> For the complete documentation index, see [llms.txt](https://george-jen.gitbook.io/data-science-and-apache-spark/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://george-jen.gitbook.io/data-science-and-apache-spark/scala-tips-for-updatestatebykey.md).

# Scala Tips for updateStateByKey

In the previous section on udateStateByKey, following we need to brush up some Scala methods:

Specifically:

```
val updateFunc = (values: Seq[Int],state: Option[Int])=>{
val currentCount = values.foldLeft(0)(_+_)
val previousCount = state.getOrElse(0)
Some(currentCount + previousCount)
}
```

A few methods we need to discuss here:

#### values.foldLeft(0)(\_+\_)

In fact, there are follow fold\* methods for any collections (such as List): 

fold(Initial:T)(func) 

foldLeft(Initial:T)(func) 

foldRight(Initial:T)(func)

*Tips: fold methods above are very similar to reduce method, except fold methods allow a initial value as argument, while reduce does not*.

#### state:Option\[Int]

this means argument state is an integer, but can be None. In Scala, it is denoted as Option\[Int]

If a variable in Scala may be None in value, it must be declared as Option\[T] (T is a datatype placeholder, such as Int, String etc)

An Option\[T] can be either Some\[T] or None object, which represents a missing value.

If a variable needs to be computed with, for example having data type  Option\[Int], to avoid computing a None value, an Optional variable has a method called getOrElse(default value), such as

state.getOrElse(0)

For the above function updateFunc, let's test it:

```
val updateFunc = (values: Seq[Int],state: Option[Int])=>{
val currentCount = values.foldLeft(0)(_+_)
val previousCount = state.getOrElse(0)
currentCount + previousCount
}

```

Then create a list x, and y:Option\[Int], assigned to None, then call updateFunc

```
val x=Seq(1,2,3,4,5)
val y:Option[Int]=None
updateFunc(x,y)

/*Output
15
*/

```

Now, replace foldLeft(0)(\_+\_) with fold(0)(\_+\_), foldRight(0)(\_+\_) and reduce(\_+\_), and test them to reinforce understanding.


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