The article showed how J solves some mathematical problems with built in functions or their built-in modifiers. But in general programming, you won't find a function that does exactly what you wanted, maybe with a little adverb. You'll usually have to write that function yourself, to describe how it works in terms of some of those pre-existing functions.
Note that your second example could be expressed as something like
x.OrderBy(y => y[1])
Ah - I see. Here[1] is quicksort in j. John scholes's videos are excellent and very illustrative, though they deal with apl rather than j; conway's game of life[2] and sudoku[3].
> The article showed how J solves some mathematical problems with built in functions or their built-in modifiers. But in general programming, you won't find a function that does exactly what you wanted, maybe with a little adverb. You'll usually have to write that function yourself, to describe how it works in terms of some of those pre-existing functions.
Right. This is true, but you generally stay much closer to the builtins in j than you do in other language.
> Note that your second example could be expressed as something like
> x.OrderBy(y => y[1])
> In C#, assuming x is int[N][M]. I understand that J's version can work with many other shapes of X, but that is hard to appreciate without seeing example of how that comes in handy when writing some other (hopefully well known) algorithm.
Right; you mentioned sort predicates, so I was giving an example of a way in which you could sort according to another relation. My point is: being able to change the layout of your data so easily can obviate sort predicates. (Though I do think a sort adverb taking a predicate would be cool.)
1. https://code.jsoftware.com/wiki/Essays/Quicksort
'[H]ow is the array sorted' is one question that computer science can be used to solve. However, assuming it has been solved (which, for many cases, it has), a much more interesting question is 'what can we do with a sorted array'; or, 'given that sorting has such-and-such time or memory complexity, how hard is this other algorithm?' The OP isn't showing off a particular sorting algorithm, he's showing off a sorting interface, because that's more novel in context.
Sorting according to a predicate could be done with outer product/table+reduction, but usually you don't need anything like that. Let's say you have an array of 3-vectors, and you want to sort the vectors according to the 2nd item in each vector. So your sort predicate is v1[1] < v2[1]. We can just sort the whole array according to the vector of the 2nd items of each vector; so: