Comment by downer | Hacker News Reader

Comment by downer | Hacker News Reader

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> like "How do your reverse an array without a temporary variable?"

You hard-code it using XOR. Note you can't even do a for-loop, as that's using an extra variable. Of course hand-unrolling a loop takes extra memory because it's so many extra lines so there's no point.

 a ^= b
 b ^= a
 a ^= b

This is very unlikely to be something that occurs to programmers who haven't had to XOR recently; if you aren't writing your own hash or crypto or codec functions then you probably haven't used it in a while. Now that you've seen this, you'll remember it and so if anyone asks, you won't be figuring it out again, just recalling.

IMO it's a pretty useless way to weed people out, kind of like testing people on chess by random arrangements of pieces. Chess experts remember actual legal positions that occur in games much better than non-players, but they are no better at remembering positions that never occur -- a bunch of kings on a chessboard, some in adjacent squares, for example.

Likewise, this kind of pattern NEVER occurs in what most people program; it's just implausible.

You can use a for loop, if you step through the array with pointer arithmetic. On the assumption that you're given the array length (or else that the array is of fixed size), you don't need to hand-unroll the loop, which wouldn't handle a variably-sized array anyway.

But I agree that it's a bad interview question.

Doing p++, len-- uses two temporary variables.

You don't need a temporary variable for "p" -- if the task is to write a function, you must have a pointer with the address of the start of the array, not an array variable as such. And yeah, you need some way to tell the length.

...and where you are in the array, hence the temp variable.

Write it out to see.

So it still doesn't work, even if you use a function to "cheat" and get a "free" pointer (which, by the way, wasn't what the task said).

Yeah, I suppose you'd need 1 temporary variable. Your solution doesn't work either, though: doing the freshman XOR trick some arbitrary fixed number of times just reduces to the XOR trick, which is plainly not what the task asked.

It does work, you do the 3-step XOR with a[0] and a[last], then a[1] and a[last-1], etc. The 3-step XOR from the OP swaps the data.

Here it is with a 4-element array, a[0] through a[3]:

 a=[100,200,300,400]

 a[0] ^= a[3]
 a[3] ^= a[0]
 a[0] ^= a[3]

 a[1] ^= a[2]
 a[2] ^= a[1]
 a[1] ^= a[2]

 a
 [400, 300, 200, 100]

(Pretend this list is an array, it's just for illustrative purposes)

Well, obviously the XOR swap works. But how do you use that to reverse an array of variable size? (i.e. where "last" isn't known at compile-time, so the number of swaps isn't known in advance).

You don't; it's basically, "here's an array, now reverse it" (keep in mind a pointer is not an array).

http://www.lysator.liu.se/faq/c-faq/c-2.html

The general point is that interviews of this type are counterproductive.

An array can be variably-sized in C (C99, that is). And thank you, I know the difference between an array and a pointer. You just seem to have arbitrarily interpreted the question to limit it to C89 and a fixed-size array in order to make your solution work.

This has nothing to do with it being statically sized or variably sized. You have to know what size it is when you are going to reverse it. The solution presented works as long as the programmer knows what the size is.

If you want to posit any kind of sane question, then you can have a sane solution.

Well, suppose I have an array `A' of size `len'. How would you write an algorithm that reverses that array in place, without using any temporary variables, and that works for arbitrary values of A and len? I think the answer is "you can't". You can't claim to have a solution for the problem if it only works for a fixed input, and if the solution itself would be different for different inputs.

> I think the answer is "you can't".

That's what I said a few posts ago.

> You can't claim to have a solution for the problem if it only works for a

> fixed input, and if the solution itself would be different for different

> inputs.

"Reverse this array". Done. "Reverse an unspecified array of arbitrary length" is a different problem :)

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