$substrCP (aggregation)（聚合）

Definition定义

$substrCP

Returns the substring of a string. The substring starts with the character at the specified UTF-8 code point (CP) index (zero-based) in the string for the number of code points specified.返回字符串的子字符串。子字符串以字符串中指定的UTF-8码点（CP）索引（从零开始）处的字符开头，对应指定的码点数。

$substrCP has the following operator expression syntax:具有以下运算符表达式语法：

{ $substrCP: [ <string expression>, <code point index>, <code point count> ] }

Field字段	Type类型	Description描述
string expression	string字符串	The string from which the substring will be extracted. 从中提取子字符串的字符串。string expression can be any valid expression as long as it resolves to a string. string expression可以是任何有效的表达式，只要它解析为字符串即可。For more information on expressions, see Expressions.有关表达式的详细信息，请参阅表达式。 If the argument resolves to a value of null or refers to a field that is missing, $substrCP returns an empty string.如果参数解析为null值或引用缺少的字段，$strCP将返回一个空字符串。 If the argument does not resolve to a string or null nor refers to a missing field, $substrCP returns an error.如果参数未解析为字符串或null，也未引用缺失的字段，$subrCP将返回错误。
code point index	number数字	Indicates the starting point of the substring. code point index can be any valid expression as long as it resolves to a non-negative integer.指示子字符串的起点。code point index可以是任何有效的表达式，只要它解析为非负整数即可。
code point count	number数字	Can be any valid expression as long as it resolves to a non-negative integer or number that can be represented as an integer (such as 2.0).可以是任何有效的表达式，只要它解析为非负整数或可以表示为整数的数字（如2.0）。

Example示例	Results结果
{ $substrCP: [ "abcde", 1, 2 ] }	"bc"
{ $substrCP: [ "Hello World!", 6, 5 ] }	"World"
{ $substrCP: [ "cafétéria", 0, 5 ] }	"cafét"
{ $substrCP: [ "cafétéria", 5, 4 ] }	"éria"
{ $substrCP: [ "cafétéria", 7, 3 ] }	"ia"
{ $substrCP: [ "cafétéria", 3, 1 ] }	"é"

Behavior行为

The $substrCP operator uses the code points to extract the substring. This behavior differs from the $substrBytes operator which extracts the substring by the number of bytes, where each character uses between one and four bytes.$strCP运算符使用代码点来提取子字符串。此行为与$strBytes运算符不同，后者按字节数提取子字符串，其中每个字符使用一到四个字节。

Example示例

db.inventory.insertMany( [
   { _id: 1, item: "ABC1", quarter: "13Q1", description: "product 1" },
   { _id: 2, item: "ABC2", quarter: "13Q4", description: "product 2" },
   { _id: 3, item: "XYZ1", quarter: "14Q2", description: null }
] )

db.inventory.aggregate(
  [
    {
      $project: {
        item: 1,
        yearSubstring: { $substrCP: [ "$quarter", 0, 2 ] },
        quarterSubtring: {
          $substrCP: [
            "$quarter", 2, { $subtract: [ { $strLenCP: "$quarter" }, 2 ] }
          ]
        }
      }
    }
  ]
)

{ _id: 1, item: "ABC1", yearSubstring: "13", quarterSubtring: "Q1" }
{ _id: 2, item: "ABC2", yearSubstring: "13", quarterSubtring: "Q4" }
{ _id: 3, item: "XYZ1", yearSubstring: "14", quarterSubtring: "Q2" }

db.food.insertMany( [
   { _id: 1, name: "apple" },
   { _id: 2, name: "banana" },
   { _id: 3, name: "éclair" },
   { _id: 4, name: "hamburger" },
   { _id: 5, name: "jalapeño" },
   { _id: 6, name: "pizza" },
   { _id: 7, name: "tacos" },
   { _id: 8, name: "寿司sushi" }
] )

db.food.aggregate(
  [
    {
      $project: {
          "name": 1,
          "menuCode": { $substrCP: [ "$name", 0, 3 ] }
      }
    }
  ]
)

{ _id: 1, name: "apple", menuCode: "app" }
{ _id: 2, name: "banana", menuCode: "ban" }
{ _id: 3, name: "éclair", menuCode: "écl" }
{ _id: 4, name: "hamburger", menuCode: "ham" }
{ _id: 5, name: "jalapeño", menuCode: "jal" }
{ _id: 6, name: "pizza", menuCode: "piz" }
{ _id: 7, name: "tacos", menuCode: "tac" }
{ _id: 8, name: "寿司sushi", menuCode: "寿司s" }