Monday, February 20, 2017

Hack 4.13 Reversing Character Strings with the REVERSE Function

SAS Programming Professionals,

Did you know that: the REVERSE function can be very useful and also a lot of fun to use?

The REVERSE function can be used to return a character string in reverse order.  For instance, consider the following example:

data testit;

x = "SAS";
Y = reverse(x);

put "x= " x;
put "y= " y;


As you can see from the log:

1    data testit;
3    x = "SAS";
4    Y = reverse(x);
6    put "x= " x;
7    put "y= " y;
9    run;

x= SAS
y= SAS

… Y is the exact reverse of X!

A more practical application of this can be found when you need to pluck out the last node in something, such as a full-path file name.  Here is an example from a SAS program that I use to get the name of previously executed SAS programs from the LOGFILE variable in LOGPARSE SAS data sets:

/* Determine the SAS program name */
logtemp = reverse(strip(logfile));

r = index(logtemp,"\");

substr(logtemp,r,length(logtemp) - r + 1) = " ";

SASprogram = tranwrd(strip(reverse(logtemp)),"log","sas");

In the example, I know that logfile has the full path name of a SAS log, such as:  "c:\production\fy02\bigprog.log".  If the name of the log is "bigprog.log", then the program that was executed must be "".  That is what I want to capture in variable SASprogram.

So, the snippet of code first reverses LOGFILE, then looks for the first "/", which was originally the last "/", but is now the first one since the full path has been reversed.  It turns the rest of the contents of LOGTEMP into blanks.  Then, it sets SASprogram equal to the strip-ed, reverse (again) of LOGTEMP while translating "log" into "sas".  Neat, clean, and effective!

Best of luck in all your SAS endeavors!

(aka Michael A. Raithel)
Author of the new cult classic for computer programmers:  It Only Hurts When I Hit <ENTER>
Print edition: 
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The hack above is an excerpt from the book:  Did You Know That?  Essential Hacks for Clever SAS Programmers

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