5 ms·

The closest I am to being famous is this question on SO, to which I gave the #1 answer. I'd been waiting years to get that piece of trivia off my chest, and it

by edanm 14y ago

The closest I am to being famous is this question on SO, to which I gave the #1 answer. I'd been waiting years to get that piece of trivia off my chest, and it was oh-so-rewarding. The answer was that you can use arrays like so: 5[arr], not just the usual way (arr[5]) Funny enough, several people gave interesting places where this kind of syntax is a good thing. Goes to show that even meaningless pieces of fun on SO still end of enriching the reader.

reeses 14y ago
Pointer arithmetic via syntactic sugar is definitely fun. I still love the occasional "variable index of a literal".
mikeash 14y ago
What places is this syntax a good thing? I read through the comments on your answer, but every single one that claimed this was either pointless (e.g. 0[x] instead of (x)[0]) or not even an example of the trick in question (e.g. "blah"[index] is not this). I was hoping for something interesting, thus my question.
- edanm 14y ago
  Honestly, I wrote this because I remembered that somebody had a use for this. A funny use, but a semi-legitimate one none the less. Unfortunately, I can't really find it now - there is a link to a question which claims to have a use for this syntax, but it's been removed. There are these 2 comments: "Don't forget "Hello World"[i]. Or i["Hello World"] – Richard Pennington Jan 3 '10 at 15:12" "Or, more usefully, "0123456789abcdef"[x & 0xf] " Their basic point is that you can use this trick to find the i'th character in a string. Which I guess is useful sometimes...
  - mikeash 14y ago
    Except that "0123456789abcdef"[x & 0xf] is just normal array dereferencing syntax, not the backwards kind you pointed out. The backwards version would be (x & 0xf)["0123456789abcdef"], which isn't really useful, I think. Oh well. If you happen to find the funny but semi-legitimate one, I'd love to see it.
    - edanm 14y ago
      You're right - that is the normal way (although not commonly seen). I'll update if I find it.
    - greyfade 14y ago
      It was used in one of the early winners of the IOCCC: http://www.ioccc.org/1984/anonymous.c http://www.ioccc.org/1984/anonymous.c It's rumored to have been written by either Kernighan or Ritchie.
  - derobert 14y ago
    The answer on the deleted question (http://stackoverflow.com/questions/469696/what-is-your-most-useful-c-c-utility/470221#470221 http://stackoverflow.com/questions/469696/what-is-your-most-...), by Josh Kelley, gives this: --- Get the number of elements in an array: #define ITEMSOF(arr) (sizeof(arr) / sizeof(0[arr])) (0[arr] is identical to arr[0] for arrays but will intentionally fail if it's used against a C++ object that overloads operator[].) He gives a C++ template version, too. --- ... 10k+ users on Stack Overflow can see deleted stuff. Or you can probably get them out of the CC data dumps (but that's a lot more work than you want, probably). Or maybe out of the data explorer.
    - mikeash 14y ago
      Ah! I saw that #define but didn't see the bit about failing on C++ objects that overload operator[]. I don't do C++ much so that really didn't occur to me. I personally prefer sizeof(*(arr)), which is the same as arr[0], but I suppose C++ lets you overload that as well.

by edanm 14y ago

reeses 14y ago

Pointer arithmetic via syntactic sugar is definitely fun. I still love the occasional "variable index of a literal".

mikeash 14y ago

What places is this syntax a good thing? I read through the comments on your answer, but every single one that claimed this was either pointless (e.g. 0[x] instead of (x)[0]) or not even an example of the trick in question (e.g. "blah"[index] is not this). I was hoping for something interesting, thus my question.

edanm 14y ago
Honestly, I wrote this because I remembered that somebody had a use for this. A funny use, but a semi-legitimate one none the less. Unfortunately, I can't really find it now - there is a link to a question which claims to have a use for this syntax, but it's been removed. There are these 2 comments: "Don't forget "Hello World"[i]. Or i["Hello World"] – Richard Pennington Jan 3 '10 at 15:12" "Or, more usefully, "0123456789abcdef"[x & 0xf] " Their basic point is that you can use this trick to find the i'th character in a string. Which I guess is useful sometimes...
- mikeash 14y ago
  Except that "0123456789abcdef"[x & 0xf] is just normal array dereferencing syntax, not the backwards kind you pointed out. The backwards version would be (x & 0xf)["0123456789abcdef"], which isn't really useful, I think. Oh well. If you happen to find the funny but semi-legitimate one, I'd love to see it.
  - edanm 14y ago
    You're right - that is the normal way (although not commonly seen). I'll update if I find it.
  - greyfade 14y ago
    It was used in one of the early winners of the IOCCC: http://www.ioccc.org/1984/anonymous.c http://www.ioccc.org/1984/anonymous.c It's rumored to have been written by either Kernighan or Ritchie.
- derobert 14y ago
  The answer on the deleted question (http://stackoverflow.com/questions/469696/what-is-your-most-useful-c-c-utility/470221#470221 http://stackoverflow.com/questions/469696/what-is-your-most-...), by Josh Kelley, gives this: --- Get the number of elements in an array: #define ITEMSOF(arr) (sizeof(arr) / sizeof(0[arr])) (0[arr] is identical to arr[0] for arrays but will intentionally fail if it's used against a C++ object that overloads operator[].) He gives a C++ template version, too. --- ... 10k+ users on Stack Overflow can see deleted stuff. Or you can probably get them out of the CC data dumps (but that's a lot more work than you want, probably). Or maybe out of the data explorer.
  - mikeash 14y ago
    Ah! I saw that #define but didn't see the bit about failing on C++ objects that overload operator[]. I don't do C++ much so that really didn't occur to me. I personally prefer sizeof(*(arr)), which is the same as arr[0], but I suppose C++ lets you overload that as well.