使用 MongoDB 聚合框架四舍五入到小数点后 2 位 [英] Rounding to 2 decimal places using MongoDB aggregation framework

问题描述

我正在使用 mongodb 聚合框架并进行如下所示的一些计算

I am using the mongodb aggregation framework and doing some calculations as shown below

db.RptAgg.aggregate( 
  {$group: {
    _id: {Region: "$RegionTxt", Mth: "$Month"},           
    ActSls: {$sum:"$ActSls"},
    PlnSls: {$sum:"$PlnSls"}
  }},
  {$project: {
    ActSls: 1,
    PlnSls: 1,
    ActToPln: {$cond: [
      {$ne: ["$PlnSls", 0]}, 
      {$multiply: [{$divide: ['$ActSls', '$PlnSls']}, 100]}, 
      0
    ]}
  }}
); 

我正在尝试找出将我的结果四舍五入到小数点后 2 位的最佳和最简单的方法.以下是我的结果

I am trying to figure out what is the best and easiest way to round my results to 2 decimal places. Following is my result

{
  "result": [{
    "_id": {
      "Region": "East",
      "Mth": 201301
    },
    "ActSls": 72,
    "PlnSls": 102,
    "ActToPln": 70.58823529411765
  }],
  "ok": 1
}

我想要ActToPln"显示 70.59 而不是 ActToPln":70.58823529411765，在聚合框架本身的结果中.我想避免在我的应用程序中进行舍入

I want "ActToPln" to show 70.59 instead of "ActToPln" : 70.58823529411765, in the results from aggegation framework itself. I want to avoid doing the rounding in my application

你能帮忙吗?

以下是我使用的数据集.

Following is the dataset i used.

{
    "_id" : ObjectId("51d67ef69557c507cb172572"),
    "RegionTxt" : "East",
    "Month" : 201301,
    "Date" : "2013-01-01",
    "ActSls" : 31,
    "PlnSls" : 51
}
{
    "_id" : ObjectId("51d67ef69557c507cb172573"),
    "RegionTxt" : "East",
    "Month" : 201301,
    "Date" : "2013-01-02",
    "ActSls" : 41,
    "PlnSls" : 51
}

提前致谢.南都

推荐答案

没有 $round 操作符，但是你可以在聚合框架中这样做——按特定顺序执行通常可以避免浮点数精度问题.

There is no $round operator but you can do this in the aggregation framework - doing it in specific order will usually avoid floating point precision issues.

> db.a.save({x:1.23456789})
> db.a.save({x:9.87654321})
> db.a.aggregate([{$project:{ _id:0, 
         y:{$divide:[
              {$subtract:[
                      {$multiply:['$x',100]},
                      {$mod:[{$multiply:['$x',100]}, 1]}
              ]},
              100]}
}}])
{ "y" : 1.23 }
{ "y" : 9.87 }

鉴于问题中存在的管道，替换:

Given the existing pipeline in the problem, replace:

{$multiply:[{$divide: ['$ActSls', '$PlnSls']},100]}

与

{$divide:[
     {$subtract:[ 
          {$multiply:[
             {$divide: ['$ActSls','$PlnSls']},
             10000
          ]}, 
          {$mod:[
             {$multiply:[{$divide: ['$ActSls','$PlnSls']}, 10000 ]},
             1]}
          ]}, 
     100
]}

使用您的示例数据点，结果如下:

With your sample data points this is the result:

{ "ActSls" : 31, "PlnSls" : 51, "ActToPln" : 60.78 }
{ "ActSls" : 41, "PlnSls" : 51, "ActToPln" : 80.39 }
{ "ActSls" : 72, "PlnSls" : 102, "ActToPln" : 70.58 }

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