readme.md

Challenge: Find Missing Letter Refactor

Instructions

In the last section, we created a function calledfindMissingLetter that takes in an array of consecutive (increasing) letters as input and returns the missing letter in the array.

I want to do that same thing using high order array methods. It is possible to do it using map, filter, reduce, or forEach.

Function Signature

/**
 * Returns the missing letter in an array of consecutive letters.
 * @param {string[]} arr - An array of consecutive letters.
 * @returns {string} - The missing letter.
 */
function findMissingLetter(arr: string[]): string;

Examples

findMissingLetter(['a', 'b', 'c', 'd', 'f']); // => "e"
findMissingLetter(['O', 'Q', 'R', 'S']); // => "P"
findMissingLetter(['t', 'u', 'v', 'w', 'x', 'z']); // => "y"

Constraints

• The input array will always contain at least two letters
• The input array will only contain letters in one case (lower or upper)
• Use only high order array methods. No for loops allowed.

Hints

• You can use map to get an array of the unicode numbers (using charCodeAt) of the letters in the input array and then use find to find the missing character code.
• You can also use filter as well as reduce

Solutions

Click For Solution 1

This solution uses the map and find methods.

function findMissingLetter(arr) {
  let start = arr[0].charCodeAt(0);
  const missingCharCode = arr
    .slice(1)
    .map((char) => char.charCodeAt(0))
    .find((current) => {
      if (current - start > 1) {
        return true;
      }
      start = current;
      return false;
    });
  return missingCharCode ? String.fromCharCode(missingCharCode - 1) : '';
}

Explanation

• Declare a variable start and assigned it the ASCII code of the first letter of the input array.
• Declare a variable missingCharCode and assigned it the result of calling the map method on the input array. The callback function passed to the map method takes in a letter and returns the ASCII code of that letter.
• Call the find method on the result of the map method. The callback function passed to the find method takes in the current ASCII code and checks if the current ASCII code minus the previous ASCII code is greater than 1. If it is, it returns true. If it is not, it returns false.
• If the find method returns a truthy value, we return the letter that is one less than the current letter.
• If the find method returns a falsy value, we return an empty string.

Click For Solution 2

This solution uses the filter method.

function findMissingLetter(arr) {
  const missingCharCode = arr.filter((char, index) => {
    if (index === 0) return false;
    const prevCharCode = arr[index - 1].charCodeAt(0);
    const currentCharCode = char.charCodeAt(0);
    return currentCharCode - prevCharCode > 1;
  })[0];

  return missingCharCode
    ? String.fromCharCode(missingCharCode.charCodeAt(0) - 1)
    : '';
}

Explanation

• Declare a variable missingCharCode and assigned it the result of calling the filter method on the input array.
• The callback function passed to the filter method takes in a letter and its index and checks if the current letter is the first letter in the array. If it is, it returns false. If it is not, it checks if the current letter's ASCII code minus the previous letter's ASCII code is greater than 1. If it is, it returns true. If it is not, it returns false.
• If the filter method returns an array with a length greater than 0, we return the letter that is one less than the current letter.
• If the filter method returns an array with a length of 0, we return an empty string.

Click For Solution 3

This solution uses the reduce method.

function findMissingLetter(arr) {
  let start = arr[0].charCodeAt(0);
  const missingCharCode = arr.reduce((missing, char) => {
    const current = char.charCodeAt(0);
    if (current - start > 1 && missing === null) {
      missing = start + 1;
    }
    start = current;
    return missing;
  }, null);
  return missingCharCode ? String.fromCharCode(missingCharCode) : '';
}

Explanation

• Declare a variable start and assigned it the ASCII code of the first letter of the input array.
• Declare a variable missingCharCode and assigned it the result of calling the reduce method on the input array. The callback function passed to the reduce method takes in an accumulator and a letter. The accumulator is the missing ASCII code. The callback function checks if the current ASCII code minus the previous ASCII code is greater than 1 and if the accumulator is null. If it is, it assigns the missing ASCII code to the accumulator. It then assigns the current ASCII code to the start variable. It then returns the accumulator.
• If the reduce method returns a truthy value, we return the letter that is one less than the current letter.
• If the reduce method returns a falsy value, we return an empty string.

Test Cases

test('Find Missing Letter', () => {
  expect(findMissingLetter(['a', 'b', 'c', 'e'])).toBe('d');
  expect(findMissingLetter(['X', 'Z'])).toBe('Y');
  expect(findMissingLetter(['m', 'n', 'o', 'q', 'r'])).toBe('p');
  expect(findMissingLetter(['F', 'G', 'H', 'J'])).toBe('I');
});