CODEWITHSUNDEEP
javascriptarraysecmascript-6
sabithpocker
sabithpocker

Reputation: 15566

Cartesian product using reduce, map and spread is grouping the result which need to be flattened

class MathSet extends Set{
  constructor(arr){
     super(arr);
     }
  union(set){
     return new MathSet([...this, ...set])
  }
  intersection(set){
     return new MathSet([...this].filter(x => set.has(x)));
  }
  difference(set){
     return new MathSet([...this].filter(x => !set.has(x)));
  }
  cartesian(set){
     return new MathSet( [...this].reduce((acc, i)=> [...acc, [...set].map(j=>[i,j])], []) )
  }
}

let x = new MathSet([1,2,3]);
let y = new MathSet([1,2,3,4,5]);

console.log(JSON.stringify([...x.cartesian(y)]));
//[
//  [[1,1],[1,2],[1,3],[1,4],[1,5]],
//  [[2,1],[2,2],[2,3],[2,4],[2,5]],
//  [[3,1],[3,2],[3,3],[3,4],[3,5]]
// ]

With cartesian function expected result is a flattened version of the above array ([[1,1],[1,2],[1,3],[1,4],[1,5],[2,1],[2,2],[2,3],[2,4],[2,5],[3,1],[3,2],[3,3],[3,4],[3,5]]), but as you can see somehow its getting grouped into three arrays. The reduce is keeping on concatenating earlier result with spreaded version of new results. Any guess on what I am doing wrong?

Upvotes: 1

Views: 47

Answers (2)

Redu
Redu

Reputation: 26191

This is a problem. I do cartesians by two nested reduces just like dealing with two arrays but can handle n arrays in fact. The major problem was to flatten the nesting up arrays in between operation. My solution would be 

Array.prototype.cartesian = function(...a){
  return a.length ? this.reduce((p,c) => (p.push(...a[0].cartesian(...a.slice(1)).map(e => a.length > 1 ? [c,...e] : [c,e])),p),[])
                  : this;
};

var arr = ['a', 'b', 'c'],
    brr = [1,2,3],
    crr = [[9],[8],[7]];
console.log(JSON.stringify(arr.cartesian(brr,crr)));

Actually on a second thought and by influencing from your code, i think using a map in the place of the second reduce would in fact be more appropriate. I modified the code accordingly.

Upvotes: 1

Bergi
Bergi

Reputation: 665000

To flatten the arrays you need just one more spread:

return new MathSet( [...this].reduce((acc, i)=> [...acc, ...[...set].map(j=>[i,j])], []) )
//                                                       ^^^

(or acc.concat(Array.from(set, j=>[i,j])))

Upvotes: 2

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