C++将子字符串解析为整数

Question

我需要一个 strtol 函数，它允许我指定字符串将被解析的范围。例如

char * number = "123456789";
std::cout << my_strtol(number, 1, 3);

应该打印“234”。

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使用一些指针算法我可以非常接近:

int32_t my_strtol(const char* str, int from_char, int to_char, int base = 10)
{
  char *end;
  auto res = strtol(str + from_char, &end, base);
  auto extra_digits = end - str - (to_char + 1);
  if(extra_digits > 0) res /= pow(10, extra_digits); 
  return res;
}

然而，如果字符串大于 LONG_MAX(不管该部分的值如何)，它就会失败。例如输入 "1234567890123456789" 调用 str_to_l(number, 1, 3) 将失败并返回 0。此外，它使用了不必要的 pow –性能至关重要。

编辑:我想避免构造新的字符串或字符串流，因为我将从同一个缓冲区中提取多个整数，更重要的是我将字面执行数以万计的此类电话。

---

对于那些对建议解决方案的(随意)分析结果感兴趣的人:

//put temp null character at end
int32_t my_strtol1(char* str, int from_char, int to_char, int base = 10)
{
    char tmp = str[to_char];
    str[to_char] = '\0';
    auto res = strtol(str + from_char, nullptr, base);
    str[to_char] = tmp;
    return res;
}

//use substr()
int32_t my_strtol2(const std::string& str,
    const std::string::size_type from_char,
    const std::string::size_type to_char,
    int base = 10) {
    return std::stol(str.substr(from_char, to_char - from_char + 1));
}

//using boost
int32_t my_strtol3(char* str, int from_char, int to_char) {
    return boost::lexical_cast<int>(str + from_char, to_char - from_char + 1);
}

//parse characters one by one
int32_t my_strtol4(const char* str, int from_char, int to_char)
{
    int32_t res = 0;
    for (int i = from_char; i < to_char; i++)
    {
        char ch = str[i];
        ch -= '0';
        if (ch > 10 || ch < 0) return 0;
        res *= 10;
        res += ch;
    }
    return res;
}

我机器上的输出(由 clock_t 测量)是:

Manipulating null character with 100000 iterations took 0.114s
Using substr() with 100000 iterations took 0.62s
Using boost::lexical_cast<T>() with 100000 iterations took 0.231s
Parsing character one by one with 100000 iterations took 0.083s

Answer 1

因此，如果您不需要 super 便携性(例如支持泰语、中文和阿拉伯数字):

int32_t my_strtol(const char* str, int from_char, int to_char, int base = 10)
{
   int32_t res = 0; 
   for(int i = from_char; i < to_char; i++)
   {
      char ch = str[i];
      if (ch > '9' && base > 10)
      {
         ch &= ~32;    /* Make it upper case */
         ch -= 'A' + 10;
      }
      else 
      {
          ch -= '0';
      }
      if (ch > base || ch < 0)   ... do some error handling ... 
      res *= base;
      res += ch;
   }
   return res;
}