$indexOfBytes (aggregation)

Definition定义

$indexOfBytes

Searches a string for an occurrence of a substring and returns the UTF-8 byte index (zero-based) of the first occurrence. 在字符串中搜索子字符串的出现次数，并返回第一次出现的UTF-8字节索引（从零开始）。If the substring is not found, returns -1.如果未找到子字符串，则返回-1。

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

{ $indexOfBytes: [ <string expression>, <substring expression>, <start>, <end> ] }

Operand	Description描述
<string expression>	Can be any valid expression as long as it resolves to a string. 可以是任何有效的表达式，只要它解析为字符串。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 string expression resolves to a value of null or refers to a field that is missing, $indexOfBytes returns null.如果字符串表达式解析为null值或引用缺少的字段，则$indexOfBytes返回null。 If the string expression does not resolve to a string or null nor refers to a missing field, $indexOfBytes returns an error.如果字符串表达式未解析为字符串或null，也未引用缺少的字段，则$indexOfBytes将返回错误。
<substring expression>	Can be any valid expression as long as it resolves to a string. 可以是任何有效的表达式，只要它解析为字符串。For more information on expressions, see Expressions.有关表达式的详细信息，请参阅表达式。
<start>	Optional. 可选。An integral number that specifies the starting index position for the search. 一个整数，指定搜索的起始索引位置。Can be any valid expression that resolves to a non-negative integral number.可以是解析为非负整数的任何有效表达式。
<end>	Optional. 可选。An integral number that specifies the ending index position for the search. 一个整数，指定搜索的结束索引位置。Can be any valid expression that resolves to a non-negative integral number. 可以是解析为非负整数的任何有效表达式。If you specify a <end> index value, you should also specify a <start> index value; otherwise, $indexOfBytes uses the <end> value as the <start> index value instead of the <end> value.如果指定<end>索引值，还应指定<start>索引值；否则，$indexOfBytes将<end>值用作<start>索引值，而不是<end>值。

Behavior行为

• If <string expression> is null, $indexOfBytes returns null.如果<string expression>为null，则$indexOfBytes返回null。
• If $indexOfBytes is called on a field that doesn't exist in the document, $indexOfBytes returns null.如果对文档中不存在的字段调用了$indexOfBytes，则$indexOfBytes将返回null。
• If <string expression> is not a string and not null, $indexOfBytes returns an error.如果<string expression>不是字符串且不为空，则$indexOfBytes返回错误。
• If <substring expression> is null, $indexOfBytes returns an error.如果<substring expression>为null，则$indexOfBytes返回错误。
• If <start> or <end> is a negative number, $indexOfBytes returns an error.如果<start>或<end>是负数，$indexOfBytes将返回错误。
• If <start> is a number greater than <end>, $indexOfBytes returns -1.如果<start>是大于<end>的数字，$indexOfBytes将返回-1。
• If <start> is a number greater than the byte length of the string, $indexOfBytes returns -1.如果<start>是一个大于字符串字节长度的数字，$indexOfBytes将返回-1。
• If <start> or <end> is given a value that is not an integer, $indexOfBytes returns an error.如果给<start>或<end>的值不是整数，$indexOfBytes将返回错误。
• If the <substring expression> is found multiple times within the <string expression>, then $indexOfBytes returns the index of the first <substring expression> found.如果在<substring expression>中多次找到<string expression>，则$indexOfBytes将返回找到的第一个<string expression>的索引。

Some short examples to highlight different behavior:一些简短的例子强调了不同的行为：

Examples示例

Consider an inventory collection with the following documents:考虑具有以下文档的inventory集合：

{ "_id" : 1, "item" : "foo" }
{ "_id" : 2, "item" : "fóofoo" }
{ "_id" : 3, "item" : "the foo bar" }
{ "_id" : 4, "item" : "hello world fóo" }
{ "_id" : 5, "item" : null }
{ "_id" : 6, "amount" : 3 }

The following operation uses the $indexOfBytes operator to retrieve the indexes at which the string foo is located in each item:以下操作使用$indexOfBytes运算符检索每个项中字符串foo所在的索引：

db.inventory.aggregate(
   [
     {
       $project:
          {
            byteLocation: { $indexOfBytes: [ "$item", "foo" ] },
          }
      }
   ]
)

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

{ "_id" : 1, "byteLocation" : "0" }
{ "_id" : 2, "byteLocation" : "4" }
{ "_id" : 3, "byteLocation" : "4" }
{ "_id" : 4, "byteLocation" : "-1" }
{ "_id" : 5, "byteLocation" : null }
{ "_id" : 6, "byteLocation" : null }