• Status: Solved
  • Priority: Medium
  • Security: Public
  • Views: 324
  • Last Modified:

Comma operator and order or precedence

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
mrwad99
Asked:
mrwad99
  • 3
  • 2
1 Solution
 
ZoppoCommented:
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
 
mrwad99Author Commented:
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
 
ZoppoCommented:
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
 
mrwad99Author Commented:
Ah-ha!  Your statement

"but the c is never assigned to anything"

plus the lovely little example perfectly explain this.

Thank you!
0
 
ZoppoCommented:
You're welcome, I'm glad I could help :o)

Have a nice day,

best regards,

ZOPPO
0
Question has a verified solution.

Are you are experiencing a similar issue? Get a personalized answer when you ask a related question.

Have a better answer? Share it in a comment.

Join & Write a Comment

Featured Post

Cloud Class® Course: SQL Server Core 2016

This course will introduce you to SQL Server Core 2016, as well as teach you about SSMS, data tools, installation, server configuration, using Management Studio, and writing and executing queries.

  • 3
  • 2
Tackle projects and never again get stuck behind a technical roadblock.
Join Now