Solved

Comma operator and order or precedence

Posted on 2013-12-11
5
292 Views
Last Modified: 2013-12-11
Ah hello.

I am sure I am having a silly moment here, but I am trying to understand the comma operator in C++ and "operator precedence" rules (which I believe are a general maths thing, hence me posting this in the Maths TA too:)).

From what I have read about the comma operator, we have two facts

1) The result of it is the right hand value, so for example a,b would yield b.
2) It has the lowest operator precedence.  I read this means "it's always the last one to bind to an expression", but I don't quite get that bit.

So, if we have

a = (b , c)

the evaulation sequence would first be (b,c) which would yield c, hence a would be assigned c.  But I am struggling with

a = b , c

This is apparently equivalent to

(a = b) , c

The result of this expression is b.  

Now, I know we have to evaluate the brackets first, which results in b, so we then have

b, c

Q1) From rule 1 above, why is the result hence not c?  Clearly something is incorrect in what I deduced above, but I cannot see it...

Q2) Can someone clarify fact 2) above and what "it's always the last one to bind to an expression" means?

TIA
0
Comment
Question by:mrwad99
[X]
Welcome to Experts Exchange

Add your voice to the tech community where 5M+ people just like you are talking about what matters.

  • Help others & share knowledge
  • Earn cash & points
  • Learn & ask questions
  • 3
  • 2
5 Comments
 
LVL 31

Expert Comment

by:Zoppo
ID: 39710942
Hi mrwad99,

1. This is because the comma seperates the two statements, so the expression a = b , c is equivalent to ( a = b ) , c

For some further info about comma-operator you can start reading http://en.wikipedia.org/wiki/Comma_operator

One maybe important fact: In C++ it's possible to override comma-operators which may lead to confusion since if it is overridden it starts acting like a function call which means there's no order defined in which left- and rightside expressions are executed. For this you can find a sample at i.e. http://madebyevan.com/obscure-cpp-features/

Hope that helps,

ZOPPO
0
 
LVL 19

Author Comment

by:mrwad99
ID: 39710950
Thanks ZOPPO, but what you say I have already observed.  The result of

( a = b ) , c

is (and this is where I might be wrong!)

b, c

which is why I cannot see why the overall answer is not c!
0
 
LVL 31

Accepted Solution

by:
Zoppo earned 200 total points
ID: 39710959
Yes, that's correct, but the c is never assigned to anything. You have two different statements, "a = b" and "c", so a is equal to b afterward, no matter in which order the statements are evaluated.

Maybe it's clear when you check this:

   x = ( a = b, c );

This results in 'a = b' and 'x = c'.
0
 
LVL 19

Author Closing Comment

by:mrwad99
ID: 39710972
Ah-ha!  Your statement

"but the c is never assigned to anything"

plus the lovely little example perfectly explain this.

Thank you!
0
 
LVL 31

Expert Comment

by:Zoppo
ID: 39710979
You're welcome, I'm glad I could help :o)

Have a nice day,

best regards,

ZOPPO
0

Featured Post

Technology Partners: We Want Your Opinion!

We value your feedback.

Take our survey and automatically be enter to win anyone of the following:
Yeti Cooler, Amazon eGift Card, and Movie eGift Card!

Question has a verified solution.

If you are experiencing a similar issue, please ask a related question

IntroductionThis article is the second in a three part article series on the Visual Studio 2008 Debugger.  It provides tips in setting and using breakpoints. If not familiar with this debugger, you can find a basic introduction in the EE article loc…
This article provides a brief introduction to tissue engineering, the process by which organs can be grown artificially. It covers the problems with organ transplants, the tissue engineering process, and the current successes and problems of the tec…
Although Jacob Bernoulli (1654-1705) has been credited as the creator of "Binomial Distribution Table", Gottfried Leibniz (1646-1716) did his dissertation on the subject in 1666; Leibniz you may recall is the co-inventor of "Calculus" and beat Isaac…
Finds all prime numbers in a range requested and places them in a public primes() array. I've demostrated a template size of 30 (2 * 3 * 5) but larger templates can be built such 210  (2 * 3 * 5 * 7) or 2310  (2 * 3 * 5 * 7 * 11). The larger templa…

717 members asked questions and received personalized solutions in the past 7 days.

Join the community of 500,000 technology professionals and ask your questions.

Join & Ask a Question