Re: [Dev-C++] Question about increment..

[Robert A. <ral...@gm...>]

Hello,

It's undefined in the standard.... the sub expressions can be
calculated in any order.... ie...

= a++ + b++ + a + b;
a++ could be calculated before or after any of the following
b++ ,
a ,
b,

Since the side effects of the a++ and b++ change the value of the
later it is undefined in C or C++

See also: http://en.wikipedia.org/wiki/Unspecified_behavior

On Thu, Sep 20, 2012 at 9:50 PM, Reid Thompson <rei...@at...> wrote:
> On 9/20/2012 9:08 PM, Mani wrote:
>> I think I might argue with one point way below.. please scroll down..
>>
>> On Thu, Sep 20, 2012 at 4:08 PM, Reid Thompson <Rei...@at...> wrote:
>>> On Thu, 2012-09-20 at 12:54 -0400, Mani wrote:
>>>> Hi,
>>>>
>>>> I recently to try some toy things in DevC++ (version I am using is 4.9.9.2)
>>>>
>>>> The code I had was this:
>>>>
>>>>      int a = 1, b = 2;
>>>>      int c = a++ + b++ + a + b;
>>>>
>>>>      cout << "a = " << a << endl;
>>>>      cout << "b = " << b << endl;
>>>>      cout << "c = " << c << endl;
>>>>
>>>> I expected the c value to be 8 (by operator precedence, ++ has higher
>>>> precedence than +, so do in order:
>>>> a++, then b++
>>>> then do (a++ + b++)
>>>> then add the previous result and a
>>>> then add the previous result and b
>>>>
>>>> but the answer I got was 6..
>>>>
>>>> what might be the reason for this..??
>>>>
>>>> thanks, murali
>>>>
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>>> [16:06:54][94s] rthompso@raker2> ~
>>> $ cat helloworld.c
>>> #include <stdio.h>
>>> #include <stdlib.h>
>>> int main(void)
>>> {
>>>      int a = 1, b = 2;
>>>      int c = a++ + b++ + a + b;
>>>
>>>      printf("c is %d\n", c);
>>>      a = 1;
>>>      b = 2;
>>>      c = ++a + ++b + a + b;
>>>
>>>      printf("c is %d\n", c);
>>>
>>>      return 0;
>>>
>>> }
>>>
>>> [16:07:01][0s] rthompso@raker2> ~
>>> $ cat Hello.cs
>>> using System;
>>>
>>> class Program
>>> {
>>>      static void Main()
>>>      {
>>>          int a = 1, b = 2;
>>>          int c = a++ + b++ + a + b;
>>>
>>>          Console.WriteLine(c);
>>>          Console.WriteLine("Hello world!");
>>>
>>>          a = 1;
>>>          b = 2;
>>>          c = ++a + ++b + a + b;
>>>          Console.WriteLine(c);
>>>          Console.WriteLine("Hello world!");
>>>
>>>
>>>          a = 1;
>>>          b = 2;
>>>          c = a++ + b++;
>>>          int d =  c + a + b;
>>>
>>>          Console.WriteLine(d);
>>>      }
>>> }
>>>
>>> [16:07:26][0s] rthompso@raker2> ~
>>> $ ./a.out
>>> c is 6
>>> c is 10
>>>
>>> [16:07:30][0s] rthompso@raker2> ~
>>> $ mono Hello.exe
>>> 8
>>> Hello world!
>>> 10
>>> Hello world!
>>> 8
>>>
>>>
>>> gcc C
>>> n++ -> increment after use
>>>>      int a = 1, b = 2;
>>>>      int c = a++ + b++ + a + b;
>>>            c = 1 + 2 + 1 + 2
>>>            c = 6
>>>
>>> ++n -> increment before use
>>>>      int a = 1, b = 2;
>>>>      int c = ++a + ++b + a + b;
>>>            c = 2 + 3 + 2 + 3
>>>            c = 10
>>>
>>> mcs csharp
>>> evidently creates an incremented copy and uses it with the original
>>> value
>>>>      int a = 1, b = 2;
>>>>      int c = a++ + b++ + a + b;
>>>            c = 2 + 3 + 1 + 2
>>>            c = 8
>>>>>> BEGIN: Murali:
>> How can you be sure that this is what happens.. I am inclined to
>> believe what happens is:
>> c = 1 + 2 + 2 + 3
>> c = 8
>>
>>>>>> END: Murali
>>> ++n -> increment before use
>>>>      int a = 1, b = 2;
>>>>      int c = ++a + ++b + a + b;
>>>            c = 2 + 3 + 2 + 3
>>>            c = 10
>>>
>>>          a = 1;
>>>          b = 2;
>>>          c = a++ + b++;
>>>          int d =  c + a + b;
>>>          c = 5
>>>          d = 5 + 1 + 2
>>>          d = 8
>>>
>>>
> ok -- you are most likely correct.
>
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