APL and J are similar but for a pretty fundamental difference: APL has nested array theory while J is flat array theory. In practice, this does not make them th

APL and J are similar but for a pretty fundamental difference: APL has nested array theory while J is flat array theory. In practice, this does not make them that different to the user, except APL's model is arguably more elegant since it's "turtles all the way down" (J array elements are not array themselves). K on the other hand, is really different! K doesn't have true multidimensional arrays, but lists of vectors. So it's ideal for its use case: finance (1d/2d numeric tables).