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From: jtr485@umich.UUCP (Johnathan Tainter)
Newsgroups: comp.lang.c
Subject: Re: Unary plus (and wysiwyg order or evaluation)
Message-ID: <105@umich.UUCP>
Date: Wed, 15-Apr-87 01:21:00 EST
Article-I.D.: umich.105
Posted: Wed Apr 15 01:21:00 1987
Date-Received: Sun, 19-Apr-87 18:06:33 EST
References: <6772@brl-adm.ARPA> <803@viper.UUCP> <752@rtech.UUCP>
Organization: EECS, University of Michigan
Lines: 55

In article <752@rtech.UUCP>, jas@rtech.UUCP writes:
> Sorry to add to a discussion that may have gone on too long; I'll be brief.
> 
> Postings like this betray some confusion:
> 
>  > It wasn't until reading about unary '+' that i knew that C might
>  > disregard my parens.
> 
> >I think parens should be followed without any micky-mousing from the compiler.
> 
> PARENTHESES AFFECT PRECEDENCE, NOT EVALUATION ORDER.  THESE ARE 2 DIFFERENT
> THINGS.  To put it another way, parentheses affect the shape of the parse
> tree for an expression; evaluation order is an order in which the parse
> tree is walked.  Unary '+' does not "override" parentheses,
Correct so far.
>							      nor may a compiler
> "disregard my parens."  Parens have a precise and essential function, entirely
> distinct from order of evaluation.
Sorry, Jim.  But, you are the one showing confusion here.
The compiler CAN disregard parens! (K&R page 49)  Unary plus exists to keep it
from doing so.
Unary plus does NOT alter order of evaluation, it just maintains precedence
specified by the parentheses.
i.e.  on a simple stack machine:
    +(x + y) + z
CANNOT generate
    push x; push y; push z; add; add
OR
    push y; push x; push z; add; add
OR
    push x; push z; push y; add; add
OR
    push y; push z; push x; add; add
OR
    push x; push z; add; push y; add
OR
    push y; push z; add; push x; add
OR
    push z; push x; add; push y; add
OR
    push z; push y; add; push x; add
but
    (x + y) + z
can.
This says nothing about whether the former is:
    push x; push y; add; push z; add
OR
    push y; push x; add; push z; add
OR
    push z; push x; push y; add; add
OR
    push z; push y; push x; add; add

--j.a.tainter
> Jim Shankland