> ## Documentation Index
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> Use this file to discover all available pages before exploring further.

# Numerical data in Atoti Server

## NaN (Not a Number)

### The danger of NaN

In Java, `NaN` has the property that `NaN != NaN`.
This is a problem when storing `NaN` in a numeric dictionary, as the dictionary uses `==` to compare values.
This means that the value will never be found and each instance of `NaN` will be mapped to a new value in the dictionary.
It leads to an ever-growing dictionary, wasting memory, and potential bad result in queries.

### How to handle NaN:

#### Do not dictionarize a field containing NaN

It is ok to have some `NaN` values in the datastore as long as the field storing the value is not dictionarized.
For instance, such field can be used as a measure.

#### Try to avoid using NaN

If you need to dictionarize the field to use it as a hierarchy, then try to avoid using `NaN`:

* Use another special value as the marker for “no value”, for instance `null`, `Double.NEGATIVE_INFINITY`, `Double.POSITIVE_INFINITY`, `Double.MAX_VALUE`...
* Use a String column with `N/A` and parse the value when you need it as double (parse `N/A` the same way you would use `NaN`)
* Use 2 columns: one `String` and one `double` (you can have one column calculated based on the other) and use the `String` column as the hierarchy and the numeric column for measures.

#### Type the column as `Object`

You can also type the column as Object instead of Double, so it uses an object dictionary which compares values with `Object.equals(...)`.
As `NaN.equals(Nan)` is `true`, the dictionary will be able to correctly match the `NaN`.