## Monday, April 29, 2013

### Scala Lists - Partition

Two very common list functions are filter and filterNot, where the former function returns a new list where all of the elements in the list hold true for a specified predicate and the latter function returns a new list where all of the elements in the list do not hold true for a specified predicate. For example, we can use these two functions to traverse a list of integers and generate a list of all positive integers, as well as a list of all negative integers.

```    val l: List[Int] = List(-12, 2, -2, -3, 4, 5, 6)

val positiveList = l.filter(x => x > 0)
val negativeList = l.filterNot(x => x > 0)
```

The list function partition combines both of these operations into a single traversal of the original list and returns two lists, one list of values where the predicate holds true and another list of values where the predicate does not hold true.

```val l: List[Int] = List(-12, 2, -2, -3, 4, 5, 6)

val positiveAndNegativeLists = l.partition(x => x > 0)
```

We can then access each list using positiveAndNegativeLists._1 and positiveAndNegativeLists._2, where the former is the list of all elements where the predicate is true and the latter contains all elements where the predicate is false.

```    for (y <- positiveAndNegativeLists._1) println(y)
for (y <- positiveAndNegativeLists._2) println(y)
```