Question

我正在尝试汇总一组食谱。该集合中的每个食谱都有一个calories字段，即一次服务中的卡路里量。我需要找到每1,2和3份最接近 1000 卡路里的食谱。我知道你不能像我在下面那样使用$ min运算符，但它只是在那里提出一个想法。

[
    {
      $project: {
        recipe: "$$ROOT",
        "caloriesInOneServing": "$calories",
        "caloriesInTwoServings": {$multiply: ["$calories", 2.0]},
        "caloriesInThereServings": {$multiply: ["$calories", 3.0]}
      }
    },
    {
       $project: {
        recipe: 1,
        "caloriesInOneServingDiff": {$subtract: ["$caloriesInOneServing", 1000],
        "caloriesInTwoServingsDiff": {$subtract: ["$caloriesInTwoServings", 1000],
        "caloriesInThereServingsDiff": {$subtract: ["$caloriesInThereServings", 1000]
      }
    },
    {
       $project: {
        recipe: 1,
        //how do i find the min value among given fields?
        minDiff: {$min: [$caloriesInOneServingDiff, caloriesInTwoServingsDiff, caloriesInThereServingsDiff]
      }
    },
    {
       $sort: { minDiff: -1 }
    }
  ]

Answer 1

这可以通过条件减法来完成：

Int operator*(Int b) const{
    return Int{value*b.value};
}

Int operator/(Int b) const{
    return Int{value/b.value};
}

Int operator-(Int b) const{
    return Int{value - b.value};
}

const Int& operator++() {
    ++value;
    return *this;
}

Int operator--(int) {
    Int copy{value};
    value--;
    return copy;
    }
};

ostream& operator<<(ostream& out, Int x) {
    out << x.value;
}

int main(){
    int a = 11; Int A{11};
    int b = 4; Int B{4};
    cout << ++a / b * b-- - a << endl; // 0
    cout << ++A / B * B-- - A << endl; // 4
}

示例：

[
    {
      $project: {
        recipe: "$$ROOT",
        "caloriesInOneServing": "$calories",
        "caloriesInTwoServings": {$multiply: ["$calories", 2.0]},
        "caloriesInThereServings": {$multiply: ["$calories", 3.0]}
      }
    },
    {
       $project: {
        recipe: 1,
        "caloriesInOneServingDiff": {$cond: [
              { $lt: ['$caloriesInOneServing', 1000] },
              { $subtract: [1000, '$caloriesInOneServing'] }, 
              { $subtract: ['$caloriesInOneServing', 1000] }
            ]},
        "caloriesInTwoServingsDiff": {$cond: [
              { $lt: ['$caloriesInTwoServings', 1000] },
              { $subtract: [1000, '$caloriesInTwoServings'] }, 
              { $subtract: ['$caloriesInTwoServings', 1000] }
            ]},
        "caloriesInThereServingsDiff": {$cond: [
              { $lt: ['$caloriesInThereServings', 1000] },
              { $subtract: [1000, '$caloriesInThereServings'] }, 
              { $subtract: ['$caloriesInThereServings', 1000] }
            ]}
      }
    },
    {
       $project: {
        recipe: 1,
        minDiff: {$min: [$caloriesInOneServingDiff, caloriesInTwoServingsDiff, caloriesInThereServingsDiff]
      }
    },
    {
       $sort: { minDiff: -1 }
    }
  ]

条件减法后

：

calories = 300
x2 = 600
x3 = 900

min = x3，最接近1000

Mongo聚合框架：在预测值内找到最小值

1 个答案: