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Interleave two arrays (card decks) using recursion... I was able to solve it without recursion but want to know what I am doing wrong

I am attempting to write a function that takes two arrays as inputs, representing the top and bottom halves of a deck of cards, and shuffles them together using recursion. I am attempting return a single array containing the elements from both input arrays interleaved, like so:

the first element should be the first element of the first input array
the second element should be the first element of the second input array,
the third element should be the second element of the first input array,
the fourth element should be the second element of the second array,

...etc.

I want to append any remaining elements to the end of the array.

This is how I solved it without recursion:

function shuffleCards(topHalf, bottomHalf) {
  let returnArray = [];
  for (let i = 0; i < Math.max(topHalf.length, bottomHalf.length); i++) {
    if (topHalf[i]) {
      returnArray.push(topHalf[i]);
    }
    if (bottomHalf[i]) {
      returnArray.push(bottomHalf[i]);
    }
  }
  return returnArray
}

and this is my attempt with recursion:

function shuffleCards(topHalf, bottomHalf) {
  let results = [];
  if (topHalf.length) {
    results.push(topHalf[0]
    } 
  if (bottomHalf.length) {
      results.push(bottomHalf[0])
    }
  return results.concat(shuffleCards(topHalf.slice(1), bottomHalf.slice(1)));
}

I keep getting the syntax error "missing ) after argument list" but I am fairly certain all of the parenthesis are in the write place.

Any tips?

Thanks!

3 months ago ·

Juan Pablo Isaza

2 answers

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Answer 1 · 2022-07-10T11:47:32.981Z

Beside missing parenthesis, you could take only the first item from the first half and call the function with swapped arrays.

function shuffleCards([value, ...topHalf], bottomHalf) {
    return value === undefined
        ? [...bottomHalf]
        : [value, ...shuffleCards(bottomHalf, topHalf)];
}

console.log(...shuffleCards([1, 2, 3, 4], [5, 6, 7, 8]));

Answer 2 · 2022-07-10T11:47:32.988Z

function shuffleCards(topHalf, bottomHalf,results = []) {
    if(topHalf.length===0&&bottomHalf.length===0)return results;
    if (topHalf.length!==0) {
    results.push(topHalf[0])
    } 
    if (bottomHalf.length!==0) {
      results.push(bottomHalf[0])
    }
    return shuffleCards(topHalf.slice(1), bottomHalf.slice(1),results);
}