Calculation based on lag

[SAS_Muggle]

I have some data like the below:

Group Sort Value 

1 1 50

1 2 55

1 3 60

1 4 70

2 1 40

2 2 60

2 3 40

2 4 90

I want to create 2 new variables x and y such that if sort=1 then x=0 and y=value, otherwise x=value from previous row y and y= x + value, so like below:

Group Sort Value x y

1 1 50 0 50

1 2 55 50 105

1 3 60 105 165

1 4 70 165 135

2 1 40 0 40

2 2 60 40 100

2 3 40 100 140

..

Can anyone help with code. Whenever I run using lag and by sort and group, the new y does update based on the previous calculation. 

[WarrenKuhfeld]

Be sure to create the lag unconditionally, even if you use the value conditionally.

Be sure to read the documentation. If you have expectations on the behavior of lag as a matrix operator, you will find that they don't apply to a function in a general language like the SAS DATA step.

https://documentation.sas.com/doc/en/pgmsascdc/9.4_3.5/lefunctionsref/n0l66p5oqex1f2n1quuopdvtcjqb.h...

[Ksharp]

data have;
input Group Sort Value; 
cards;
1 1 50
1 2 55
1 3 60
1 4 70
2 1 40
2 2 60
2 3 40
2 4 90
;
data want;
 set have;
 retain _y;
 if sort=1 then do;x=0;y=value;_y=value;end;
  else do;x=_y;y=x+value;_y=y; end;
 drop _y;
run;

[mkeintz]

Do you really want to check for SORT=1,  or check for start of a new group.  Of course they are the same if SORT=1 is reliably the first (and only) observation of each incoming group.   There is a way to use LAG, as here:

data have;
input Group Sort Value  ; 
cards;
1 1 50
1 2 55
1 3 60
1 4 70
2 1 40
2 2 60
2 3 40
2 4 90
;run;

data want;
  set have;
  by group;
  x+lag(value);  if first.group then x=0;
  y+value;       if first.group then y=value;
run;

Of course you could use  "IF sort=1"  instead of "IF first.group".