Applying logical and to list of boolean values

Question

Consider the following list of Boolean values in Scala

List(true, false, false, true)

How would you using either foldRight or foldLeft emulate the function of performing a logical AND on all of the values within the list?

Saleable

Maybe a general tutorial on folds would help: stackoverflow.com/questions/2293592 – Oxidize

Answer 1

val l = List(true, false, false, true)
val andAll = l.foldLeft(true)(_ && _)

Conjure

reduceOption helps deal with the degenerate case (see answer below) – Douville

Answer 2

Instead of using foldLeft/Right, you can also use forall(identity) for the logical AND, or exists(identity) for the logical OR.

edit: The benefit of these functions is the early exit. If forall hits a false or exists a true, they will immediately return.

Wrote

Becareful of the degenerate case. It can be counter-intuitive. Basically, both of the following expressions are true: List.empty[Boolean].forall(identity) == true and List.empty[Boolean].exists(identity) == false. – Karen

True, but you would get the same results for folding with && or ||. – Wrote

These behaviours are perfectly logical. Given a predicate p and an empty set of values S = {}, of course the statement "p(x) for all in x in S" is true, and likewise "there exists x in S such that p(x)" is false, regardless of the definition of p. – Rhinestone

Answer 3

Without initial value as in foldLeft,

List(true, false, false, true).reduce(_&&_)

Yet this works not for List.empty[Boolean].

Danieledaniell

Answer 4

I like the forAll approach best so long as it fits your use case. It exits early which is great. However, if that doesn't fit here's another, only marginally more complex, approach.

With reduceOption you get no early exit, but you can clearly specify the value for the case where the list is empty.

val l = List(true, false, false, true)
val andAll = l.reduceOption(_ && _).getOrElse(false)

Douville

Answer 5

The OP specifically asks for a solution using foldRight or foldLeft, so the accepted answer is strictly correct. The answer currently with most votes - using forall - has better performance due to early exit. Here's another solution - using contains - with early exit too and probably more readable.

For the logical AND:

!List(true, false, false, true).contains(false)

For the logical OR it's even simpler, no negation is needed:

List(true, false, false, true).contains(true)

A note about the "degenerate" empty list case, with logical AND:

!List.empty[Boolean].contains(false)
// Returns true

If that doesn't fit your use case, a marginally more complex solution:

Some(List.empty[Boolean]).filter(_.nonEmpty).exists(!_.contains(false))
// Returns false

Kucera