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# Product of cross-ness

Marshal
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Please have a look at this thread, where somebody is trying to create a Set of tuples from other sets. Is that a Cartesian product, a cross‑product or what? To save me the bother of finding my copy of your book, how many of those types of product do you describe?

Saloon Keeper
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Just had a look at that topic. It is unclear what OP wants: het starts off with having a Set and getting all possible combinations from it. and ends with two sets of which he wants a Cartesian product. The first is simply drawing from a Set with replacement and getting all possibilities when drawing 0, 1, 2, ... times. Paul mentiones a name, never knew that that had a name.

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Hey, fun question! (Though this is not something covered in Math for Programmers... perhaps a future book!)

The power set of a set is the set of all of its subsets; it is a set of sets.  Sets by their nature don't come with orders.    It sounds like this is asking for all possible lists of length <= n whose entries are taken from some set.  This is related to cartesian products and cartesian power sets more so than ordinary power sets.

Here's an example.  If S is the set of possible values, like {4,5,6,7} then there's one singleton list for every element of S: [4], [5], [6], [7]

The cartesian product S x S is the set of all ordered pairs taken from the list, consisting of [4,4], [4,5], [4,6], [4,7], [5,4], [5,5], ... and so on (there are 4*4 = 16 of these)

The cartesian product S x S x S is the set of all ordered triples of elements of S, so [4,4,4], [4,4,5], and so on...

We also write S^n to be the n-fold cartesian product, so the cartesian product of n copies of S which looks like S x S x S x ... x S.  This is the set of all ordered n-tuples of elements of S.  For example, S^2 = S x S and S^4 = S x S x S x S, the set of all ordered 4-tuples.  S^0 contains a 0-tuple of elements of S, i.e. an empty list.  (I'm not sure how to write superscripts in this forum but, S^n is written like S to the nth power, indicating that it's a repeated product).

If you want, say, all possible lists of length <= 4 what you want to do is take the union of the carteisan powers S^0, S^1, S^2, S^3, and S^4, as in the combination of all elements of these sets.  This is

So, the point is, this problem looks expressible in terms of set terminology, but perhaps not using the power set.  Here's a Python solution, where the first function computes the nth cartesian power of a list elts of inputs, and then the second function computes their union

Piet Souris
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hi Paul,

it is always amazing how short these python codes are, given one is used to java!

I use a Map<Integer, List<List<Integer>>> where the key is the length of the lists in the value, and I fill it using recursion, although that is not required. You could start with key = 0.

To make it generic, I have a method that sees all the integers in the map as indices into some List<T>.

But the code is a "few" lines longer than yours...

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