Why does this function change the order of the array items?

June 22, 2022

The function should return the earliest complete pair that equals the sum (s);

function sumPairs(ints, s, arr = ints.slice(), result) {
  let lastIndex = arr.length;
  for (let int of arr) {
    for (let i = 1; i < arr.length; i++) {
      if (int + arr[i] === s && i < lastIndex) {
        result = [int, arr[i]];
        lastIndex = i;
      }

    }
   arr.splice(0, 1); 
  }
  return result;
}

  console.log(sumPairs([10, 5, 2, 3, 7, 5], 10)); // [3,7] is expected
  // console.log(sumPairs([1, 2, 3, 4, 1, 0], 2));
  // console.log(sumPairs([0, 0, -2, 3], 2));
  // console.log(sumPairs([1, 4, 8, 7, 3, 15], 8));
   console.log(sumPairs([1, -2, 3, 0, -6, 1], -6)); // [0,-6] is expected
  // console.log(sumPairs([4, 3, 2, 3, 4], 6));

For the examples that are returning oddly, I have listed the expected results next to the function call/console log.

>Solution :

By splicing inside the loop, you’re removing one element from the beginning of the array while you’re iterating over the array. This results in the int variable being iterated over being only every other value, eg:

sumPairs([10, 5, 2, 3, 7, 5], 10)
          ^^     ^     ^

function sumPairs(ints, s, arr = ints.slice(), result) {
  let lastIndex = arr.length;
  for (let int of arr) {
    console.log(int);
    for (let i = 1; i < arr.length; i++) {
      if (int + arr[i] === s && i < lastIndex) {
        result = [int, arr[i]];
        lastIndex = i;
      }

    }
    arr.splice(0, 1);
  }
  return result;
}

sumPairs([10, 5, 2, 3, 7, 5], 10)

So the first item in the returned array is never an odd index.

Why splice? It’s not doing anything for you at all, and doesn’t make sense for the algorithm.

To return the first pair that exists, construct a Set of the numbers needed to be found to sum to the desired amount. For example, when iterating over 3 with a target of 10, add 7 to the Set.

function sumPairs(ints, target) {
  const set = new Set();
  for (const num of ints) {
    if (set.has(num)) return [target - num, num];
    set.add(target - num);
  }
}

console.log(sumPairs([10, 5, 2, 3, 7, 5], 10)); // [3,7] is expected
console.log(sumPairs([1, 2, 3, 4, 1, 0], 2));
console.log(sumPairs([0, 0, -2, 3], 2));
console.log(sumPairs([1, 4, 8, 7, 3, 15], 8));
console.log(sumPairs([1, -2, 3, 0, -6, 1], -6)); // [0,-6] is expected
console.log(sumPairs([4, 3, 2, 3, 4], 6));