Question

I am trying to use assoc in racket to create a memo table and want to associate an ordered pair (x,y) with a value but I am a little unclear of the syntax.

For example I have:

[f (lambda (x y)
               (let ([ans (assoc [x y] memo)])

But it is not correct.

Answer 1

The assoc procedure receives as its first parameter the “key” to the element that you’re looking for, and as second parameter a list of associations – in this context, an association is just a key-value pair. It will return the first association that corresponds to the given key or #f if none was found. For example:

(assoc 'x '((a 1) (b 2) (x 3) (c 4)))
> '(x 3)

If you need to use pairs as keys, that’s fine and it will work like this:

(assoc (list 1 2) '(((1 2) x) ((3 4) y)))
> '((1 2) x)