The toughest interview question

July 29, 2014

What is the output of the following C code snippet?

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int n = 1;
printf("%d %d %d\n", n++, ++n, n++ + ++n);

This is a tricky question. Any attempt to answer this question requires either a deep understanding of your compiler’s parser or a complete ignorance of C. Even if you fall under the first category and know the ins and outs of your favorite compiler, how do you know which is your interviewer’s favorite compiler?

The ANSI C language specification, you see, specifies neither the evaluation order of function arguments nor the exact side effect evaluation time of prefix and postfix operators. The answer to the above problem depends on the compiler being used.

Unfortunately, these kinds of puzzles form the bulk of programming interview questions for IT companies in India. They are even common in term papers in colleges.

I could never fathom the utility of such questions. The correct answer, Depends on the compiler is never an option for this question. What am I supposed to do with this? Should I just evaluate from left to right and calculate the side effects ASAP or should I try to second guess my interviewer and assume that he/she is terribly confused by the C function call stack convention and thinks that the compiler always evaluates from right to left?

One day I chanced upon a book that actually taught that C compilers evaluate function arguments from right to left. This gem of a book is quite famous in India. It is called Let Us C, written by the renowned Yashwant Kanetkar, who according to his Wikipedia page,

” … has created, moulded and groomed many IT professionals in the last decade and half.”

How poetic …

If there is a single most important factor behind the stagnation of the Indian IT industry, I think it is this book. Forget about complying to ANSI C standards, it professes and quizzes on the areas of C that have deliberately been left to the compiler. Professors and interviewers who unfortunately never got a chance to study K&R C think of Let Us C as the authority on the C programming language. Most of their questions are a straight copy-paste from this book.

This is a sad state of affairs. Mainly because the people who actually know C are never able to answer this question correctly. If you are one of them, Congrats! You are on the right path towards mastering C.


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