$substrCP (aggregation)

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定义
行为
例子

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.

$substrCP 具有以下操作符表达式语法：

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

字段	类型	说明
`string expression`	字符串	要从中提取子字符串的字符串。 `string expression`可以是任何有效的表达式，只要它解析为字符串即可。有关表达式的更多信息，请参阅表达式操作符。如果参数解析为`null` 的值或引用了缺失的字段，则`$substrCP` 会返回空字符串。如果参数未解析为字符串或`null` ，也未引用缺失字段，则`$substrCP` 将返回错误。
`code point index`	数字	Indicates the starting point of the substring. `code point index` can be any valid 表达式(expression) as long as it resolves to a non-negative integer.
`code point count`	数字	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（例如 2.0）。

例子	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"`

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.

例子

单字节字符集

考虑包含以下文档的 inventory 集合：

{ "_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 }

The following operation uses the $substrCP operator to separate the quarter value into a yearSubstring and a quarterSubstring. The quarterSubstring field represents the rest of the string from the specified byte index following the yearSubstring. It is calculated by subtracting the byte index from the length of the string using $strLenCP.

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

操作返回以下结果：

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

单字节和多字节字符集

使用以下文档创建 food 集合：

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" }
 ]
)

The following example uses the $substrCP operator to create a three byte menuCode from the name value:

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

操作返回以下结果：

{ "_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" }

另请参阅：

$substrBytes

后退

$substrBytes

来年

$subtract