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POWhich of the following combinations of post & pre-increment operators have undefined behaviour in C?
 

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  1. POWhich of the following combinations of post & pre-increment operators have undefined behaviour in C?
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    <p>I've read, <a href="https://stackoverflow.com/questions/949433/could-anyone-explain-these-undefined-behaviors-i-i-i-i-i-etc">Could anyone explain these undefined behaviors (i = i++ + ++i , i = i++, etc...)</a> and <em>tried understanding</em> <a href="http://c-faq.com/expr/seqpoints.html" rel="nofollow noreferrer">Sequence points on "comp.lang.c FAQ"</a> after wasting more than 2 hours of time trying to explain the following results by gcc compiler.</p> <pre><code>expression(i=1;j=2) i j k k = i++ + j++; 2 3 3 k = i++ + ++j; 2 3 4 k = ++i + j++; 2 3 4 k = ++i + ++j; 2 3 5 k = i++ + i++; 3 2 k = i++ + ++i; 3 4 k = ++i + i++; 3 4 k = ++i + ++i; 3 6 i = i++ + j++; 4 3 i = i++ + ++j; 5 3 i = ++i + j++; 4 3 i = ++i + ++j; 5 3 i = i++ + i++; 4 i = i++ + ++i; 5 i = ++i + i++; 5 i = ++i + ++i; 6 </code></pre> <p><strong>Question:</strong></p> <ol> <li><p>I want to know if all the expressions shown (in 4 groups) in above figure have undefined behavior? If only some of them have undefined behavior which ones does and which ones doesn't? </p></li> <li><p>For defined behaviour expressions, kindly can you <em>show (not explain)</em> how compiler evaluates them. Just to make sure, if I got this pre-increment &amp; post increment correctly.</p></li> </ol> <p><strong>Background:</strong></p> <p>Today, I've attended a campus interview, in which I was asked to explain the results of <code>i++ + ++i</code> for a given value of <code>i</code>. After compiling that expression in gcc, I realized that the answer I gave in interview was wrong. I decided not to make such mistake in future and hence, tried to compile <em>all possible combinations</em> of pre and post increment operators and compile them in gcc and then try to explain the results. I struggled for more than 2 hours. I couldn't find single behaviour of evaluation of these expressions. So, I gave up and turned to stackoverflow. After little bit of reading archives, found that there is something like <code>sequence point</code> and undefined behaviour.</p>
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    1. COI think the undefined groups are 2, 3, and 4? They all increment and use the same variable in the same expression, with no intervening sequence point.
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    2. COIf you post code in a question, please post it as a code block (without line numbers), not as an image.
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      1. POWhich of the following combinations of post & pre-increment operators have undefined behaviour in C?
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    3. CO@KeithThompson: Done sir!
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