I have two arrays (X and Y) and I need to create array Z that contains all elements from array X except those, that are present in array Y p times where p is a prime number. I am trying to write this in JS.

For Example:
Array X: [2, 3, 9, 2, 5, 1, 3, 7, 10]
Array Y: [2, 1, 3, 4, 3, 10, 6, 6, 1, 7, 10, 10, 10]
Array Z: [2, 9, 2, 5, 7, 10]

So far I have this:

const arrX = [2, 3, 9, 2, 5, 1, 3, 7, 10]
const arrY = [2, 1, 3, 4, 3, 10, 6, 6, 1, 7, 10, 10, 10]
const arrZ = []
const counts = [];

// count number occurrences in arrY
for (const num of arrY) {
  counts[num] = counts[num] ? counts[num]   1 : 1;
}

// check if number is prime
const checkPrime = num => {
    for (let i = 2; i < num; i  ) if (num % i === 0) return false
    return true
}

console.log(counts[10]);
// returns 4

Any hint or help appreciated. Thanks!

CodePudding user response：

You're on the right track. counts should be an object mapping elements in arrY to their number of occurrences. It's easily gotten with reduce.

The prime check needs a minor edit, and the last step is to filter arrX. The filter predicate is just a prime check on the count for that element.

// produce an object who's keys are elements in the array
// and whose values are the number of times each value appears
const count = arr => {
  return arr.reduce((acc, n) => {
    acc[n] = acc[n] ? acc[n] 1 : 1;
    return acc;
  }, {})
}

// OP prime check is fine, but should handle the 0,1 and negative cases:
const checkPrime = num => {
    for (let i = 2; i < num; i  ) if (num % i === 0) return false
    return num > 1;
}

// Now just filter with the tools you built...
const arrX = [2, 3, 9, 2, 5, 1, 3, 7, 10]
const arrY = [2, 1, 3, 4, 3, 10, 6, 6, 1, 7, 10, 10, 10]
const counts =  count(arrY);
const arrZ = arrX.filter(n => checkPrime(counts[n]));
console.log(arrZ) 

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