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$atan2 (aggregation)

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$atan2

New in version 4.2.

Returns the inverse tangent (arc tangent) of y / x, where y and x are the first and second values passed to the expression respectively.

$atan2 has the following syntax:

copy 
{ $atan2: [ <expression 1>, <expression 2> ] }

$atan2 takes any valid expression that resolves to a number.

$atan2 returns values in radians. Use $radiansToDegrees operator to convert the output value from radians to degrees.

By default $atan2 returns values as a double. $atan2 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.

Behavior

null and NaN

If either argument given to $atan2 is null, the expression returns null. If either argument is NaN, the expression returns NaN. If one argument is null and the other is NaN, the expression returns null.

Example Results
{ $atan2: [ NaN, <value> ] } NaN
{ $atan2: [ <value>, NaN ] } NaN
{ $atan2: [ null, <value> ] } null
{ $atan2: [ <value>, null ] } null
{ $atan2: [ NaN, null ] } null
{ $atan2: [ null, NaN ] } null

Example

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

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

The following aggregation operation uses the $atan2 expression to calculate the angle adjacent to side_a and add it to the input document using the $addFields pipeline stage.

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db.trigonometry.aggregate([
  {
    $addFields : {
      "angle_a" : {
        $radiansToDegrees : {
          $atan2 : [ "$side_b", "$side_a" ]
        }
      }
    }
  }
])

The $radiansToDegrees expression converts the radian value returned by $atan2 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 $atan2 is a 128-bit decimal.

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

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

The following aggregation operation uses the $atan2 expression to calculate the angle adjacent to side_a and add it to the input document using the $addFields pipeline stage.

copy 
db.trigonometry.aggregate([
  {
    $addFields : {
      "angle_a" : {
        $atan2 : [ "$side_b", "$side_a" ]
      }
    }
  }
])

The command returns the following output:

{
  "_id" : ObjectId("5c50782193f833234ba90d85"),
  "side_a" : NumberDecimal("3"),
  "side_b" : NumberDecimal("4"),
  "hypotenuse" : NumberDecimal("5"),
  "angle_a" : NumberDecimal("0.9272952180016122324285124629224287")
}

Since side_b and side_a are stored as 128-bit decimals, the output of $atan2 is a 128-bit decimal.

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