Convert function just using array methods

Question

Imagine I have the following array of objects:

 const object = [
  {Name: "A", Nr: "01"},
  {Name: "B", Nr: "02"},
  {Name: "C", Nr: "04"},
  {Name: "D", Nr: "06"},
  {Name: "E", Nr: "07"},
 ];

And I have the following function:

const findCurrentNumber = (obj) => {
    let numbers = obj.map((a) => {
      return parseInt(a["Nr"]);
    });

    numbers.sort((a, b) => a - b);
    let current = numbers[numbers.length - 1];
    for (let i = numbers.length - 1; i > 0; i--) {
      if (numbers[i - 1] !== numbers[i] - 1) current = numbers[i - 1];
    }

    return current + 1;
  };

I will get the value 3 as return. But I want to do the same just using array methods (map, sort, reduce etc). I was trying and I stopped here:

const obj = object.map(a => parseInt(a["Nr"])).sort((a, b) => a - b);

I don't know how to continue from the part I declared the current variable.

Is it possible? If so, how can I do it?

Thank you!

It looks like the for loop is just grabbing the element at [0] and adding 1 to it. Not sure why you need a loop to do that — Taplar
– Taplar, Commented Nov 12, 2020 at 15:15
@Taplar it might be the first element or not. It's the last element (in regards to the loop, it's going as close as possible to the beginning of the array) that is not 1 larger than the element following it. — VLAZ
– VLAZ, Commented Nov 12, 2020 at 15:17
@Taplar I edited the question, I guess you can have a better understanding now — Lit2000hania
– Lit2000hania, Commented Nov 12, 2020 at 15:17
@Polalas actually, it's even less clear. You don't even have numbers, you're not adding one to current, you're doing string concatenation. I'm still not sure what the point is. — VLAZ
– VLAZ, Commented Nov 12, 2020 at 15:18

Nina Scholz · Accepted Answer · 2020-11-12 16:49:14Z

1

You could take an approach for How to write the code with less time complexity for finding the missing element in given array range? and take the numerical values of Nr.

function getMissing(array) {
    var min = array[0],
        max = array[0],
        missing = new Set;
    
    array.forEach(v => {
        if (missing.delete(v)) return;                   // if value found for delete return
        if (v < min) while (v < --min) missing.add(min); // add missing min values
        if (v > max) while (v > ++max) missing.add(max); // add missing max values
    });
    return missing.values().next().value;                // take the first missing value
}

const
    data = [{ Name: "A", Nr: "01" }, { Name: "B", Nr: "02" }, { Name: "C", Nr: "04" }, { Name: "D", Nr: "06" }, { Name: "E", Nr: "07" }],
    first = getMissing(data.map(({ Nr }) => +Nr));

console.log(first);

Classic approach. By checking an ordered array of numbers and get the one where the next value has a delta of more than one. Finally add one for the missing number.

const
    data = [{ Name: "A", Nr: "01" }, { Name: "B", Nr: "02" }, { Name: "C", Nr: "04" }, { Name: "D", Nr: "06" }, { Name: "E", Nr: "07" }],
    first = 1 + data
        .map(({ Nr }) => +Nr)
        .sort((a, b) => a - b)
        .find((l, i, { [i + 1]: r = Number.MAX_VALUE }) => r >= l + 2);

console.log(first);

edited Nov 12, 2020 at 16:49

answered Nov 12, 2020 at 15:36

Nina Scholz

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4 Comments

Lit2000hania Over a year ago

Thank you, Nina! I appreciate you showing me this method with less time complexity. I will not set your response as the questions' answer because that is not specifically what I wanted, but I really appreciate! Danke

VLAZ Over a year ago

I commented on the original but I'd like to bring attention to the same thing here. This first algorithm is actually O(n*m) where n is the input size and m is the largest gap between the numbers. For example [1, 3] will only generate 2 but [0, 9001] will generate the entire range from 1 to 9000 because of the gap between the two elements.

Lit2000hania Over a year ago

@NinaScholz in this example jsbin.com/velobip/edit?js,console it doesn't return the exact value it was expect to return, which is 2

Nina Scholz Over a year ago

@Polalas, thank you for the hint. i changed the items, now it checks the next value or a max value.

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