www.stata-journal.com/sjpdf.h...iclenum=pr0031

[Nick Cox]

cond(mod(_n , 25) == 0, 1, .)

[Carlo Lazzaro]

Thanks, Nick.

[Nick Cox]

Note further that if what is desired is to flag (select, do something special) to every 25th observation that no new variable is necessary as

if mod(_n, 25) == 0

or

if !mod(_n, 25)

will do it.

(These can be put into words as flagging if the observation number is divisible by 25.)

If you do prefer an indicator variable then a 1 or 0 variable is usually easier to work with than a 1 or missing variable.

[Sue Farb]

Thank you for your help and your solutions!

[Marcos Almeida]

Nick's advice is the ultimate one, definitely.

Just for the sake of amusing myself, keeping up with the learning process and presenting an alternative, albeit less elegant, I gather Sue could also try this:

(assuming she already has "oldvar" as a sequence)

. gen newvar = 1 if oldvar == 25

Below, just a description of the steps, assuming she needs to create the data set from scratch:

The first 2 lines are just to create a data set with 1000 observations and a sequence of number from 1 to 25

. set obs 1000
number of observations (_N) was 0, now 1,000

. egen float myseq = seq(), from(1) to(25) block(1)

*/ To get 1 for every time the sequence is equal to 25
. gen tag25 = 1 if myseq ==25
(960 missing values generated)

*/ Shall you have sequences longer than 25, say, the double (50):
. egen float myseq2 = seq(), from(1) to(50) block(1)

*/ you will still get the same result, as wished; below, just the same, with a different name for the variable
. gen tag25_b = 1 if myseq2 ==25
(980 missing values generated)

. count if tag25 !=.
  40

. count if tag25_b !=.
  20