Method 1

#include <cmath>  int countDecimalPlaces(double num) 
{  if (num == 0) return 0;  // 避免除以零错误  return std::floor(std::log10(std::abs(num - std::floor(num)))) + 1;  
}

Method 2

#include <string>  
#include <sstream>  int countDecimalPlaces(double num) 
{  std::ostringstream oss;  oss << std::fixed << num;  // 保留固定的小数位数  std::string str = oss.str();  return str.find('.') - str.find('e') > 0 ? str.find('.') + 1 : str.find('.');  
}

Method 3

#include <cmath>  int countDecimalPlaces(double num) 
{  if (num == 0) return 0;  // 避免除以零错误  return std::abs(std::modf(num, nullptr)) == 0 ? 0 : std::floor(std::log10(std::abs(std::modf(num, nullptr)))) + 1;  
}