How do I match numbers and create 2-dimensional array?

Question

I have an array like so [1.2, 1.3, 1.4, 2.2, 2.3, 3.0, 4.3, 4.4, 4.5, 4.6, 4.7]

How can I turn it into a 2-dimensional array that's based on matching numbers before the decimal?

ex.[[1.2, 1.3, 1.4], [2.2, 2.3], [3.0], [4.3, 4.4, 4.5, 4.6, 4.7]]

Answer 1

You can use Array.reduce to do this, building an array using indexes based on the integer portion of each value:

arr = [1.2, 1.3, 1.4, 2.2, 2.3, 3.0, 4.3, 4.4, 4.5, 4.6, 4.7];

let res = arr.reduce((c, v) => {
  i = Math.trunc(v) - 1;
  c[i] = c[i] || [];
  c[i].push(v);
  return c;
}, []);

console.log(res);

If you could have values starting at 0, or gaps in the values, you can use a similar piece of code and then use Object.values() to just extract the filled in arrays:

arr = [0.2, 1.3, 1.4, 2.2, 2.3, 3.0, 4.3, 4.4, 4.5, 6.6, 8.7];

res = arr.reduce((c, v) => {
  i = Math.trunc(v);
  c[i] = c[i] || [];
  c[i].push(v);
  return c;
}, []);
res = Object.values(res)
console.log(res);

Answer 2

Since it's always sorted, you can use this imperative approach:

const arr = [1.2, 1.3, 1.4, 2.2, 2.3, 3.0, 4.3, 4.4, 4.5, 4.6, 4.7];

let curr = -Infinity;
let currArr = null;
const res = [];

for (let n of arr) {
  if (Math.trunc(n) > curr) {
    res.push(currArr = [curr = n]);
  } else {
    currArr.push(n);
  }
}

console.log(res);

It just tracks the latest number, and its current sub-array. If the next integer is greater than the current number, it creates a new array, otherwise it pushes into the current sub-array.

Some may not like using the result of an assignment as a value. If not, it can easily be reworked with a couple extra lines of code.