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A freshman's question

Started bywhl <wanghailunmail@gmail.com>
First post2011-10-18 20:08 -0700
Last post2011-10-20 03:55 -0700
Articles 4 on this page of 24 — 10 participants

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  A freshman's question whl <wanghailunmail@gmail.com> - 2011-10-18 20:08 -0700
    Re: A freshman's question Patricia Shanahan <pats@acm.org> - 2011-10-19 04:48 +0100
      Re: A freshman's question whl <wanghailunmail@gmail.com> - 2011-10-19 00:54 -0700
        Re: A freshman's question Patricia Shanahan <pats@acm.org> - 2011-10-19 09:20 +0100
          Re: A freshman's question whl <wanghailunmail@gmail.com> - 2011-10-19 03:38 -0700
            Re: A freshman's question Lars Enderin <lars.enderin@telia.com> - 2011-10-19 17:48 +0200
              Re: A freshman's question Tim Slattery <Slattery_T@bls.gov> - 2011-10-19 12:24 -0400
                Re: A freshman's question whl <wanghailunmail@gmail.com> - 2011-10-20 03:55 -0700
              Re: A freshman's question whl <wanghailunmail@gmail.com> - 2011-10-20 03:50 -0700
            Re: A freshman's question Lew <lewbloch@gmail.com> - 2011-10-19 09:58 -0700
              Re: A freshman's question Patricia Shanahan <pats@acm.org> - 2011-10-19 18:47 +0100
              Re: A freshman's question whl <wanghailunmail@gmail.com> - 2011-10-20 03:48 -0700
          Re: A freshman's question whl <wanghailunmail@gmail.com> - 2011-10-19 03:49 -0700
            Re: A freshman's question Patricia Shanahan <pats@acm.org> - 2011-10-19 18:58 +0100
              Re: A freshman's question whl <wanghailunmail@gmail.com> - 2011-10-20 02:13 -0700
        Re: A freshman's question Arved Sandstrom <asandstrom3minus1@eastlink.ca> - 2011-10-19 06:05 -0300
          Re: A freshman's question whl <wanghailunmail@gmail.com> - 2011-10-19 03:38 -0700
      Re: A freshman's question Travers Naran <tnaran@gmail.com> - 2011-10-20 07:30 -0700
        Re: A freshman's question Gene Wirchenko <genew@ocis.net> - 2011-10-20 10:08 -0700
          Re: A freshman's question Patricia Shanahan <pats@acm.org> - 2011-10-20 18:22 +0100
          Re: A freshman's question Lew <lewbloch@gmail.com> - 2011-10-20 13:59 -0700
          Re: A freshman's question Joshua Cranmer <Pidgeot18@verizon.invalid> - 2011-10-20 17:38 -0500
    Re: A freshman's question Roedy Green <see_website@mindprod.com.invalid> - 2011-10-19 23:56 -0700
      Re: A freshman's question whl <wanghailunmail@gmail.com> - 2011-10-20 03:55 -0700

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#9055

FromLew <lewbloch@gmail.com>
Date2011-10-20 13:59 -0700
Message-ID<10920796.1086.1319144380777.JavaMail.geo-discussion-forums@prng5>
In reply to#9040
Wirchenko wrote:
>      That comes from C.  I have a question for anyone thinking it
[ i = i++; ]
> valid Java.  What is it supposed to do of use?  (If nothing, why even
> use it?)  I suspect the whole thing got started by someone not
> understanding that ++ causes an assignment to occur.

I have a question for anyone thinking it is not valid Java.

Why haven't you read the Java Language Specification on this matter?
<http://java.sun.com/docs/books/jls/third_edition/html/expressions.html>

-- 
Lew

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#9056

FromJoshua Cranmer <Pidgeot18@verizon.invalid>
Date2011-10-20 17:38 -0500
Message-ID<j7q7sf$qhp$1@dont-email.me>
In reply to#9040
On 10/20/2011 12:08 PM, Gene Wirchenko wrote:
> On Thu, 20 Oct 2011 07:30:31 -0700, Travers Naran<tnaran@gmail.com>
> wrote:
>
> [snip]
>
>> This is off topic, but I just recently learned that in C++, the
>> result of i=i++ is officially undefined.  It's interesting a
>> question related to what I learned comes up again in a completely
>> different forum.
>
> That comes from C.  I have a question for anyone thinking it valid
> Java.  What is it supposed to do of use?  (If nothing, why even use
> it?)  I suspect the whole thing got started by someone not
> understanding that ++ causes an assignment to occur.

I'm guessing that the largest reason for this issue lies in the fact 
that ++/-- is largely tied to increment/decrement addressing modes (so 
*p++ would translate to "Load from p, auto-increment the pointer") [1]. 
In cases where it would compile to an autoincrement address mode, the 
effect would take place immediately after the access, much as it does in 
Java. However, on machines without this mode, it probably made more 
sense to translate it into "*p; p += 1;". This is, I believe, the reason 
why the C committee made |i=i++| invalid.

In contrast, Java sought to leave nothing undefined in its semantics, so 
it has to pick a particular point in time at which the ++ takes place. 
If you're not tied to any overt architecture, the time that makes the 
most sense is to do it is immediately after getting the value (even 
before its use!). That means that the following function has a 
well-defined output:

int testWhen() {
   int i = 0;
   try {
     int j = 5 / i++;
   } catch (Exception e) { return i; }
   return i;
}

It is 1: the increment happens before the division.

Note that the only point of |i=i++| is to point out that how and when 
the increment occurs is not fully specified in C, while it is in Java.

[1] Apparently, this isn't why they were originally developed, according 
to <http://cm.bell-labs.com/cm/cs/who/dmr/chist.html>. Kind of... the 
PDP-7 had some memory slots which autoincremented on load, which may 
have been the spark that caused them to be introduced in the language.

-- 
Beware of bugs in the above code; I have only proved it correct, not
tried it. -- Donald E. Knuth

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#9017

FromRoedy Green <see_website@mindprod.com.invalid>
Date2011-10-19 23:56 -0700
Message-ID<1dhv97lae1vhgj95pmo0pd8btte7ca3tus@4ax.com>
In reply to#8971
On Tue, 18 Oct 2011 20:08:05 -0700 (PDT), whl
<wanghailunmail@gmail.com> wrote, quoted or indirectly quoted someone
who said :

> I don't kown the
>statement i=i++ operation sequence.

This sort of code is not found in real code. It pushes the edges of
the definition of the language.  

You would see
i++
or
i+=2
-- 
Roedy Green Canadian Mind Products
http://mindprod.com
It should not be considered an error when the user starts something
already started or stops something already stopped. This applies
to browsers, services, editors... It is inexcusable to 
punish the user by requiring some elaborate sequence to atone,
e.g. open the task editor, find and kill some processes.

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#9021

Fromwhl <wanghailunmail@gmail.com>
Date2011-10-20 03:55 -0700
Message-ID<abd9cdc3-bae3-44b5-aa34-7bca01a76981@z28g2000pro.googlegroups.com>
In reply to#9017
On 10月20日, 下午2时56分, Roedy Green <see_webs...@mindprod.com.invalid>
wrote:
> On Tue, 18 Oct 2011 20:08:05 -0700 (PDT), whl
> <wanghailunm...@gmail.com> wrote, quoted or indirectly quoted someone
> who said :
>
> > I don't kown the
> >statement i=i++ operation sequence.
>
> This sort of code is not found in real code. It pushes the edges of
> the definition of the language.  
>
> You would see
> i++
> or
> i+=2
> --
> Roedy Green Canadian Mind Productshttp://mindprod.com
> It should not be considered an error when the user starts something
> already started or stops something already stopped. This applies
> to browsers, services, editors... It is inexcusable to
> punish the user by requiring some elaborate sequence to atone,
> e.g. open the task editor, find and kill some processes.

thank you ,I see!

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