Iterate an array as a pair (current, next) in Java

Question

In the question Iterate a list as pair (current, next) in Python, the OP is interested in iterating a Python list as a series of current, next pairs. I have the same problem, but I'd like to do it in JavaScript in the cleanest way possible, perhaps using lodash.

It is easy to do this with a simple for loop, but it doesn't feel very elegant.

for (var i = 0; i < arr.length - 1; i++) {
  var currentElement = arr[i];
  var nextElement = arr[i + 1];
}

Lodash almost can do this:

_.forEach(_.zip(arr, _.rest(arr)), function(tuple) {
  var currentElement = tuple[0];
  var nextElement = tuple[1];
})

The subtle problem with this that on the last iteration, nextElement will be undefined.

Of course the ideal solution would simply be a pairwise lodash function that only looped as far as necessary.

_.pairwise(arr, function(current, next) {
  // do stuff 
});

Are there any existing libraries that do this already? Or is there another nice way to do pairwise iteration in JavaScript that I haven't tried?

---

Clarification: If arr = [1, 2, 3, 4], then my pairwise function would iterate as follows: [1, 2], [2, 3], [3, 4], not [1, 2], [3, 4]. This is what the OP was asking about in the original question for Python.

Answer 1

We can wrap Array.reduce a little to do this, and keep everything clean. Loop indices / loops / external libraries are not required.

If the result is required, just create an array to collect it.

function pairwiseEach(arr, callback) {
  arr.reduce((prev, current) => {
    callback(prev, current)
    return current
  })
}

function pairwise(arr, callback) {
  const result = []
  arr.reduce((prev, current) => {
    result.push(callback(prev, current))
    return current
  })
  return result
}

const arr = [1, 2, 3, 4]
pairwiseEach(arr, (a, b) => console.log(a, b))
const result = pairwise(arr, (a, b) => [a, b])

const output = document.createElement('pre')
output.textContent = JSON.stringify(result)
document.body.appendChild(output)

Answer 2

Not sure why you want this but you can just make the "ugly" part on a function and then it looks nice:

arr = [1, 2, 3, 4];

function pairwise(arr, func){
    for(var i=0; i < arr.length - 1; i++){
        func(arr[i], arr[i + 1])
    }
}

pairwise(arr, function(current, next){
    console.log(current, next)
})

You can even slightly modify it to be able to make iterate all i, i+n pairs, not just the next one:

function pairwise(arr, func, skips){
    skips = skips || 1;
    for(var i=0; i < arr.length - skips; i++){
        func(arr[i], arr[i + skips])
    }
}

pairwise([1, 2, 3, 4, 5, 6, 7], function(current,next){
    console.log(current, next) // displays (1, 3), (2, 4), (3, 5) , (4, 6), (5, 7)
}, 2)

Answer 3

This answer is inspired by an answer I saw to a similar question but in Haskell: https://stackoverflow.com/a/4506000/5932012

We can use helpers from Lodash to write the following:

const zipAdjacent = function<T> (ts: T[]): [T, T][] {
  return zip(dropRight(ts, 1), tail(ts));
};
zipAdjacent([1,2,3,4]); // => [[1,2], [2,3], [3,4]]

(Unlike the Haskell equivalent, we need dropRight because Lodash's zip behaves differently to Haskell's`: it will use the length of the longest array instead of the shortest.)

The same in Ramda:

const zipAdjacent = function<T> (ts: T[]): [T, T][] {
  return R.zip(ts, R.tail(ts));
};
zipAdjacent([1,2,3,4]); // => [[1,2], [2,3], [3,4]]

Although Ramda already has a function that covers this called aperture. This is slightly more generic because it allows you to define how many consecutive elements you want, instead of defaulting to 2:

R.aperture(2, [1,2,3,4]); // => [[1,2], [2,3], [3,4]]
R.aperture(3, [1,2,3,4]); // => [[1,2,3],[2,3,4]]