4 ms·
If you want to be picky, I'ld go with: if ((x >= 'a' && x <= 'z') || (x >= 'A' && x <= 'Z') || (x >= '0' && x <= '9')) As in a regular tex
by bbrizzi 15y ago
If you want to be picky, I'ld go with:
if ((x >= 'a' && x <= 'z') ||
(x >= 'A' && x <= 'Z') ||
(x >= '0' && x <= '9'))
As in a regular text file you're more likely to hit an alphanumerical character than a number. If the first OR statement is true, any good compiler will skip the next two clauses.
- viraptor 15y agoIf you want to be super-picky, then char[256] with 1/0 values for each character will be faster than 6 comparisons in the worst case.
- DrJokepu 15y agoAnd if you want to be super-super picky, int[256] on 32-bit or long[256] on 64-bit would be even faster. (Although I'm not sure if lookup would be faster at all than a few comparisons, considering caches and everything, but this is not my domain.)
- viraptor 15y agoA jump address table could be even faster since it's not branching...
- mikeash 15y agoWould it? Loading a single byte should still be fast (in the absolute worst case, it's a single shift and a mask on top of the cost of loading 4 or 8 bytes) and using less memory means it's much more likely for the table to remain in cache (and evict less other stuff). In fact, I'd go so far as to wager that a uint8_t[16] (or uint64_t[4] or whatever) with one bit per character, manually extracted, would be the fastest way to do it. Not that I've tested this or anything, and I could certainly be wrong.
- maximilianburke 15y agoI hate to break up perfectly good hand-waving with numbers but I put together a test. On my Core i7 with an 18mb input, which is a large novel duplicated many times, the performance factors, normalized to the if-based version are: if: 1x byte-based lookup: 0.83x int-based lookup: 0.84x bit-vector based lookup: 0.93x http://pastebin.com/5twfXfEt http://pastebin.com/5twfXfEt
- ori_b 15y agoUnless it bloats your data to the point where other stuff falls out of L1 cache.
- schrototo 15y ago> If the first OR statement is true, any good compiler will skip the next two clauses. This has nothing to do with the compiler, it's part of the language specification. If your compiler doesn't comply with this, it's broken.