Computing third variable using id and another variable

[desireatem]

Data A;

Imput id A;

Cards;

1. 4

1  7

1. 11
2. 1

2    10

3   11

3  13

Run;

I want to create a  variable B such
that

1. 4         4-4=0(id=1)

1    7           7-4=3

1. 11       11-7=4
2. 1         1-1=0(id=2)

2  10          10-1=9

3 11            11-11=0(id=3)

3  13          13-11=2(id=3)

[stat_sas · Accepted solution]

data want;

set A;

by id;

B=a-lag(a);

if first.id then B=0;

run;

[Astounding]

Not as well known, but equally effective here, is a cousin of the LAG function.  These two statements produce identical results:

B=a-lag(a);

B=dif(a);