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

Definition定义

$indexOfCP

Searches a string for an occurrence of a substring and returns the UTF-8 code point 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码点索引（从零开始）。如果未找到子字符串，则返回-1。

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

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

Field字段	Type类型	Description描述
<string>	string字符串	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, $indexOfCP returns null.如果字符串表达式解析为null值或引用缺少的字段，$indexOfCP将返回null。 If the string expression does not resolve to a string or null nor refers to a missing field, $indexOfCP returns an error.如果字符串表达式未解析为字符串或null，也未引用缺失的字段，则$indexOfCP将返回错误。
<substring>	string字符串	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>	integer整数	Optional. 可选。An integer, or a number that can be represented as integers (such as 2.0), that specifies the starting index position for the search. Can be any valid expression that resolves to a non-negative integral number.一个整数，或一个可以表示为整数的数字（如2.0），指定搜索的起始索引位置。可以是解析为非负整数的任何有效表达式。 If unspecified, the starting index position for the search is the beginning of the string.如果未指定，则搜索的起始索引位置是字符串的开头。
<end>	integer整数	Optional. 可选。An integer, or a number that can be represented as integers (such as 2.0), that specifies the ending index position for the search. 一个整数，或一个可以表示为整数的数字（如2.0），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, $indexOfCP uses the <end> value as the <start> index value instead of the <end> value.如果指定了<end>索引值，则还应指定<start>索引值；否则，$indexOfCP使用<end>值作为<start>索引值，而不是<end>的值。 If unspecified, the ending index position for the search is the end of the string.如果未指定，则搜索的结束索引位置是字符串的末尾。

Behavior行为

If the <substring expression> is found multiple times within the <string expression>, then $indexOfCP returns the index of the first <substring expression> found from the starting index position.如果在<string expression>中多次找到<substring expression>，则$indexOfCP返回从起始索引位置找到的第一个<substring expression>的索引。

$indexOfCP returns null:返回null：

• If <string expression> is null, or如果<string expression>为空，或
• If <string expression> refers to a non-existing field in the input document.如果<string expression>引用了输入文档中不存在的字段。

$indexOfCP returns an error:返回错误：

• If <string expression> is not a string and not null, or如果<string expression>不是字符串且不是null，或者
• If <substring expression> is null or is not a string or refers to a nonexistent field in the input document, or如果<substring expression>为null或不是字符串，或引用输入文档中不存在的字段，或
• If <start> or <end> is a negative integer (or a value that can be represented as a negative integer, like -5.0).如果<start>或<end>是负整数（或可以表示为负整数的值，如-5.0）。

$indexOfCP returns -1:返回-1：

• If the substring is not found in the <string expression>, or如果在<string expression>中找不到子字符串，或者
• If <start> is a number greater than <end>, or如果<start>是一个大于<end>的数字，或者
• If <start> is a number greater than the byte length of the string.如果<start>是一个大于字符串字节长度的数字。

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 $indexOfCP operator to return the code point index at which the string foo is located in each item string:以下操作使用$indexOfCP运算符返回每个item字符串中字符串foo所在的代码点索引：

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

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

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