first difference data (selection)

[Golf]

Hello, 

   I have the following data set.

city     Year     D87      X     dx

  1        82      0           a       -

  2        87      1          b       (b-a)

  3        82      0          c       -

  4        87      1          d      (d-c)

  5        82      0          e       -

  6        87      1          f       (f-e)

What procedure should be used to achieve dx ?

Thank You

[PGStats]

In a data step:

pX = lag(X);
if D87 then dx = X - pX;
drop pX;

[Golf]

Thank You.

[Golf]

L_x = lag(x);
dx = x-L_x;
if d87 = 0 then dx = .;

[mkeintz]

Or you could use the DIF function, defined as X-lag(X):

   dx=dif(x);

   if d87=0 then dx=.;

Or slightly more economical. 

    dx= ifn(d87=1,dif(x),.) ;

[Golf · Accepted solution]

Thank you for your suggestion.
Best,

[FreelanceReinh]

Hello @Golf,

Glad to see that mkeintz's solution worked for you. Then it would be fair and help later readers if you marked his helpful reply as the accepted solution, not your own "thank you" post. Could you please change that? It's very easy: Select his post as the solution after clicking "Not the Solution" in the option menu (see icon below) of the current solution.

[Kurt_Bremser]

Using the IFN() and DIF() functions, as suggested by @mkeintz, is the most economical way.