Ruby/RoR - Count occurrence of an element in an array

Question

I'v a hash

{1=>true, 7=>false, 6=>true, 4=>false}

or an array like

[1, true], [7, false], [6, true], [4, false]]

or

[true, false, true, false].

How can I find the number of trues in the array?

Answer 1

In order to count the elements, you obviously have to iterate over the collection. Since iterating over a Hash yields two-element Arrays, the first two are actually exactly the same:

{ 1 => true, 7 => false, 6 => true, 4 => false }.count(&:last)
[[1, true], [7, false], [6, true], [4, false]].count(&:last)

For the simple Array case, you could do something like this:

[true, false, true, false].count(true)

This Array is of course also the same as the Hash#values from your Hash above, so you could use the same method on that:

{ 1 => true, 7 => false, 6 => true, 4 => false }.values.count(true)

If you don't know which one of three you will get, you could use something like this:

{ 1 => true, 7 => false, 6 => true, 4 => false }.flatten.count(true)
[[1, true], [7, false], [6, true], [4, false]].flatten.count(true)
[true, false, true, false].flatten.count(true)

Answer 2

With Enumerable#count:

hash.values.count(true)
array_of_pairs.map { |k, v| v }.count(true)
plain_array.count(true)

More verbose, but does not create intermediate arrays:

hash_or_array_of_pairs.inject(0) { |acc, (k, v)| acc + (v == true ? 1 : 0) }

Answer 3

Simpler:

hash.values.count(true)

array.flatten.count(true)

This works with all the above cases.