In my recent post on how to understand character vectors in SAS/IML, I left out an important topic: How can you allocate a character vector of a specified length? In this article, "length" means the maximum number of characters in an element, not the number of elements in a vector.

In the SAS DATA step, you can use the LENGTH statement to specify the maximum number of characters that can be stored in a variable. Thus the length of a variable can be greater than the length of any string that it contains. However, the SAS/IML language does not support the LENGTH statement. This leads to an interesting problem: How can you create a vector in SAS/IML such that each element can contain up to k characters?

This problem comes up in statistical programming because sometimes you need to enforce a limit on the length of a character vector. For example, if you are creating strings that will be used to name SAS variables, the strings must not exceed 32 characters.

There are a couple of possible solutions. In SAS/IML 12.3, which was released with SAS 9.4, you can use the BLANKSTR function to create a blank string with a given number of characters. The following statements create a string that contains 10 space characters. You can then use the J function or the REPEAT function to create a character vector.

proc iml;
/* generate blank strings of an arbitrary size */
maxChars = 10;                /* maximum number of characters to store */
Blanks = BlankStr(maxChars);  /* string of length 10 (SAS/IML 12.3)    */
c = j(1, 5, Blanks);          /* allocate 1x5 vector of blank strings  */

If you do not have SAS/IML 12.3, use the SUBPAD function in Base SAS. The following statement duplicates the functionality of the BlankStr function.

Blanks = subpad(" ", 1, maxChars); /* return string of length 10 */

I'll leave it as an exercise to figure out why this call to the SUBPAD function creates a string with 10 blank characters!

Have you ever faced the problem of generating a string with a specified number of characters? What was your solution?