4 ms·

I would think a "if (x % 2 == 1) //odd" would be within the realm of most CS applicants.

kazinator 9y ago
If x is a signed type, and negative, are you sure the modulus is positive 1? Since ISO C99, C's division operator has been required to truncate toward zero. The % operator harmonizes with that and so it yields negative remainders south of zero. In C90, it was implementation-defined, IIRC. You really just want to test for zero/nonzero, rather than x % 2 == 1.
- craftyguy 9y ago
  Good point. Interestingly enough, Python seems to handle this appropriately: >>> -4%2 0 >>> -8%2 0 >>> -9%2 1
  - panic 9y ago
    Yeah, Python is one of the few languages which actually defines sane behavior for integer division and modulus rather than leaving it up to the hardware. I wish more languages would follow its example!
    - triska 9y ago
      Prolog is also like this: ?- x is -4 mod 2. X = 0. ?- X is -8 mod 2. X = 0. ?- X is -9 mod 2. X = 1. Using declarative integer arithmetic, we can even express very generally that a number X is a multiple of 2: ?- X #= 2*_. This is true iff there is any integer (indicated by an anonymous variable _) that, multiplied by 2, yields X. It works for positive and negative integers. For example: ?- -4 #= 2*_. true. ?- -9 #= 2*_. false. A very nice reference on the general topic of bitwise operations is Donald Knuth's Bitwise Tricks and Techniques in Volume 4 of The Art of Computer Programming.
    - bakery2k 9y ago
      Guido has written a blog post explaining "Why Python's Integer Division Floors" [1]. Python also uses floor division, and the corresponding "sign-of-modulo matches sign-of-divisor", for floating-point numbers. The blog post claims that this can be problematic in certain cases (compared to C's "sign-of-modulo matches sign-of-dividend"), but does not give any concrete examples. [1] http://python-history.blogspot.com.au/2010/08/why-pythons-integer-division-floors.html http://python-history.blogspot.com.au/2010/08/why-pythons-in...
    - kazinator 9y ago
      ISO C doesn't leave it up to the hardware. Integer division is required to truncate toward zero; the remainder harmonizes with that. (If someone designs a new HLL which leaves some arithmetic to the hardware that C doesn't, that is indeed silly.) The sane language here Common Lisp: it does it in all the ways rather than picking one approach with some hand-waving justification about why it's the good choice. It has truncate, round, ceiling and floor functions that all return a quotient and remainder. There are also mod and rem functions that calculate just the remainder from a floor and truncate, respectively.
LandR 9y ago
In C# this would be wrong (-123 % 2 == 1).Dump(); Prints False. % in C# is the remainder operator, not canonical modulus.
- deleted 9y ago
  [deleted]
- craftyguy 9y ago
  Well that's not confusing at all. Why would C# change the definition of a common operator character?
  - bjoli 9y ago
    I think you would be surprised if you started looking around to see how how % is handled.
- tonyarkles 9y ago
  Javascript as well: >> -2 % 2 -0 >> -3 % 2 -1 Edit: well, at least in Firefox JS. I have no idea if that's actually standard-mandated behaviour or if it's an implementation quirk. The -0 is especially weird (I had written odd but...)
  - justin66 9y ago
    This is pretty interesting. I don't use js but I took a quick look and apparently (x % 2) == 0 as a test for even integers should still work properly there, since a comparison of -0 and 0 will consider them equivalent.
justin66 9y ago
> I would think a "if (x % 2 == 1) //odd" would be within the realm of most CS applicants. Yeah, if they can pass a fizzbuzz question they can probably get that. At which point it behooves the interviewer to explain why (x % 2) == 0 to find an even number is a better test...

If x is a signed type, and negative, are you sure the modulus is positive 1? Since ISO C99, C's division operator has been required to truncate toward zero. The % operator harmonizes with that and so it yields negative remainders south of zero. In C90, it was implementation-defined, IIRC. You really just want to test for zero/nonzero, rather than x % 2 == 1.

craftyguy 9y ago
Good point. Interestingly enough, Python seems to handle this appropriately: >>> -4%2 0 >>> -8%2 0 >>> -9%2 1
- panic 9y ago
  Yeah, Python is one of the few languages which actually defines sane behavior for integer division and modulus rather than leaving it up to the hardware. I wish more languages would follow its example!
  - triska 9y ago
    Prolog is also like this: ?- x is -4 mod 2. X = 0. ?- X is -8 mod 2. X = 0. ?- X is -9 mod 2. X = 1. Using declarative integer arithmetic, we can even express very generally that a number X is a multiple of 2: ?- X #= 2*_. This is true iff there is any integer (indicated by an anonymous variable _) that, multiplied by 2, yields X. It works for positive and negative integers. For example: ?- -4 #= 2*_. true. ?- -9 #= 2*_. false. A very nice reference on the general topic of bitwise operations is Donald Knuth's Bitwise Tricks and Techniques in Volume 4 of The Art of Computer Programming.
  - bakery2k 9y ago
    Guido has written a blog post explaining "Why Python's Integer Division Floors" [1]. Python also uses floor division, and the corresponding "sign-of-modulo matches sign-of-divisor", for floating-point numbers. The blog post claims that this can be problematic in certain cases (compared to C's "sign-of-modulo matches sign-of-dividend"), but does not give any concrete examples. [1] http://python-history.blogspot.com.au/2010/08/why-pythons-integer-division-floors.html http://python-history.blogspot.com.au/2010/08/why-pythons-in...
  - kazinator 9y ago
    ISO C doesn't leave it up to the hardware. Integer division is required to truncate toward zero; the remainder harmonizes with that. (If someone designs a new HLL which leaves some arithmetic to the hardware that C doesn't, that is indeed silly.) The sane language here Common Lisp: it does it in all the ways rather than picking one approach with some hand-waving justification about why it's the good choice. It has truncate, round, ceiling and floor functions that all return a quotient and remainder. There are also mod and rem functions that calculate just the remainder from a floor and truncate, respectively.

LandR 9y ago

In C# this would be wrong (-123 % 2 == 1).Dump(); Prints False. % in C# is the remainder operator, not canonical modulus.

deleted 9y ago
[deleted]
craftyguy 9y ago
Well that's not confusing at all. Why would C# change the definition of a common operator character?
- bjoli 9y ago
  I think you would be surprised if you started looking around to see how how % is handled.
tonyarkles 9y ago
Javascript as well: >> -2 % 2 -0 >> -3 % 2 -1 Edit: well, at least in Firefox JS. I have no idea if that's actually standard-mandated behaviour or if it's an implementation quirk. The -0 is especially weird (I had written odd but...)
- justin66 9y ago
  This is pretty interesting. I don't use js but I took a quick look and apparently (x % 2) == 0 as a test for even integers should still work properly there, since a comparison of -0 and 0 will consider them equivalent.

justin66 9y ago

> I would think a "if (x % 2 == 1) //odd" would be within the realm of most CS applicants. Yeah, if they can pass a fizzbuzz question they can probably get that. At which point it behooves the interviewer to explain why (x % 2) == 0 to find an even number is a better test...