ftp.cs.arizona.edu/ftp.cs.arizona.edu.tar/ftp.cs.arizona.edu/icon/newsgrp/group96a.txt/000131_icon-group-sender _Mon Jun 10 17:45:25 1996.msg

Internet Message Format | 1996-09-05 | 2KB

Received: by cheltenham.cs.arizona.edu; Tue, 11 Jun 1996 07:45:48 MST
To: [e-mail]
Date: Mon, 10 Jun 1996 17:45:25 GMT
From: [e-mail] (Julian Pardoe LADS LDN X1428)
Message-Id: <[e-mail]>
Organization: Merrill Lynch Europe
Sender: [e-mail]
References: <[e-mail]>
Reply-To: [e-mail]
Subject: Re: m3 should have SWAP(a,b)
Errors-To: [e-mail]
Status: O

In article <[e-mail]>, [e-mail] (Lee Odegard) writes:
-->[e-mail] (Dave Schaumann) writes:
-->
-->>As you say.  For my part, the only time I've found 'swap' to be a useful
-->>operation is when I am writing sort algorithm (or sort-related algorithms,
-->>like heap manipulation).
-->
-->In my own work, I've seen many instances where algorithms could be described
-->more succintly with
-->            designator := expr_designator := expression ;
-->to abbreviate
-->            designator := expr_designator ;
-->                  expr_designator := expression ;
-->In this notation, a swap would appear
-->
-->            VAR temp : type_of_designator ;  BEGIN
-->              temp := item1 := item2 := temp ;  END ;
-->--Lee

This notation might confuse programmers used to right-associative assigment,
where
    a := b := c
abbreviates
    b := c; a :=b

Algol68C had the `displacement operator', :=:= which yield the old value of the
LHS rather than the new value (as yielded by :=).  Thus a swap could be written
    a := b :=:= a

This also gave us the equivalent of C's i++:
    i +:=:= 1
and some neat syntax for list operations:
    next OF new item := list head :=:= new item

Whether you like such things is a matter of taste.  I did because it meant I could
expression a logically single operation as one statement (or `unit' in A68C-talk).
Likewise, (assuming a C-like stdio library):
    fclose (file :=:= NULL)
meaning "close the file and note that we have no open file" as a single operation.

Of course, you can write some pretty bizarre stuff if you want to.  My favourite
was
    ( b | x +:= 1; y | x ) +:= 1

-- jP --