Path: utzoo!utgpu!water!watmath!clyde!rutgers!sdcsvax!ucbvax!ARIZONA.EDU!gudeman
From: gudeman@ARIZONA.EDU ("David Gudeman")
Newsgroups: comp.lang.icon
Subject: Substring positions
Message-ID: <8712302319.AA15498@megaron.arizona.edu>
Date: 30 Dec 87 23:19:17 GMT
References: <2321@ihuxy.ATT.COM>
Sender: daemon@ucbvax.BERKELEY.EDU
Distribution: inet
Organization: The ARPA Internet
Lines: 29


   From: ihnp4!ihuxy!lied@ucbvax.Berkeley.EDU  (Bob Lied)

   ....  In an example and a solution in chapter 5 of the book (I
   assume you know the book I refer to), the size of a substring is
   calculated with

	   *line[i:j]
   where I would intuitively expect to use
	   i-j

   I am inferring from this that string positions are
   like other languages' pointers in that their numerical
   values are not to be trusted, and I should limit their
   use to assignments, comparisons, and defined operations
   like subscripting.  Am I right?

Well, not really.  String positions are integer positions _between_
the characters of the string, where 1 is the position at the beginning
of the string.  However, non-positive integers are also valid string
positions (with 0 at the end of the string).  So if i or j is
non-positive, it is not true that "*line[i:j] = j-i" (I assume you
meant "j-i" rather than "i-j").

In the example in the book, both i and j are guaranteed to be
positive, so "j-i" works.  Notice though, that in most languages the
positive integers refer to character positions, so the correct formula
for the length of a substring from i to j is "j-i+1".  Perhaps the
authors wanted to avoid confusing readers with this subtlety.