dips - 2 years ago 74

Scala Question

I have a list of lists in Scala as follows.

`val inputList:List[List[Int]] = List(List(1, 2), List(3, 4, 5), List(1, 9))`

I want a list of cross products of all the sub-lists.

`val desiredOutput: List[List[Int]] = List(`

List(1, 3, 1), List(1, 3, 9),

List(1, 4, 1), List(1, 4, 9),

List(1, 5, 1), List(1, 5, 9),

List(2, 3, 1), List(2, 3, 9),

List(2, 4, 1), List(2, 4, 9),

List(2, 5, 1), List(2, 5, 9))

The number of elements in inputList as well as the sublist are not fixed. What is the Scala way of doing this?

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Answer Source

If you use `scalaz`

, this may be a suitable case for `Applicative Builder`

:

```
import scalaz._
import Scalaz._
def desiredOutput(input: List[List[Int]]) =
input.foldLeft(List(List.empty[Int]))((l, r) => (l |@| r)(_ :+ _))
desiredOutput(List(List(1, 2), List(3, 4, 5), List(1, 9)))
```

I am not very familiar with scalaz myself, and I expect it has some more powerful magic to do this.

**Edit**

As Travis Brown suggest, we just write

```
def desiredOutput(input: List[List[Int]]) = input.sequence
```

And I find the answers of this question very helpful for understanding what `sequence`

does.

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