A howl on the wind…

C++ 16 Feb 2010 19:58:03

C++ Convert String to Int Speed

Comments (9)

9 Responses to “C++ Convert String to Int Speed”

1. on 20 Mar 2010 at 03:47:49 1.Alex said …

   It’s funny.
   But why boost::lexical_cast is so slow?

2. on 20 Mar 2010 at 10:28:03 2.Tino Didriksen said …

   boost::lexical_cast is so slow because it wraps around a std::stringstream and performs many extra checks on top of that, such as ensuring that the entire input was used for conversion.

3. on 09 Mar 2011 at 13:18:58 3.Tomalak Geret'kal said …

   Does MSVS2010 use a C++0x stdlib by default?

4. on 09 Mar 2011 at 13:57:42 4.Tino Didriksen said …

   Yes it does.

5. on 20 Apr 2011 at 01:40:45 5.graphitemaster said …

   the naive method is the fastest – always is.

6. on 13 Jul 2011 at 21:30:04 6.phax said …

   static uint64_t decdigits[100] =
   {
   0ll,0ll,0ll,0ll,0ll,0ll,0ll,0ll,0ll,0,
   1ll,10ll,100ll,1000ll,10000ll,100000ll,1000000ll,10000000ll,100000000ll,1000000000ll,
   2ll,20ll,200ll,2000ll,20000ll,200000ll,2000000ll,20000000ll,200000000ll,2000000000ll,
   3ll,30ll,300ll,3000ll,30000ll,300000ll,3000000ll,30000000ll,300000000ll,3000000000ll,
   4ll,40ll,400ll,4000ll,40000ll,400000ll,4000000ll,40000000ll,400000000ll,400000000ll,
   5ll,50ll,500ll,5000ll,50000ll,500000ll,5000000ll,50000000ll,500000000ll,5000000000ll,
   6ll,60ll,600ll,6000ll,60000ll,600000ll,6000000ll,60000000ll,600000000ll,6000000000ll,
   7ll,70ll,700ll,7000ll,70000ll,700000ll,7000000ll,70000000ll,700000000ll,7000000000ll,
   8ll,80ll,800ll,8000ll,80000ll,800000ll,8000000ll,80000000ll,800000000ll,8000000000ll,
   9ll,90ll,900ll,9000ll,90000ll,900000ll,9000000ll,90000000ll,900000000ll,9000000000ll
   };

   long long int naive2(const char * __restrict__ p)
   {
   bool neg((*p==’-‘)?1:0);
   register int64_t num(0);
   register size_t pos(strlen(p+neg)-1);

   neg && ++p;

   while (*p)
   num+= decdigits[(*p++ – ‘0’)* 10 + pos–];
   return (neg?-num:num);
   }

7. on 14 Jul 2011 at 15:06:49 7.phax said …

   Another naive() in asm though, only tested on Linux g++ 4.4.3, not sure if VC++ supports this semantics for inlining asm.

   long long int __inline__ naive2(const char * p)
   {
   int result(0);
   int neg(0);

   // result -> ecx, p -> ebx
   asm (
   “movl %0, %%ecx\t\n”
   “movl %2, %%ebx\t\n”
   “movl $0, %%edx\t\n”
   “movb (%%ebx),%%dl\t\n”
   “subb $45, %%dl\t\n”
   “jnz loop1\t\n”
   “movl $1, %1\t\n”
   “incl %%ebx\t\n”
   “loop1:\t\n”
   “movl $0, %%edx\t\n”
   “movb (%%ebx), %%dl\t\n”
   “testb %%dl,%%dl\t\n”
   “jz exitloop1\t\n”
   “subb $48, %%dl\t\n”
   “movl %%ecx, %%eax\t\n”
   “shll $3,%%ecx\t\n”
   “addl %%eax, %%ecx\t\n”
   “addl %%eax, %%ecx\t\n”
   “addl %%edx,%%ecx\t\n”
   “incl %%ebx\t\n”
   “jmp loop1\t\n”
   “exitloop1:\t\n”
   “movl %%ecx, %0\t\n”
   :
   “=r” (result), “=m” (neg)
   :
   “r”(p),”0″(result), “m” (neg)
   : “%ebx”, “%eax”, “%ecx”, “%edx”
   );
   return neg?-result:result;
   }

8. on 25 Oct 2015 at 04:39:23 8.Ariel said …

   Could you please help me with something I can’t understand?
   define inf 5000000000LL
   I don’t really get what is that meaning.
   Why there are two ‘LL’ behind the number?
   thank you

9. on 25 Oct 2015 at 11:02:03 9.Tino Didriksen said …

   It means “long long”, see http://en.cppreference.com/w/cpp/language/integer_literal. You’d be better off asking such questions on IRC: Freenode’s ##C++ or QuakeNet’s #C++

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