Sets and Lists, again
Date: Fri, 19 May 2006 14:04:51 GMT
Message-ID: <7gkbg.4$J95.2_at_trndny05>
Recently, in a thread on implementing both threads and lists in a programming language, the example of lists or sets of Presidents arose. I mentioned that in a list of presidents, Grover Cleveland would appear once, but in a list of presidencies, he would appear twice.
Bob Badour asked what purppose would be served by a list of presidents, or words to that effect. I'm interested.
If one could have a set of presidents, why would one ever want a list? In general, if a language implements sets, why would the same language need to also implement lists? What does it buy you?
I'm thinking of Lisp, which implemented lists, but not sets. MDL (aka Muddle) implemented arrays, and that's one step closer to implementing sets, but not all the way.
SQL implemented sets, but not lists. Although local extensions of SQL do implement lists, e.g. "Segmented Strings" in DEC Rdb (aka Oracle/Rdb), it's not really part of the language as such.
I'm also thinking of Pascal, which implemented sets, (as bitmaps), and also lists, albeit implicitly. What I mean is that you can combine the concepts of "record" and "pointer" in Pascal to construct dynamic linked lists of whatevers. But Pascal was primarily for teaching and learning programming, and may have implemented both for precisely that purpose.
So, if you have sets, why do you need lists? Received on Fri May 19 2006 - 16:04:51 CEST