naz@hslrswi.hasler.ascom.ch (Norman H. Azadian) (02/26/91)
Recently I found out the hard way about something I consider to be
an anomaly in string indexing. I consider it an anomaly because
it was, at least for me, totally unexpected. To wit:
s := "-D"
x := (s[3:0] | "default")
"Obviously" accessing the 3rd character in a 2-character string
should fail and x would get the string "default". It turns out,
non-intuitively enough, that I get an empty string, exactly as
if I had tried s[1:1].
Thinking about it, I have to admit that it makes a certain amount
of sense. In fact, it makes a lot of sense when I think about it.
However it was totally unexpected and so I thought I'd mention it
to perhaps help some other hapless soul from falling into the same
pit.
NHA
--
PAPER: Norman Azadian; Ascom AG; Belpstrasse 23; 3000 Berne 14; Switzerland
INTERNET: naz%hslrswi.uucp@uunet.uu.net
UUCP: ...{uunet,ukc,mcvax,...}!chx400!hslrswi!naz
VOICE: +41 31 63 2178 BITNET: naz%hslrswi.UUCP@cernvax.BITNETgoer%sophist@GARGOYLE.UCHICAGO.EDU (Richard Goerwitz) (02/26/91)
Recently I found out the hard way about something I consider to be
an anomaly in string indexing. I consider it an anomaly because
it was, at least for me, totally unexpected. To wit:
s := "-D"
x := (s[3:0] | "default")
"Obviously" accessing the 3rd character in a 2-character string
should fail and x would get the string "default". It turns out,
non-intuitively enough, that I get an empty string, exactly as
if I had tried s[1:1].
Icon's string indexing methods are definitely non-intuitive for people
who acquire it as a second, third, etc. language (which means virtually
everyone). I guess the idea that people need to catch on to is that
subscripts deal with string positions, and not the index of characters
within strings. What throws people off is that s[3] would work as in
some other languages (producing the third character in s). In reality,
though, this is shorthand in Icon for s[3+:1], i.e. s[3:4].
Of course, if we want to talk about unintuitive schemes, let's not for-
get languages where s[0] indexes the _first_ array element in s.
s := "-D"
s ? { x := move(3) | "default" }
s := "-D"
s ? { x := (tab(-3), tab(0)) | "default" }
s := "-D"
x := ("" ~== s[3:0]) | "default"
-Richard (goer@sophist.uchicago.edu)