Technology Tales

Using SAS FILENAME statement to extract directory file listings into SAS

The filename statement’s pipe option allows you to direct the output of operating system commands into SAS for further processing. Usefully, the Windows dir command (with its /s switch) and the UNIX and Linux equivalent ls allow you get a file listing into SAS. For example, here’s how you extract the list of files in your UNIX or Linux home directory into SAS:

filename DIRLIST pipe 'ls ~';
data dirlist;
length filename $200;
infile dirlist length=reclen;
input buffer $varying200. reclen;
run;

Using the ftp option on the filename statement allows you to get a list of the files in a directory on a remote server, even one with a different operating system to that used on the client (PC or server), very useful for cases where cross-platform systems are involved. Here’s some example code:

filename dirlist ftp ' ' ls user='user' host='host' prompt;
data _null_;
length filename $200;
infile dirlist length=reclen;
input buffer $varying200. reclen;
run;

The PROMPT option will cause SAS to ask you for a password and the null string is where you would otherwise specify the name of a file.

Finding the number of observations in a SAS dataset

There are a number of ways of finding out the number of observations (also known as records or rows) in a SAS data set and, while they are documented in a number of different places, I have decided to collect them together in one place. At the very least, it means that I can find them again.

First up is the most basic and least efficient method: read the whole data set and increment a counter to pick up its last value. The END option allows you to find the last value of count without recourse to FIRST.x/LAST.x logic.

data _null_;
set test end=eof;
count+1;
if eof then call symput(”nobs”,count);
run;

The next option is a more succinct SQL variation on the same idea. The colon prefix denotes a macro variable whose value is to be assigned in the SELECT statement; there should be no surprise as to what the COUNT(*) does…

proc sql noprint;
select count(*) into :nobs from test;
quit;

Continuing the SQL theme, accessing the dictionary tables is another route to the same end and has the advantage of needing to access the actual data set in question. You may have an efficiency saving when you are testing large datasets, but you are still reading some data here.

proc sql noprint;
select nobs into :nobs from dictionary.tables where libname=”WORK” and memname=”TEST”;
quit;

The most efficient way to do the trick is just to access the data set header. Here’s the data step way to do it:

data _null_;
if 0 then set test nobs=nobs;
call symputx(”nobs”,nobs);
stop;
run;

The IF/STOP logic stops the data set read in its tracks so that only the header is accessed, saving the time otherwise used to read the data from the data set. Using the SYMPUTX routine avoids the need to explicitly code a numeric to character transformation; it’s a SAS 9 feature, though.

I’ll finish with the most succinct and efficient way of all: the use of macro and SCL functions. It’s my preferred option, and you don’t need a SAS/AF licence to do it, either.

%let dsid=%sysfunc(open(work.test,in));
%let nobs=%sysfunc(attrn(&dsid,nobs));
%if &dsid > 0 %then %let rc=%sysfunc(close(&dsid));

The first line opens the data set, and the last one closes it; this is needed because you are not using data step or SCL and could leave a data set open, causing problems later. The second line is what captures the number of observations from the header of the data set using the SCL ATTRN function called by %SYSFUNC.