In MongoDB, the $abs
aggregation pipeline operator returns the absolute value of a number.
Example
Suppose we have a collection called data
with the following document:
{ "_id" : 1, "a" : 20, "b" : -20 }
We can use the $abs
operator to return the absolute values of the a
and b
fields.
db.data.aggregate(
[
{ $project: {
_id: 0,
a: { $abs: [ "$a" ] },
b: { $abs: [ "$b" ] }
}
}
]
)
Result:
{ "a" : 20, "b" : 20 }
Absolute values don’t have include any signs, and so we can see the negative sign has been removed from the b
value.
You can think of an absolute value of a number as being the distance, on the number line, of that number from zero.
Null Values
Null values return null
when using the $abs
operator.
Suppose we add the following document to our collection:
{ "_id" : 2, "a" : 0, "b" : null }
Let’s run the the $abs
operator against that document:
db.data.aggregate(
[
{ $match: { _id: 2 } },
{ $project: {
_id: 0,
a: { $abs: [ "$a" ] },
b: { $abs: [ "$b" ] }
}
}
]
)
Result:
{ "a" : 0, "b" : null }
We can see that b
resolved to null
.
We can also see that 0
resolves to 0
.
NaN Values
If the argument resolves to NaN
, $abs
returns NaN
.
Example:
db.data.aggregate(
[
{ $match: { _id: 2 } },
{ $project: {
_id: 0,
a: { $abs: [ "$a" ] },
b: { $abs: [ 1 * "g" ] }
}
}
]
)
Result:
{ "a" : 0, "b" : NaN }
In this case I tried to multiple a number by a string, which resulted in NaN
being returned.
Non-Existent Fields
If the $abs
operator is applied against a field that doesn’t exist, null
is returned.
Example:
db.data.aggregate(
[
{ $match: { _id: 2 } },
{ $project: {
_id: 0,
c: { $abs: [ "$c" ] }
}
}
]
)
Result:
{ "c" : null }
Combined With Other Operators
In this example I combine $abs
with $subtract
in order to calculate the magnitude of difference between fields a
and b
:
db.data.aggregate(
[
{ $match: { _id: 1 } },
{ $project: {
_id: 0,
a: 1,
b: 1,
result: {
$abs: {
$subtract: [ "$a", "$b" ]
}
}
}
}
]
)
Result:
{ "a" : 20, "b" : -20, "result" : 40 }