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Learn which values of an array are non-unique? #918

MeoMix opened this issue Feb 3, 2015 · 2 comments


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commented Feb 3, 2015


I have an array that is: [1,1,2,3,6,7,7,9] and I would like to transform this into [1,7]

I was a bit surprised that there doesn't seem to be a clear path to achieving this. Am I missing something really obvious? _.uniq is close, but doesn't tell me which ones it removed. I'd think I should be able to diff _.uniq and original, but that'll remove all of the elements...

Right now I'm rocking something ugly:

var nonUnique = _.chain([1, 1, 2, 3, 6, 7, 7, 9])
  .groupBy(function (val) { return val; })
  .filter(function (arr) { return arr.length > 1; })
  .map(function (arr) { return arr[0]; })

or maybe

var foo = [1, 1, 2, 3, 6, 7, 7, 9];
var nonUnique = _.transform(_.groupBy(foo), function(data, value, key) {
  if (value.length > 1) {
}, []);

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commented Feb 3, 2015

Hi @MeoMix. You can do:

_.transform(_.countBy(array), function(result, count, value) {
  if (count > 1) result.push(value);
}, []);

@jdalton jdalton closed this Feb 3, 2015

@jdalton jdalton added the question label Feb 3, 2015


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commented Jan 20, 2019

This thread has been automatically locked since there has not been any recent activity after it was closed. Please open a new issue for related bugs.

@lock lock bot locked as resolved and limited conversation to collaborators Jan 20, 2019

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