topic Re: First.variable and Last.Variable initial value before execution in SAS Programming

First.variable and Last.Variable initial value before execution

nkm1 — Wed, 07 Oct 2015 20:22:53 GMT

Hi,

Is it normal to set initial value of first. and last. to 1.

Here is example.

data temp;
store_id=101;
sale_eff_date = '01may2015'd;
sale_end_date = '15may2015'd;
total_amount = 100;
output;
sale_eff_date=101;
sale_eff_date = '17may2015'd;
sale_end_date = '30may2015'd;
total_amount = 200;
output;
format sale_eff_date sale_end_date mmddyy10.;
run;
data temp1;
put "before exec " _all_;
set temp;
by store_id;
put "after exec " _all_;
run;

before exec store_id=. sale_eff_date=. sale_end_date=. total_amount=. FIRST.store_id=1 LAST.store_id=1 _ERROR_=0 _N_=1

after exec store_id=101 sale_eff_date=05/01/2015 sale_end_date=05/15/2015 total_amount=100 FIRST.store_id=1 LAST.store_id=0 _ERROR_=0 _N_=1

Why first.store_id and last.store_id is set to 1 before execution. Isn't that should be set to missing ? Why SAS set initial value to 1 for first. and last.variable ?

Thanks

Re: First.variable and Last.Variable initial value before execution

hbi — Wed, 07 Oct 2015 21:52:58 GMT

Here's a fun experiment to try ...

/* create empty dataset */
data temp2;
  var1 = 0;
  delete;
run;

data temp3;
  put "before exec " _all_;
  set temp2;
  by var1;
  put "after exec " _all_;
run;

Log output:

18         /* create empty dataset */
19         data temp2;
20           var1 = 0;
21           delete;
22         run;

NOTE: The data set WORK.TEMP2 has 0 observations and 1 variables.
NOTE: DATA statement used (Total process time):
      real time           0.00 seconds
      cpu time            0.00 seconds
      
23         
24         data temp3;
25           put "before exec " _all_;
26           set temp2;
27           by var1;
28           put "after exec " _all_;
29         run;

before exec var1=. FIRST.var1=1 LAST.var1=1 _ERROR_=0 _N_=1
NOTE: There were 0 observations read from the data set WORK.TEMP2.
NOTE: The data set WORK.TEMP3 has 0 observations and 1 variables.
NOTE: DATA statement used (Total process time):
      real time           0.00 seconds
      cpu time            0.00 seconds

Re: First.variable and Last.Variable initial value before execution

Patrick — Wed, 07 Oct 2015 22:40:28 GMT

First and Last variables are flags and boolean in their nature. They should only have two values (True, False).

You have your put statement before the SET so I would assume what you get is how SAS initializes these temporay variables.

It's interesting to know to what SAS sets the values during initialization but I can't think of any use case where this really matters or would cause issues.

Re: First.variable and Last.Variable initial value before execution

nkm1 — Wed, 07 Oct 2015 23:57:36 GMT

It matters when using it in loop.

data temp;
store_id=101;
sale_eff_date = '01may2015'd;
sale_end_date = '15may2015'd;
total_amount = 100;
output;
store_id=101;
sale_eff_date = '17may2015'd;
sale_end_date = '30may2015'd;
total_amount = 200;
output;
store_id=102;
sale_eff_date = '17may2015'd;
sale_end_date = '30may2015'd;
total_amount = 200;
output;
format sale_eff_date sale_end_date  mmddyy10.;
run;


data temp1;
put "before exec " _all_;
do until(last.store_id);
set temp;
by store_id;
total = sum(total,total_amount);
first_sd=first.store_id;
last_id=last.store_id;
output;
put "after exec " _all_;
end;
run;

data temp2;
put "before exec " _all_;
do while(last.store_id);
set temp;
by store_id;
total = sum(total,total_amount);
first_sd=first.store_id;
last_id=last.store_id;
output;
put "after exec " _all_;
end;
run;

Re: First.variable and Last.Variable initial value before execution

RW9 — Thu, 08 Oct 2015 08:55:20 GMT

It may matter then, but then the reason is why are you doing it that way in the first place? To attain the end result, this would be the way to code it, with a retain statement:

data temp1;
  set temp;
  by store_id;
  retain total;
  if first.store_id then total=0;
  total=sum(total,total_amount);
run;

Re: First.variable and Last.Variable initial value before execution

MikeZdeb — Thu, 08 Oct 2015 15:03:27 GMT

Hi, you could also use a SUM statement and make use of the automatic retain of variables created in that statement (plus initialization at zero, not missing). The following makes use of the value of FIRST.STORE_ID ...

data temp;
input store_id total_amount @@;
datalines;
1 10 1 30 1 50 2 40 2 70 2 90 3 10 3 20
;

data temp1;
set temp;
by store_id;
total + total_amount - (first.store_id * total);
run;

data set TEMP1 ...

total_
Obs store_id amount total

1         1         10        10
2         1         30        40
3         1         50        90
4         2         40        40
5         2         70       110
6         2         90       200
7         3         10        10
8         3         20        30

Tips:Between and Within Group Counters

http://www.sascommunity.org/wiki/Tips:Between_and_Within_Group_Counters