eolas/zk/Multiple_pointers.md

---
tags:
  - design-patterns
---

# Multiple pointers

The multiple pointers pattern can be useful for moving through an array and
comparing elements against each other in a time-efficient manner and without
costly multiple loops.

## Example: moving pairs

In this example we shuffle through an array creating a pair from each element
encountered. Thus for an array such as `[4, 3, 2, 1, 3, 6, 7]` we would return
`[ [ 4, 3 ], [ 3, 2 ], [ 2, 1 ], [ 1, 3 ], [ 3, 6 ], [ 6, 7 ] ]`.

```js
function movingPairs(arr) {
  const store = [];
  let left = 0;
  let right = 1;
  for (left; left < arr.length - right; left++) {
    store.push([arr[left], arr[right + left]]);
  }
  return store;
}
```

## Example: identify unique values

An example application of the above pattern would be identifying the number of
unique values in an array that contains duplicates. One way to do this in
JavaScript would be to use a Set however this is a more generic approach that
only requires a single loop:

```js
function uniqueValues(arr) {
  if (!arr.length) {
    return 0;
  } else {
    let count = 1;
    let left = 0;
    let right = 1;
    for (right; right < arr.length; right++) {
      if (arr[left] !== arr[right]) {
        count++;
      }
      left++;
    }

    return count;
  }
}
```

This works by having two pointers at either end of the array; one moving from
the right and one moving from the left. Whilst the right pointer is less than
the array length, the left pointer is incremented as the right moves closer to
it. At each incrementation, the values of pointers (used as indexes) are
compared for uniqueness, and a count is kept.

## Example: identify duplicates

This is very similar to the above but this time we are looking for duplicates
rather than unique values.

```js
function areThereDuplicatesPointer(...input) {
  let start = 0;
  let next = 1;
  while (next < input.length) {
    let same = input[start] === input[next];
    if (same) {
      return true;
    }
    start++;
    next++;
  }

  return false;
}
console.log(areThereDuplicatesPointer(1, 2, 3)); // false
console.log(areThereDuplicatesPointer(1, 2, 2)); // true
```