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$atan (aggregation)

$atan

New in version 4.2.

Returns the inverse tangent (arc tangent) of a value.

$atan has the following syntax:

{ $atan: <expression> }

$atan takes any valid expression that resolves to a number.

$atan returns values in radians. Use $radiansToDegrees operator to convert the output value from radians to degrees.

By default $atan returns values as a double. $atan can also return values as a 128-bit decimal as long as the <expression> resolves to a 128-bit decimal value.

For more information on expressions, see Expressions.

If the argument resolves to a value of null or refers to a field that is missing, $atan returns null. If the argument resolves to NaN, $tan returns NaN.

Example
Results
{ $atan: NaN }
NaN
{ $atan: null }
null

The trigonometry collection contains a document that stores three sides of a right-angle triangle:

{
"_id" : ObjectId("5c50782193f833234ba90d85"),
"side_a" : NumberDecimal("3"),
"side_b" : NumberDecimal("4"),
"hypotenuse" : NumberDecimal("5")
}

The following aggregation operation uses the $atan expression to calculate the angle adjacent to side_a and add it to the input document using the $addFields pipeline stage.

db.trigonometry.aggregate([
{
$addFields : {
"angle_a" : {
$radiansToDegrees : {
$atan : {
$divide : [ "$side_b", "$side_a" ]
}
}
}
}
}
])

The $radiansToDegrees expression converts the radian value returned by $atan to the equivalent value in degrees.

The command returns the following output:

{
"_id" : ObjectId("5c50782193f833234ba90d85"),
"side_a" : NumberDecimal("3"),
"side_b" : NumberDecimal("4"),
"hypotenuse" : NumberDecimal("5"),
"angle_a" : NumberDecimal("53.13010235415597870314438744090658")
}

Since side_b and side_a are stored as 128-bit decimals, the output of $atan is a 128-bit decimal.

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