javascript - Find the opposite number in the array

Question

I'm trying to find the opposite number in the array.

Let's say we have this array: [1,3,4,-3]

What I want is:

{ '1': 1, '3': 2, '4': 1 } // as we have two 3

and what I tried is :

  const arr = [1,3,4,-3]
  let holder = {}
  arr.map(a => {
    holder[a] = (holder[a] ||0)+1
  })
      
  console.log(holder)

question from:https://stackoverflow.com/questions/65910492/find-the-opposite-number-in-the-array

Answer 1

The rules can be summed up as follow:

Let v be the array Let x a positive number in the array

1. if x is in v but not -x, keep {[x]:1}
2. if x is in v and -x also, keep {[x]:2}

Let y a negative number in the array

• if y is in v and -y also do nothing (symetry of 2.)

3. if y is in v but not -y, keep {[y]:1}

Now it is pretty straightforward:

• build a set of values of v
• check all the positive keys
• foreach one, check if there exists its opposite and apply 1. or 2.
• delete any opposite keys
• for the remaining keys (which are then negative and have no opposite) apply 3.

const v = [1,1,1,-2, 3,3,-3, 4,-4,-4]
const output = {}
const s = new Set(v)
const positives = [...new Set(v.filter(x => x>= 0))]
positives.forEach(p => {
  if (s.has(-p)) {
    output[p] = 2
    s.delete(-p)
  } else {
    output[p] = 1
  }
  s.delete(p)
})
;[...s].forEach(negative => {
  output[negative] = 1
})

console.log('output', output)