Cartesian Products with sets in java

Question

I am trying to create a cartesian product method in java that accepts sets as arguments and returns a set pair. The code I have coverts the argumented sets to arrays and then does the cartesian product but i can't add it back to the set pair that i want to return. Is there an easier way to do this? Thanks in advance.

public static <S, T> Set<Pair<S, T>> cartesianProduct(Set<S> a, Set<T> b) {
    Set<Pair<S, T>> product = new HashSet<Pair<S, T>>();


    String[] arrayA = new String[100];
    String[] arrayB= new String[100];

    a.toArray(arrayA);
    b.toArray(arrayB);

    for(int i = 0; i < a.size(); i++){
        for(int j = 0; j < b.size(); j++){
            product.add(arrayA[i],arrayB[j]);
        }
    }
    return product;
}

Where are you getting the `Pair` class from? Apache Commons? In any event, I think you'll need to create a `Pair` object to use as the argument to `product.add`. There is no `add` method for sets that takes two arguments. Also, I assume `arrayA` should be an array of `S`, not `String`... Also, you're not using `toArray` correctly; it will not change the length of the array if `a` is shorter than 100 and won't do anything useful if `a` is larger. Better is `arrayA = a.toArray(new S[0]);`, similarly for `b`. — ajb, Feb 08 '14 at 00:54

Camilo · Accepted Answer · 2014-02-08T02:58:56.503

2

this looks simpler,

public static <S, T> Set<Pair<S, T>> cartesianProduct(Set<S> a, Set<T> b) {
    Set<Pair<S, T>> product = new HashSet<Pair<S, T>>();

    for(S s : a) {
        for(T t : b) {
            product.add(new ImmutablePair<S, T>(s,t));
        }
    }

    return product;
}

edited Feb 08 '14 at 02:58

answered Feb 08 '14 at 01:01

Camilo

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If this `Pair` is from Apache Commons there's no two-argument constructor, unless I'm looking at an obsolete version of the documentation. I don't think Java 7 defines a `Pair` class anywhere. Did they add this to Java 8? – ajb Feb 08 '14 at 01:09
you're right, just edited, didn't think it was really relevant, he might as well simply implement his own Pair class – Camilo Feb 08 '14 at 03:00

ajb · Answer 2 · 2014-02-08T01:10:52.610

Assuming you're using Pair from Apache Commons, then I think you want the add to be

product.add(Pair.of(arrayA[i],arrayB[j]));

There's no add method for sets that takes two arguments. You have to create the Pair to add to the set. If that doesn't compile, try

product.add(Pair<S,T>.of(arrayA[i],arrayB[j]));

Also, I'm assuming you meant S and T for your arrays, instead of String. There's no reason to preallocate a certain number of elements. Furthermore, the way you've written it, if there are more than 100 elements in either set, toArray will return an all new array with the desired size, but you're not using the function result so that array will be lost. I'd prefer:

S[] arrayA = a.toArray(new S[0]);
T[] arrayB = b.toArray(new T[0]);

The zero-length arrays are just "dummies" whose purpose is to get toArray to return arrays with the correct element type, instead of Object[].

EDIT: Using an enhanced for loop is a lot better than using arrays. See Camilo's answer.

Cartesian Products with sets in java

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