$indexOfBytes (expression operator)（表达式运算符）

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. If the substring is not found, returns -1.在字符串中搜索子字符串的出现，并返回第一个出现的UTF-8字节索引（从零开始）。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操作数	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 string expression resolves to a value of null or refers to a field that is missing, $indexOfBytes returns null.如果字符串表达式解析为null值或引用缺少的字段，$indexOfBytes将返回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. For more information on expressions, see Expressions.可以是任何有效的表达式，只要它解析为字符串。有关表达式的更多信息，请参阅表达式。
<start>	Optional 可选的。An integral number that specifies the starting index position for the search. Can be any valid expression that resolves to a non-negative integral number.指定搜索起始索引位置的整数。可以是解析为非负整数的任何有效表达式。
<end>	Optional 可选的。An integral number that specifies the ending index position for the search. Can be any valid expression that resolves to a non-negative integral number. 指定搜索结束索引位置的整数。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, $indexOfBytes uses the <end> value as the <start> index value instead of the <end> value.如果指定了<end>索引值，则还应指定<start>索引值；否则，$indexOfBytes使用<end>值作为<start>索引值，而不是<end>的值。

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, $indexOfBytes returns null.如果对文档中不存在的字段调用$indexOfBytes，$indexOfBytes将返回null。
• If <string expression> is not a string and not null, $indexOfBytes returns an error.如果<string expression>不是字符串且不为null，则$indexOfBytes将返回错误。
• 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>, $indexOfBytes returns -1.如果<start>是一个大于<end>的数字，$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, $indexOfBytes returns an error.如果给<start>或<end>一个不是整数的值，$indexOfBytes将返回错误。
• If the <substring expression> is found multiple times within the <string expression>, then $indexOfBytes returns the index of the first <substring expression> found.如果在<string expression>中多次找到<substring expression>，则$indexOfBytes将返回找到的第一个<substring expression>的索引。

Some short examples to highlight different behavior:一些简短的例子来突出不同的行为：

Examples示例

Consider an inventory collection with the following documents:考虑使用以下文件进行inventory集合：

db.inventory.insertMany( [
   { _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 }