Explaination of behavior in C

This is a discussion on Explaination of behavior in C within the C Programming forums, part of the General Programming Boards category; Given the code Code: int i = 1; int a[4] = {0, 10, 20, 30}; a[i++] = i++; // a[i] ...

  1. #1
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    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.
    Mr. C: Author and Instructor

  2. #2
    and the hat of wrongness Salem's Avatar
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    > I never really thought about this until now
    No, you asked about it 2 years ago and have forgotten
    Some C test questions: challenge

    Not to mention this more recent missive
    Evaluating prefix/posfix short cut
    If you dance barefoot on the broken glass of undefined behaviour, you've got to expect the occasional cut.
    If at first you don't succeed, try writing your phone number on the exam paper.
    I support http://www.ukip.org/ as the first necessary step to a free Europe.

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    Quote Originally Posted by Salem
    > I never really thought about this until now
    No, you asked about it 2 years ago and have forgotten
    Some C test questions: challenge

    Not to mention this more recent missive
    Evaluating prefix/posfix short cut
    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

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    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.


    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
    Last edited by Mister C; 08-14-2004 at 09:03 AM.
    Mr. C: Author and Instructor

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    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

  6. #6
    & the hat of GPL slaying Thantos's Avatar
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    These threads do not give the answer to the question.
    Actually the first link does answer the question repeatidly.

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    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

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    S Sang-drax's Avatar
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    Quote Originally Posted by Salem
    No, you asked about it 2 years ago and have forgotten
    Some C test questions: challenge
    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?
    Last edited by Sang-drax; 08-14-2004 at 01:05 PM.
    Last edited by Sang-drax : Tomorrow at 02:21 AM. Reason: Time travelling

  9. #9
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    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

  10. #10
    & the hat of GPL slaying Thantos's Avatar
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    Quote Originally Posted by Mister C's Signature
    Mr. C- C, C++ and Java instructor
    What the heck is C-?

  11. #11
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    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...

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    & the hat of GPL slaying Thantos's Avatar
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    Ah I prefer the colon in that situation:
    Thantos: Overlord of Something

  13. #13
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    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.

    Enrollments are stronger in the other languages

    Lot of good questions at Javaranch.

    Mr. C.
    Mr. C: Author and Instructor

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    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.

  15. #15
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    I think he meant it as a joke notice the
    Woop?

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