$substrCP (aggregation)

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代码点（CP）索引（从零开始）处的字符开始，该索引用于指定的代码点数量。

$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	The string from which the substring will be extracted. 将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. For more information on expressions, see Expressions.string expression可以是任何有效的表达式，只要它解析为字符串即可。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, $substrCP returns an empty string.如果参数解析为null值或引用了丢失的字段，$substrCP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, $substrCP returns an error. 如果参数没有解析为字符串或null，也没有引用丢失的字段，那么$substrCP将返回一个错误。
code point index	number	Indicates the starting point of the substring. 指示子字符串的起始点。code point index can be any valid expression as long as it resolves to a non-negative integer.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	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. $substrCP运算符使用代码点来提取子字符串。This behavior differs from the $substrBytes operator which extracts the substring by the number of bytes, where each character uses between one and four bytes.此行为不同于$substrBytes运算符，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实例

{ "_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" }