使用RapidJSON设置浮点精度 [英] Set floating point precision using rapidjson

问题描述

有没有一种方法可以控制使用RapidJSON生成的JSON中的输出精度?

Is there a way to control the output precision in JSON generated using rapidjson?

例如:

writer.String("length");
writer.Double(1.0 / 3.0);

这会生成类似以下内容的

This generates something like:

{ length: 0.33333333 }

我要发送很多值，几个值只需要小数点后两位.

I'm sending a lot of values and only need two decimal places for several values.

推荐答案

来源

Writer& Double(double d)
{ 
   Prefix(kNumberType);
   WriteDouble(d);
   return *this; 
}

//! \todo Optimization with custom double-to-string converter.
void WriteDouble(double d) {
        char buffer[100];
#if _MSC_VER
    int ret = sprintf_s(buffer, sizeof(buffer), "%g", d);
#else
    int ret = snprintf(buffer, sizeof(buffer), "%g", d);
#endif
    RAPIDJSON_ASSERT(ret >= 1);
    for (int i = 0; i < ret; i++)
         stream_.Put(buffer[i]);
}

对于g转换样式，将转换为e或f样式 执行.

For the g conversion style conversion with style e or f will be performed.

f:精度指定出现在后面的最小位数 小数点字符.默认精度为6.

f: Precision specifies the minimum number of digits to appear after the decimal point character. The default precision is 6.

此处

有一个变体，可以编写新的Writer类并编写自己的Double函数实现.

There is variant, to write new Writer class and write your own Double function realisation.

最后一种情况的简单示例

Simple example of last case

template<typename Stream>
class Writer : public rapidjson::Writer<Stream>
{
public:
   Writer(Stream& stream) : rapidjson::Writer<Stream>(stream)
   {
   }
   Writer& Double(double d)
   {
      this->Prefix(rapidjson::kNumberType);
      char buffer[100];
      int ret = snprintf(buffer, sizeof(buffer), "%.2f", d);
      RAPIDJSON_ASSERT(ret >= 1);
      for (int i = 0; i < ret; ++i)
         this->stream_.Put(buffer[i]);
      return *this;
   }
};

用法类似

int main()
{
   const std::string json =
      "{"
      "\"string\": 0.3221"
      "}";
   rapidjson::Document doc;
   doc.Parse<0>(json.c_str());

   rapidjson::FileStream fs(stdout);
   Writer<rapidjson::FileStream> writer(fs);
   doc.Accept(writer);
}

结果:{"string":0.32}

result: {"string":0.32}

当然，如果您手动使用Writer，则可以使用精度参数编写函数Double.

Of course if you use Writer manually, you can writer function Double with precision parameter.

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