Explaination of behavior in C

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08-13-2004
Mister C

Explaination of behavior in C

Given the code

Code:

int i = 1; int a[4] = {0, 10, 20, 30}; a[i++] = i++; // a[i] = i; i = i + 1; i = i + 1; printf(“%d %d %d %d %d\n”,a[0],a[1],a[2],a[3],i);

The output is in C: 0 1 20 30 3

However, the output in Java and C# is (with printf statment replace in Java and C#): 0 2 20 30 3

Just curious as to how C evaluates this vs. Java and C#.
(Order of evaluation vs. precedence)

I never really thought about this until now. I really assumed all these languages that evaluate expression in the same way.

Any thoughts?

Mr. C. :)
08-13-2004
Salem

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

Not to mention this more recent missive
http://cboard.cprogramming.com/showthread.php?t=54442
08-14-2004
Dave Evans

Quote:

Originally Posted by Salem

> I never really thought about this until now
No, you asked about it 2 years ago and have forgotten :rolleyes:
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//

Quote:

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
08-14-2004
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.

Quote:

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

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

jc :(
08-14-2004
Dave Evans

Quote:

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
08-14-2004
Thantos

Quote:

These threads do not give the answer to the question.

Actually the first link does answer the question repeatidly.
08-14-2004
Dave Evans

Quote:

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
08-14-2004
Sang-drax

Quote:

Originally Posted by Salem

No, you asked about it 2 years ago and have forgotten :rolleyes:
http://cboard.cprogramming.com/showthread.php?t=24397

Heh I actually remember that thread, at that time Mr. C didn't want to accept the fact that such statements are undefined in C.

Quote:

Originally Posted by Mister C

Well the evaluation may be undefined in C

Great, only took two years! :)

Seriously, how does Java handle the evaluation of such statements? What are the rules?
08-14-2004
Perspective

Quote:

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....

Quote:

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 :)
08-14-2004
Thantos

Quote:

Originally Posted by Mister C's Signature

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

What the heck is C-?
08-14-2004
Perspective

Quote:

Originally Posted by Thantos

What the heck is C-?

thats part of his nick plus a hyphen. kinda like "Thantos- toss master" or something... ;)
08-14-2004
Thantos

Ah :) I prefer the colon in that situation:
Thantos: Overlord of Something
08-14-2004
Mister C

Here is the discussion at JavaRanch on it:

http://saloon.javaranch.com/cgi-bin/ubb/ultimatebb.cgi?ubb=get_topic&f=24&t=000775

Here is the JLS
http://java.sun.com/docs/books/jls/s....doc.html#4779

The code I posted above does evaluate in Java.

Like I said I spend so much time with Java/C# I now forget evaluation in C/C++.

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

Enrollments are stronger in the other languages

Lot of good questions at Javaranch.

Mr. C.
08-15-2004
sean

Quote:

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.
08-15-2004
prog-bman

I think he meant it as a joke notice the :p