CODEWITHSUNDEEP
javascriptrecursionmultidimensional-array
ham17
ham17

Reputation: 45

How to return the entire array in a recursive JavaScript function?

I have a recursive function to work with nested arrays that are multi-level deep. The code below is supposed to select random elements, one from each level, and combine them into an array (in the order they are printed out in the console). However, the resulting array only contains elements of the highest levels (A, B or C, D or C), nothing below that. What is the reason for that?

const arry = [
  ["A"],
  ["B", "C"],
  ["D", "E"],
  [
    [
      ["F1", "F2"],
      ["G1", "G2"],
      [
        "H1",
        "H2",
        [
          ["I1", "I2", "I3"],
          ["J1", "J2"],
          ["K1", "K2", "K3"],
        ],
      ],
    ],
  ],
];

function rndmElementSelection(array) {
  rndElm = array[Math.floor(Math.random() * array.length)];
  return rndElm;
}

function recursion(array, resultAry = []) {
  array.forEach((element) => {
    if (typeof element === "string") {
      console.log(element);
      resultAry.push(element);
    } else {
      nE = rndmElementSelection(element);
      if (typeof nE === "string") {
        console.log(nE);
        resultAry.push(nE);
      } else {
        recursion(nE);
      }
    }
  });
  return resultAry;
}

console.log(recursion(arry));

Upvotes: 1

Views: 1600

Answers (3)

Scott Sauyet
Scott Sauyet

Reputation: 50787

This is perhaps a cleaner approach:

const pickSome = (xs) =>
  Array .isArray (xs)
    ? xs .every (x => Array .isArray (x))
      ? xs .flatMap (pickSome)
      : pickSome (xs [~~ (Math .random () * xs .length)]) 
    : [xs]

const arry = [["A"], ["B", "C"], ["D", "E"], [[["F1", "F2"], ["G1", "G2"], ["H1", "H2", [["I1", "I2", "I3"], ["J1", "J2"], ["K1", "K2", "K3"]]]]]]

for (let n = 0; n < 10; n ++) console .log (`['${pickSome (arry) .join ("', '")}'']`)
.as-console-wrapper {max-height: 100% !important; top: 0}

We test if our input is an array. If it's not, we simply wrap it in an array and return it. So pickSome ('G1') return ['G1']. If it is an array, then we check whether all its children are arrays. If they are we return the result of recursively flatMapping our function over its elements. If they are not, then we randomly choose one element and call pickSome on that. (This handles your ['H1', 'H2', [['I1', 'I2'...], ...]] case.)

Note that ~~ acts as an integer truncation operator, an alternative to Math .floor for positive numbers.

Upvotes: 0

X3R0
X3R0

Reputation: 6334

Try this:


function recursion(array, resultAry = []) {
  array.forEach((element) => {
    if (typeof element === "string") {
      console.log(element);
      resultAry.push(element);
    } else {
      nE = rndmElementSelection(element);
      if (typeof nE === "string") {
        console.log(nE);
        resultAry.push(nE);
      } else {
        resultAry = [...resultAry, ...recursion(nE)];
      }
    }
  });
  return resultAry;
}

Upvotes: 1

trincot
trincot

Reputation: 350252

The problem is that your recursive call does not pass the second argument.

Without passing it, each recursive call will just populate its own, new array. It does return that array to the caller, but the caller (making the recursive call) ignores that returned value, so all the work of the recursive call is for nothing.

So the easy fix is to change this:

} else {
    recursion(nE);

to this:

} else {
    recursion(nE, resultAry);

Upvotes: 2

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