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?

Maybe a general tutorial on folds would help: http://stackoverflow.com/questions/2293592 — Rex Kerr, Mar 19 '14 at 21:47

drexin · Answer 1 · 2014-03-19T22:33:46.760

65

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.

edited Mar 19 '14 at 22:33

answered Mar 19 '14 at 21:54

drexin

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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`. – Randall Schulz Mar 19 '14 at 22:49
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True, but you would get the same results for folding with `&&` or `||`. – drexin Mar 19 '14 at 22:55
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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. – yonil Jul 14 '16 at 16:28

score 16 · Accepted Answer · answered Mar 19 '14 at 21:50

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

answered Mar 19 '14 at 21:50

Lee

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reduceOption helps deal with the degenerate case (see answer below) – sfosdal May 23 '16 at 21:35

score 9 · Answer 3 · answered Mar 20 '14 at 07:39

9

Without initial value as in foldLeft,

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

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

answered Mar 20 '14 at 07:39

elm

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score 5 · Answer 4 · edited Jun 01 '17 at 19:22

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)

score 0 · Answer 5 · answered Dec 12 '22 at 20:06

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

Applying logical and to list of boolean values

5 Answers5

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