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Comment by iliis | Hacker News Reader

Yeah, this is really cool :) A favorite of mine is the xor-swap: While not useful in pratice for me, you can get quite stunned faces if you tell your friends, that a swap is possible without a third variable.

  x = x xor y
  y = x xor y
  x = x xor y

It's also possible to use addition and subtraction instead of xor.

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FWIW, pretty much every 3rd (penultimate) year engineering student in India knows the xor-swap, I should think (only they might not call it that, because they've just memorised the procedure, and have very little idea why it works... /cynical)

This is one of the most common questions in 'technical interviews' when big IT companies are recruiting fresh grads.

(See http://www.allinterview.com/showanswers/17145.html)

I am not sure where your cynicism or contempt is coming from. There is not much to a deeper understanding of xor-swap. If one knows the xor truth table one knows why it works, and knowing the truth table is essentially an act of memorization. I hope they are exposed to digital logic way before 3rd year. On the other hand cases where this will not work in the current form, perhaps needs a somewhat deeper understanding of bit lay out. Say if one were to swap two unicode characters from void * pointers to them.

My cynicism (not contempt - I know that the students are under various sorts of pressures) stems from personal experience.

It's not much good explaining how simple it is to understand the xor-swap - it doesn't change the fact that the majority of engineering students go to the link like the one I posted, compile a list of questions and answers, and rattle off answers before the question is even completed in the interviews.

It's like knowing [any interesting method] can be used to accomplish a task, without knowing why said method works.

edit: I realise all this is OT, but I was downvoted for my earlier post and felt the need to defend it.

Am I the only one that thinks this would be a really BAD interview question? There'd be no way to tell if the candidate had memorised the answer or had worked it out on the fly.

To elaborate on its usefullness: An XOR-swap is only useful when you do not have space for a new pointer, otherwise, making a new temporary variable will on most processors be faster than XOR-swapping.

I would assume you need to be using regesters for three XOR's to be slower than a temperary variable.

been used in many-a-obfuscate-a-bug-contests for what happens when x == y.

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