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From: Keith Thompson <kst-u@mib.org>
Newsgroups: comp.lang.c
Subject: Re: Remembering C's operator precedence rules
Date: Sat, 09 Mar 2019 14:17:57 -0800
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Ben Bacarisse <ben.usenet@bsb.me.uk> writes:
> Keith Thompson <kst-u@mib.org> writes:
>> Ben Bacarisse <ben.usenet@bsb.me.uk> writes:
>> [...]
>>> Within the big block of relational operators, another grouping that
>>> looks odd at first is that == and != have their own precedence level
>>> just below <, >, <= and >=.  This is another case that it's probably
>>> wise to assume not everyone knows, but /you/ might remember it if you
>>> consider that == and != make sensible logical connectives, rather like
>>> && and ||.  For example, to test if four variables are in order --
>>> either ascending or descending -- you can write
>>>
>>>   a <= b  ==  b <= c  ==  c <= d
>>>
>>> Compare that with
>>>
>>>   a <= b  &&  b <= c  &&  c <= d
>>>
>>> for a single order.
>>
>> I think you forgot about associativity.
>>     x == y == z
>> means
>>     (x == y) == z
>> not
>>     (x == y) && (y == z)
>
> Thanks.  I should have looked for a better example as this one gets
> messy when corrected.  Do you know of an example that would motivate the
> otherwise rather surprising separation between relational an equality
> operators?

You could use the same method to determine whether 3 values are in
either ascending or descending order:
    a <= b == b <= c
It just breaks down when you need more than one "==" operator.

(I'd parenthesize that myself.)

-- 
Keith Thompson (The_Other_Keith) kst-u@mib.org  <http://www.ghoti.net/~kst>
Will write code for food.
void Void(void) { Void(); } /* The recursive call of the void */