> The idea of creating a vector of things, applying a true/false test to every element to create another vector of 1s and 0s, and summing those or using them to

> The idea of creating a vector of things, applying a true/false test to every element to create another vector of 1s and 0s, and summing those or using them to pick elements out of another vector is a super powerful tool that lets you express a lot of ideas in a compact form that describes the algorithm without getting bogged down in writing explicit loops. Exactly. APL's syntax and terminology is a bit odd. But what you're describing there should be a concept known to anyone familiar with a functional language. It seems you've just described what I would call mapping a function over an array. Indeed, a very powerful construct, often lacking from more procedural languages. Or, sadly, often eschewed by programmers even in the languages that do have it. APL makes me think of Forth. Because of its idiosyncratic terminology (e.g. functions are "words") and unusual syntax (strict RPN), it is considered more exotic than it really is.