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From: ark@alice.UucP (Andrew Koenig)
Newsgroups: net.lang.c
Subject: Re: What should be added to C
Message-ID: <5521@alice.uUCp>
Date: Sun, 25-May-86 02:24:33 EDT
Article-I.D.: alice.5521
Posted: Sun May 25 02:24:33 1986
Date-Received: Mon, 26-May-86 01:30:36 EDT
References: <2600053@ccvaxa>
Organization: Bell Labs, Murray Hill
Lines: 35

> Why not go all the way and implement Dijkstra's concurrent assignment?

>    i,j := j,i

> for a swap. Course, couldn't use commas, but you get the idea. 
> Compilers can generate good code for this, and it makes programs with
> complicated pointer manipulations much clearer; I used to teach it, and the
> associated code generation algorithm, to U1s in classes I TAed, and it
> greatly reduced the noise level of assignments in the wrong order.

Two comments:

	1. This is really not the same as a swap.  I would like
	   to be able to say something like

		swap (a[i++], a[j++]);

	   and have it do the right thing, effectively

		a[i],a[j] = a[j],a[i];
		i++, j++;

	   This is definitely not equivalent to

		a[i++], a[j++] = a[j++], a[i++];

	2. Can you give me a simple code generation algorithm
	   for multiple assignments that gets things like

		i, a[i] = j, a[j];

	   correct and never uses extra temporaries?  I have this
	   nagging feeling that even without subscripted variables,
	   the problem is NP-complete.