$strLenBytes (aggregation)
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Definition
$strLenBytes
New in version 3.4.
Returns the number of UTF-8 encoded bytes in the specified string.
$strLenBytes
has the following operator expression syntax:{ $strLenBytes: <string expression> } The argument can be any valid expression as long as it resolves to a string. For more information on expressions, see Expressions.
If the argument resolves to a value of
null
or refers to a missing field,$strLenBytes
returns an error.
Behavior
The $strLenBytes
operator counts the number of UTF-8
encoded bytes in a string where each character may use between one
and four bytes.
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. Chinese, Japanese and Korean characters typically require three bytes, and other planes of unicode (emoji, mathematical symbols, etc.) require four bytes.
The $strLenBytes
operator differs from
$strLenCP
operator which counts the
code points
in the specified string regardless of how many bytes each character
uses.
Example | Results | Notes | |
---|---|---|---|
| 5 | Each character is encoded using one byte. | |
| 12 | Each character is encoded using one byte. | |
| 9 | Each character is encoded using one byte. | |
| 11 | é is encoded using two bytes. | |
| 0 | Empty strings return 0. | |
| 7 | € is encoded using three bytes.
λ is encoded using two bytes. | |
| 6 | Each character is encoded using three bytes. |
Example
Single-Byte and Multibyte Character Set
Create a food
collection with the following documents:
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:
db.food.aggregate( [ { $project: { "name": 1, "length": { $strLenBytes: "$name" } } } ] )
The operation returns the following results:
{ "_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. The document with _id: 8
contains two Japanese characters
that are encoded using three bytes each. This makes the length
greater than the number of characters in name
for the documents
with _id: 3
, _id: 5
and _id: 8
.