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Converting Strings to Lists

Sometimes it is useful to convert text strings to lists. For example, to get substantial input, read-line is most convenient, but it returns a string. If the objective is to parse the input, it is much more convenient to have a list of words than a string.

Here is an example of code to convert a string to a list:

(defun string-to-list (str)
   (do* ((stringstream (make-string-input-stream str))
         (result nil (cons next result))
         (next (read stringstream nil 'eos)
               (read stringstream nil 'eos)))
        ((equal next 'eos) (reverse result))))
> (string-to-list "this is a string of text")
Because of its reliance on read, this function will not work with certain kinds of punctuation. For example:

> (string-to-list "Commas cause problems, see")  

Error: A comma has appeared out of a backquote.
Error signalled by READ.
Broken at READ.  Type :H for Help.
If punctuation is likely to appear in input, then it is necessary to use read-char, which reads one character at a time. It is then possible to inspect each character and process it appropriately if it is problematic. Exactly how to do this will not be covered here as it makes a nice exercise to develop your understanding of Lisp input processing.

© Colin Allen & Maneesh Dhagat
March 2007