Filtering two arrays for matching values into new array in Javascript

Question

I'm trying to match two arrays and extract the matching values into a new array while keeping it in the same order as the second array. For example:

let strArr1 = ['a', 'p', 'p', 'l', 'e']; 
let strArr2 = ['p','p','a','e', 'l', 'l', 'k'];
let newArr = [];
// Wanted output: ['p', 'p', 'a', 'e', 'l']

So what I'm trying to do here is remove the extra characters in the second array. The code I've come up with so far:

for (let i=0; i < strArr1.length; i++){
for (let j=0; j < strArr2.length; j++){
if (strArr1[i]==strArr2[j]){
  newArr.push(strArr2[i])
  }
 }
}

For some reason, the output becomes:

['p', 'p', 'p', 'a', 'a', 'e', 'e', 'l']

I've also tried the filter method. It's closer to the results I want.

const newArr = strArr2.filter(value => strArr1.includes(value));

But the output becomes:

['p', 'p', 'a', 'e', 'l', 'l']

Muhammad Usman · Accepted Answer · 2021-08-05 06:11:58Z

1

const newArr = strArr2.filter(value => strArr1.includes(value) && arr.splice(arr.indexOf(value), 1));

This works :)

answered Aug 5, 2021 at 6:11

Muhammad Usman

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Comments

Aniket Bhange · Accepted Answer · 2021-08-05 05:58:52Z

1

You can try something like this

strArr1 = strArr1.reduce((acc, v)=> ({ ...acc, [v]: (acc[v] ? (++acc[v]) : 1) }), {})
strArr2.filter(v => !!(strArr1[v]--)  )

answered Aug 5, 2021 at 5:58

Aniket Bhange

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Comments

Md Sabbir Alam · Accepted Answer · 2021-08-05 06:11:47Z

There are two mistakes in your code though, you are pushing strArr2[i] in the newArr although, you used j to iterate over strArr2. And when you find a match to push into newArr, you need to break the loop and move to the next element in strArr2.

In order to keep the order in strArr2 you need to change the order of the for loop,

for (let i=0; i < strArr2.length; i++){
  for (let j=0; j < strArr1.length; j++){
     if (strArr1[j]==strArr2[i]){
        newArr.push(strArr2[i])
        j += strArr1.length;
     }
  }
}

The complexity is O(n^2) You can optimize the code by doing the following,

let strArr1 = ['a', 'p', 'p', 'l', 'e']; 
let strArr2 = ['p','p','a','e', 'l', 'l', 'k'];
let newArr = [];
// Wanted output: ['p', 'p', 'a', 'e', 'l']

newArr = strArr2.filter(item => strArr1.includes(item));

console.log(newArr);

['p', 'p', 'a', 'e', 'l', 'l'] is correct output as there is two l in the strArr2.

If you want to match each value once, you can remove the value from strArr1 after finding a match.

let strArr1 = ['a', 'p', 'p', 'l', 'e']; 
let strArr2 = ['p','p','a','e', 'l', 'l', 'k'];
let newArr = [];
console.log('abc');

newArr = strArr2.reduce((prev, curr) => {

            if(strArr1.includes(curr)) {
               prev.push(curr);
               strArr1.splice(strArr1.indexOf(curr), 1);
            }
            return prev;
         }, []);
         
console.log(newArr);

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