How can I INTERSECT N arrays in Javascript at a reasonably fast time?
Ie.
arr1 = [1,2,3,4,5];
arr2 = [1,2,3,4];
arr3 = [4];
arr4 = [4,5];
Result: [4]
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1They seem to be asking about the intersection of the lists.Carcigenicate– Carcigenicate2018-07-14 18:11:02 +00:00Commented Jul 14, 2018 at 18:11
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1rather than revinvent the wheel, check this out: stackoverflow.com/questions/37320296/…james emanon– james emanon2018-07-14 18:13:13 +00:00Commented Jul 14, 2018 at 18:13
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1What exactly is NJOIN? Do you mean intersection?jfriend00– jfriend002018-07-14 18:18:09 +00:00Commented Jul 14, 2018 at 18:18
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Woops! Was thinking of SQLstark0323– stark03232018-07-14 18:30:38 +00:00Commented Jul 14, 2018 at 18:30
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In the future, if you mean to respond to a specific person's comment, then use put @username in your comment. That's the ONLY way they will get notified that you've done something you want them to see. Otherwise, they'll never see your comment.jfriend00– jfriend002018-07-15 23:04:46 +00:00Commented Jul 15, 2018 at 23:04
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2 Answers
You could take an intersection function for common elements with a Set and Array#filter.
function common(a, b) {
return b.filter(Set.prototype.has.bind(new Set(a)));
}
console.log([[1, 2, 3, 4, 5], [1, 2, 3, 4], [4], [4, 5]].reduce(common));5 Comments
jfriend00
Is this "reasonably fast" as the OP asked? It creates a
new Set() a lot.
Nina Scholz
for the given data, it is fast to write ... o_O
james emanon
Nina, can you explain this bit: Set.prototype.has.bind(new Set(a))
Nina Scholz
@jamesemanon, what i want, is to take a function as callback. in this case
has from Set, but it is not possible to use new Set([...]).has, because it returns an error Set.prototype.has: 'this' is not a Set object. therefore i took the prototype and with bind it takes a set as this. voilajames emanon
imho, that is pretty clever
I would simply use filter() and includes() unless I knew my arrays were going to be very large. This runs "reasonably fast" and is probably faster than creating a bunch of sets when you will be joining a lot of normal-sized arrays, however for fewer, very large arrays, the new Set() could prove faster :
let arr1 = [1,2,3,4,5];
let arr2 = [1,2,3,4,5];
let arr3 = [4];
let arr4 = [4,5];
let r = [arr1, arr2, arr3, arr4].reduce((a, c) => a.filter(i => c.includes(i)))
console.log(r)