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javascriptarraysalgorithmrecursioncapitalize

Write a recursive function called capitalizeFirst. Given an array of strings, capitalize the first letter of each string in the array

I see this similar algorithm was posted on stackoverflow, nevertheless I cannot understand, so I decided to post once more.

function capitalizeFirst(arr) {
  if (arr.length === 1) {
    return [arr[0].toUpperCase()]
  }
  let res = capitalizeFirst(arr.slice(0, -1))
  res.push(arr.slice(arr.length - 1)[0].toUpperCase())
  return res
}

console.log(capitalizeFirst(['dog', 'car', 'horse']))

Things I do not understand...

  1. Why it is inside square brackets return [arr[0].toUpperCase()] why not just return arr[0].toUpperCase()
  2. Why "arr" is getting sliced twice:

here

let res = capitalizeWords(arr.slice(0,-1)

and here

res.push(arr.slice(arr.length-1)[0].toUpperCase())

Overall, I am lost, please help

Solution

  • I would forget about what that code does and concentrate on the steps you need to take to make your function work.

    1. Recursive - so the function needs to call itself but you need to find a way to identify which element you're working on.

    2. You need a way to break out of the recursion when you reach the end of the array.

    3. You need a way to separate out the first letter of an element from all the rest, and update the element with a transformed string.

    Here's how I might approach it.

    // Pass in the array, and initialise an index
// variable
function capitalizeFirst(arr, index = 0) {
  
  if (!arr.length) return 'Empty array';

  // If we're at the end of the array
  // return the array
  if (index === arr.length) return arr;
  
  // If the element is not empty
  if (arr[index].length) {
  
    // Get the first letter, and place all
    // the other letters in an array called `rest`
    // You can use destructuring here because strings
    // are iterable
    const [first, ...rest] = arr[index];

    // Update the element at the current index
    // with the new string making sure you join up `rest`
    arr[index] = `${first.toUpperCase()}${rest.join('')}`;
  
  }

  // Call the function again increasing the index
  return capitalizeFirst(arr, ++index);
}

console.log(capitalizeFirst(['dog', 'car', 'horse']));
console.log(capitalizeFirst([]));
console.log(capitalizeFirst(['dog', '', 'horse']));
console.log(capitalizeFirst(['dog', 'o', 'horse']));

    Additional documentation