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$strLenBytes (aggregation)

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Definition定义

$strLenBytes

Returns the number of UTF-8 encoded bytes in the specified string.返回指定字符串中UTF-8编码的字节数。

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

{ $strLenBytes: <string expression> }

The argument can be any valid expression as long as it resolves to a string. For more information on expressions, see Expressions.参数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 missing field, $strLenBytes returns an error.如果参数解析为null值或引用了丢失的字段，$strLenBytes将返回错误。

Behavior行为

The $strLenBytes operator counts the number of UTF-8 encoded bytes in a string where each character may use between one and four bytes.$strLenBytes运算符统计字符串中UTF-8编码的字节数，其中每个字符可以使用一到四个字节。

For example, US-ASCII characters are encoded using one byte. Characters with diacritic markings and additional Latin alphabetical characters (i.e. Latin characters outside of the English alphabet) are encoded using two bytes. 例如，US-ASCII字符使用一个字节进行编码。带有变音符号标记的字符和附加的拉丁字母字符（即英语字母表之外的拉丁字符）使用两个字节进行编码。Chinese, Japanese and Korean characters typically require three bytes, and other planes of unicode (emoji, mathematical symbols, etc.) require four bytes.中文、日文和韩文字符通常需要三个字节，而unicode的其他平面（表情符号、数学符号等）需要四个字节。

The $strLenBytes operator differs from $strLenCP operator which counts the code points in the specified string regardless of how many bytes each character uses.$strLenBytes运算符不同于$strLenCP运算符，后者统计指定字符串中的代码点，而不管每个字符使用多少字节。

Example示例Results结果Notes备注
 
{ $strLenBytes: "abcde" }
5Each character is encoded using one byte.每个字符都使用一个字节进行编码。
{ $strLenBytes: "Hello World!" }
12Each character is encoded using one byte.每个字符都使用一个字节进行编码。
{ $strLenBytes: "cafeteria" }
9Each character is encoded using one byte.每个字符都使用一个字节进行编码。
{ $strLenBytes: "cafétéria" }
11é is encoded using two bytes.使用两个字节进行编码。
{ $strLenBytes: "" }
0Empty strings return 0.空字符串返回0。
{ $strLenBytes: "$€λG" }
7€ is encoded using three bytes. 使用三个字节进行编码。λ is encoded using two bytes.使用两个字节进行编码。
{ $strLenBytes: "寿司" }
6Each character is encoded using three bytes.每个字符使用三个字节进行编码。

Example实例

Single-Byte and Multibyte Character Set单字节和多字节字符集

Create a food collection with the following documents: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" }
 ]
)

The following operation uses the $strLenBytes operator to calculate the length of each name value:以下操作使用$strLenBytes运算符来计算每个name值的length：

db.food.aggregate(
  [
    {
      $project: {
        "name": 1,
        "length": { $strLenBytes: "$name" }
      }
    }
  ]
)

The operation returns the following results:该操作返回以下结果：

{ "_id" : 1, "name" : "apple", "length" : 5 }
{ "_id" : 2, "name" : "banana", "length" : 6 }
{ "_id" : 3, "name" : "éclair", "length" : 7 }
{ "_id" : 4, "name" : "hamburger", "length" : 9 }
{ "_id" : 5, "name" : "jalapeño", "length" : 9 }
{ "_id" : 6, "name" : "pizza", "length" : 5 }
{ "_id" : 7, "name" : "tacos", "length" : 5 }
{ "_id" : 8, "name" : "寿司", "length" : 6 }

The documents with _id: 3 and _id: 5 each contain a diacritic character (é and ñ respectively) that requires two bytes to encode. _id:3和_id:5的文档分别包含一个变音符号（é和ñ），需要两个字节进行编码。The document with _id: 8 contains two Japanese characters that are encoded using three bytes each. _id为8的文档包含两个日语字符，每个字符使用三个字节进行编码。This makes the length greater than the number of characters in name for the documents with _id: 3, _id: 5 and _id: 8.这使得_id:3、_id:5和_id:8的文档的长度大于name中的字符数。

Tip

See also: 另请参阅：