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You will create a program that will find the missing letter from a string and add it. If there is no missing letter, the program should return undefined. There is currently no test case for the string missing more than one letter, but if there was one, recursion would be used. Also, the letters are always provided in order so there is no need to sort them.

• String global object
• JS String Prototype CharCodeAt
• String.fromCharCode

You will need to convert from character to ASCII code using the two methods provided in the description.

try to solve the problem now

You will have to check for the difference in ASCII code as they are in order. Using a chart would be very helpful.

try to solve the problem now

You will need to figure out where to insert the letter and how to do it, along with handling the case that there is not missing letter as it needs an specific return value.

try to solve the problem now

Solution ahead!

function fearNotLetter(str) {

  for(var i = 0; i < str.length; i++) {
    /* code of current character */
    var code = str.charCodeAt(i);

    /* if code of current character is not equal to first character + no of iteration
    hence character has been escaped */
    if (code !== str.charCodeAt(0) + i) {

      /* if current character has escaped one character find previous char and return */
      return String.fromCharCode(code - 1);
    }  
  }
  return undefined;
}

// test here
fearNotLetter("abce");

🚀 Run Code

• This solutions makes use of a for loop.
• Code of encountered character is stored in code.
• It is checked if code of current character is the expected one (no characters are skipped) by using the logic - code of current character = code of first character + number of iterations.
• If a character is missing, the missing character is found and the final string is returned.
• undefined is returned if there is no missing character in the string.

• JS For Loops Explained
• String.length

// Adding this solution for the sake of avoiding using 'for' and 'while' loops.
// See the explanation for reference as to why. It's worth the effort.

function fearNotLetter(str) {
  var compare = str.charCodeAt(0), missing;

  str.split('').map(function(letter,index) {
    if (str.charCodeAt(index) == compare) {
      ++compare;
    } else {
      missing = String.fromCharCode(compare);
    }
  });

  return missing;
}

// test here
fearNotLetter("abce");

🚀 Run Code

• First we define variables to store the character code for the first letter in the string, and to store whatever missing letters we may find.
• We turn the string to an array in order to map through it instead of using for and while loops.
• As we map through our letters' character codes, we go comparing with the one that should be in that position.
• If the current letter matches, we move the comparison variable to its next position so we can compare on the next cycle.
• If not, the missing letter will be assigned to the missing variable, which will be returned after the map is finished.
• If there are no missing characters, return undefined.

• JS String Prototype Split
• JS Array Prototype Map

function fearNotLetter(str) {
  var allChars = '';
  var notChars = new RegExp('[^'+str+']','g');

  for (var i = 0; allChars[allChars.length-1] !== str[str.length-1] ; i++)
    allChars += String.fromCharCode(str[0].charCodeAt(0) + i);

  return allChars.match(notChars) ? allChars.match(notChars).join('') : undefined;
}

// test here
fearNotLetter("abce");

🚀 Run Code

• A new string allChars is created.
• Create a regular expression notChars which selects everything except str.
• The for loop is used to add all the letters in the range to allChars.
• match() is used to strip off the str letters from the newly created string and it is returned.
• If there are no missing characters, return undefined.

• JS Regex Resources
• JS Ternary
• JS String Prototype Match
• JS Array Prototype Join

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Thanks @Rafase282 @rohitnwn @sabahang @Hallaathrad for your help with Algorithm: Missing Letters

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