Better way for accumulating a value than using inject/reduce with conditional operator?
There's a built-in count method which handles your example cleanly:

names.count {|name| triangles.include?(name.points) }


Also, the accumulator version should use +, not +=. reduce uses the block's return value; there's no need to modify acc.

In general, though, there's no simpler way to accumulate values than reduce. It's not very clear, but all of the alternatives are worse.

Don't be afraid of the conditional operator. It's just a different way to spell if...else.