Remove unique elements from a 2d array and return the repeating element

Question

I want to remove unique elements from a 2d array and return the repeating elements. For Ex:

let arr = [ [ 'Alexandra', 'Female', 'Senior', 'CA', 'English', 'Drama Club' ],
  [ 'Andrew', 'Male', 'Freshman', 'SD', 'Math', 'Lacrosse' ],
  [ 'Andrew', 'Male', 'Freshman', 'SD', 'Math', 'Lacrosse' ],
  [ 'Andrew', 'Male', 'Freshman', 'SD', 'Math', 'Lacrosse' ],
  [ 'Benjamin', 'Male', 'Senior', 'WI', 'English', 'Basketball' ],
  [ 'Carl', 'Male', 'Junior', 'MD', 'Art', 'Debate' ]]

Output should be:

[[ 'Andrew', 'Male', 'Freshman', 'SD', 'Math', 'Lacrosse' ],
  [ 'Andrew', 'Male', 'Freshman', 'SD', 'Math', 'Lacrosse' ],
  [ 'Andrew', 'Male', 'Freshman', 'SD', 'Math', 'Lacrosse' ]]

What have you tried and what challenge are you having?

PeterKA
– PeterKA

2022-05-24 11:30:18 +00:00
Commented May 24, 2022 at 11:30 — PeterKA
– PeterKA, Commented May 24, 2022 at 11:30

Yuri Khristich · Accepted Answer · 2022-05-24 12:22:01Z

It can be something like this:

let arr = [ [ 'Alexandra', 'Female', 'Senior', 'CA', 'English', 'Drama Club' ],
  [ 'Andrew', 'Male', 'Freshman', 'SD', 'Math', 'Lacrosse' ],
  [ 'Andrew', 'Male', 'Freshman', 'SD', 'Math', 'Lacrosse' ],
  [ 'Andrew', 'Male', 'Freshman', 'SD', 'Math', 'Lacrosse' ],
  [ 'Benjamin', 'Male', 'Senior', 'WI', 'English', 'Basketball' ],
  [ 'Carl', 'Male', 'Junior', 'MD', 'Art', 'Debate' ]
]

const get_not_uniq_elements = arr => arr.filter(x => arr.indexOf(x) !== arr.lastIndexOf(x));
const flatt_2d_array = arr => arr.map(x => x.join('\t'));
const unflat_array = arr => arr.map(x => x.split('\t'));

var new_arr = flatt_2d_array(arr);
new_arr = get_not_uniq_elements(new_arr);
new_arr = unflat_array(new_arr);

console.log(new_arr);

Or the same algorithm in one line:

let new_arr = arr.map(x => x.join('\t'))
  .filter((x,_,a) => a.indexOf(x) !== a.lastIndexOf(x))
  .map(x => x.split('\t'));

PeterKA · Accepted Answer · 2022-05-24 11:41:21Z

You can use Array#filter as in the demo below.

const arr = [ [ 'Alexandra', 'Female', 'Senior', 'CA', 'English', 'Drama Club' ],
  [ 'Andrew', 'Male', 'Freshman', 'SD', 'Math', 'Lacrosse' ],
  [ 'Andrew', 'Male', 'Freshman', 'SD', 'Math', 'Lacrosse' ],
  [ 'Andrew', 'Male', 'Freshman', 'SD', 'Math', 'Lacrosse' ],
  [ 'Benjamin', 'Male', 'Senior', 'WI', 'English', 'Basketball' ],
  [ 'Carl', 'Male', 'Junior', 'MD', 'Art', 'Debate' ]],
      
      output = arr.filter(
          ([firstName,gender,classOf,state,course,club],i,a) => 
          a.find(
              ([fn,g,co,st,cs,cb],j) =>
              i !== j && fn === firstName &&
                         g  === gender &&
                         co === classOf && 
                         st === state &&
                         cs === course &&
                         cb === club
          )
      );
       
console.log( output );

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