How can I use lodash utilities in order to check an array for duplicate values?
I want to input ['foo', 'foo', 'bar'] and have the function return true. And input ['foo', 'bar', 'baz'] and have the function return false.
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Related, not quite a duplicate though: stackoverflow.com/q/31681732General Grievance– General Grievance2025-09-25 17:17:56 +00:00Commented Sep 25 at 17:17
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6 Answers
You can use uniq to check:
function hasDuplicates(a) {
return _.uniq(a).length !== a.length;
}
var a = [1,2,1,3,4,5];
var b = [1,2,3,4,5,6];
document.write(hasDuplicates(a), ',',hasDuplicates(b));<script src="http://cdnjs.cloudflare.com/ajax/libs/lodash.js/3.1.0/lodash.min.js"></script>
Comments
As of ES6 you can simply use Set so this becomes:
let hasDuplicates = arr => new Set(arr).size != arr.length
console.log(hasDuplicates([5,3,2,1,2,1,2,1]))
console.log(hasDuplicates([1,2,3,4,5]))Which somewhat negates the use of lodash in this particular case.
Comments
You could check that there is _.some element in the array which does not return its own location when looked up in the array. In other words, there is at least one element which has a match earlier in the array.
function hasDuplicates(array) {
return _.some(array, function(elt, index) {
return array.indexOf(elt) !== index;
});
}
Perhaps this is faster than the _.uniq solution, since it will identify the first duplicated element right away without having to compute the entire unique-ified array.
Or, depending on your coding style and desire for readability, and if you want to use ES6 arrow functions for brevity:
var earlierMatch = (elt, index, array) => array.indexOf(elt) !== index;
var hasDuplicates = array => _.some(array, earlierMatch);
2 Comments
tokland
Nit-pick:
indexOf (O(n)) within a some (O(n)) -> O(n2).
Kaleo Brandt
Yeah, this is an O(n^2) algorithm for checking for duplicates. I wouldn't call that a nitpick, more of a major flaw.
Well, there's always. lodash's _.uniq() function. That function actually returns a new array that only contains unique values, so checking to see if the length of the array has changed would get you your 'true' or 'false' value to return yourself, I believe.
Comments
I don't know lodash but I submit:
_.any(_.countBy(['foo', 'foo', 'bar']), function(x){ return x > 1; });
The problem with all the solutions proposed so far is that the entire input array needs processing to get an answer, even if the answer is obvious from the first 2 elements of the array.
1 Comment
random_user_name
As of version 4 of lodash,
_.any is deprecated in favor of _.someconst array = [1, 2, 1]
array.some((el, idx) => array.lastIndexOf(el) !== idx)
// true
1 Comment
General Grievance
This is O(n^2), but O(n) solution exists: stackoverflow.com/a/56317910