Compare array and change if values are different

Question

I have the next situation in JavaScript:

const arr1 = [1,2,3];  // by default it is empty `[]`
const arr2 = [2,1,8,9];

arr1 can be changed dynamicly. So sometimes it could be [1,2,3,4] or [4,2,3] and so on.

The idea is next:

Depending by the arr1 to change the arr2 like,
if in arr1 is a number which is equal with a number from arr2,
then the number from arr2 that is equal should be deleted,
but if the new number that is added in arr1 is not exists in arr2 then it should be added in arr2:
EX:

arr1 = []; arr2=[2,1,8,9] // nothing happens
arr1 = [1]; arr2=[2,8,9] // 1 from arr2 is deleted
arr1 = [1,8,9,7]; arr2=[1,2,7] // 1,8,9 from arr2 is deleted, 7 is added

arr1 and / or arr2 can be very big array sized ( more than 1000 elements ? , 10000, 100000... ) ? — Mister Jojo
– Mister Jojo, Commented Feb 26, 2021 at 0:08

The Bomb Squad · Accepted Answer · 2021-02-25 23:23:10Z

2

Let me explain the logic.. for each elem in arr1, if arr2 has the elem(arr2.indexOf(a)!=-1), run a while loop that will stop when ALL elems in arr2 that match the specific elem in arr1 are removed
Lastly the else logic would activate if arr2 DOES NOT contain specific elem

const arr1 = [1,2,3];  // by default it is empty `[]`
const arr2 = [2,1,8,9];

function updateArr(arr1,arr2){
  arr1.forEach(a=>{
    let index=arr2.indexOf(a)
    if(index!=-1){
      while(index!=-1){arr2.splice(index,1);index=arr2.indexOf(a)}
    }
    else{arr2.push(a)}
  })
  return arr2
}

//function that takes in 2 vars, uses the first to edit the second

console.log(updateArr([2,3,7,9], [2,1,8,9]))
console.log(updateArr(arr1,arr2))
console.log("No cap it works\n",arr2)

edited Feb 25, 2021 at 23:23

answered Feb 25, 2021 at 22:59

The Bomb Squad

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4 Comments

Asking Over a year ago

could you show also an example using react js hooks like useState, like const [arr2, setArr2] = useState([1,2,3]). How to change the state in this situation?

The Bomb Squad Over a year ago

ok im gonna come clean with u.. i havent really learned react(secondary school year gonna be over soon so that gonna change) but I CAN make a function

Asking Over a year ago

could you help with the function?

The Bomb Squad Over a year ago

oh.. just made a slight syntax error.. my bad.. check it now

Mister Jojo · Accepted Answer · 2021-02-26 00:31:52Z

0

In a simple words, you want to keep only uniques elements of Arr1 and Arr2, and give the result in Arr2 (not in a new array)

function updArr2(arr1, arr2)
  {
  let set1 = new Set(arr1)
    , plus = arr1.filter(x=>!arr2.includes(x))

  for(let i=arr2.length;i--;)  
    if (set1.has(arr2[i])) arr2.splice(i,1)

  plus.forEach(e => arr2.push(e) )
  }

const
  arr_1 = [1,2,3] 
, arr_2 = [2,1,8,9]
;
updArr2(arr_1, arr_2)


console.log( JSON.stringify(arr_2) )

answered Feb 26, 2021 at 0:31

Mister Jojo

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