Thread: Explaination of behavior in C

Originally Posted by Salem

> I never really thought about this until now
No, you asked about it 2 years ago and have forgotten
http://cboard.cprogramming.com/showthread.php?t=24397

Not to mention this more recent missive
http://cboard.cprogramming.com/showthread.php?t=54442

These threads do not give the answer to the question.

The question is: How does C evaluate the expression

Code:

a[i++] = i++;

The answer is: The c language standard states that the behavior is undefined, since the order of evaluation is undefined. There are two subexpressions that must be evaluated. Since each has a side effect that affects the value of the other, behavior of the whole expression is undefined.

Here's a quote from the FAQ at comp.lang.c for an even simpler expression

http://www.faqs.org/faqs/C-faq/faq//

3.1: Why doesn't this code:

a[i] = i++;

work?

A: The subexpression i++ causes a side effect -- it modifies i's
value -- which leads to undefined behavior since i is also
referenced elsewhere in the same expression, and there's no way
to determine whether the reference (in a[i] on the left-hand
side) should be to the old or the new value. (Note that
although the language in K&R suggests that the behavior of this
expression is unspecified, the C Standard makes the stronger
statement that it is undefined -- see question 11.33.)

References: K&R1 Sec. 2.12; K&R2 Sec. 2.12; ISO Sec. 6.3; H&S
Sec. 7.12 pp. 227-9.

It doesn't matter what any particular compiler does with this code, so don't bother telling me what yours does. The behavior is undefined

Dave

Originally Posted by Mister C

Dave,

That is what I was looking for. The specific reference on the evaluation.

Well the evaluation may be undefined in C but they aren't in in Java and C#-where I spend most of my time now.



So I hope you were talking about C/c++ compilers-because in the JLS it is not.

jc

I'm sorry if it was not clear that I meant C and C++. The original post did refer to other languages; I did not comment on them, since this is the C board.

(I should have said, "don't bother telling me what your C or C++ compiler does. The behavior is undefined.")

Dave


Dave

These threads do not give the answer to the question.

Actually the first link does answer the question repeatidly.

Originally Posted by Thantos

Actually the first link does answer the question repeatidly.

You are correct, of course; I didn't read all of the responses. Sorry

Dave

Originally Posted by Mister C

Well the evaluation may be undefined in C but they aren't in in Java and C#-where I spend most of my time now.

hmm. i was under the impression that modifying a variable twice in one statement is undefined in Java as well....

Originally Posted by Sang-drax

Seriously, how does Java handle the evaluation of such statements? What are the rules?

i am quite curious as well, enlighten us Mr C

Originally Posted by Mister C's Signature

Mr. C- C, C++ and Java instructor

What the heck is C-?

Originally Posted by Thantos

What the heck is C-?

thats part of his nick plus a hyphen. kinda like "Thantos- toss master" or something...

Ah I prefer the colon in that situation:
Thantos: Overlord of Something

Well this will be the last semester I teach C-for a while.

There's that C- again. This time I don't get it however.