Find Missing Character in a Character Array (javascript)

Question

I am trying to write a function that takes in an array of individual characters (eg.['a','b','d']) and returns the first character that is missing (eg. 'c'). I am not sure why my current function doesn't work as described.

const alph = "abcdefghijklmnopqrstuvwxyz";
const findMissingLetter = (arr) => {
  arr.forEach((l, i, a) => {
    const ltrIdx = alph.indexOf(l); //a's index in the alph is 0
    const arrNxtLtr = a[i+1]; //the next ltr after a is c
    if(arrNxtLtr !== alph[ltrIdx + 1]) return alph[ltrIdx + 1] //return the letter that is missing from the arr
  })
  return -1 //return -1 if there is no missing char
}

console.log(findMissingLetter(['a','c']))

ps. I've seen similar approaches to solve this general problem, I am just simply wondering what I've done wrong with my function so I can learn.

Thank you!

You're returning inside a closure that isn't what's returned from the outer closure, for one thing. So it's always `-1`. — Jared Farrish, Jun 03 '21 at 22:13
Does this answer your question? [Short circuit Array.forEach like calling break](https://stackoverflow.com/questions/2641347/short-circuit-array-foreach-like-calling-break) — tevemadar, Jun 03 '21 at 22:14
The proposed duplicate answers the "what I've done wrong with my function so I can learn" part. You can't easily stop a `forEach()`, use practically any other loop and it will work. — tevemadar, Jun 03 '21 at 22:15
I would use `arr.reduce()` for one thing, `const findMissingLetter = arr => arr.reduce(...)` — Jared Farrish, Jun 03 '21 at 22:15
Are you looking for the order too? `['a','c','b']` would be `'b'` or `-1`? — tevemadar, Jun 03 '21 at 22:20

tevemadar · Accepted Answer · 2021-06-03T23:21:57.373

If you simply want to find the first mismatch between two strings, then just compare them character by character until you find a mismatch or reach the end of the input string:

const alph = "abcdefghijklmnopqrstuvwxyz";
const findMissingLetter = (arr) => {
  for(let i=0; i<arr.length; i++) {
    if(arr[i] !== alph[i]) {
      return alph[i]; // found the first mismatch
    }
  }
  return -1 // return -1 if there is no missing char
}

console.log(findMissingLetter([]),"-1?");
console.log(findMissingLetter(['a']),"-1?");
console.log(findMissingLetter(['b']),"a?");
console.log(findMissingLetter(['a','b']),"-1?");
console.log(findMissingLetter(['a','c']),"b?");

And avoid forEach() if you want to return from inside a loop as it was commented already.

And if the input string does not have to start at the beginning of the "big" string, locate it's first character and do the comparison from there:

const alph = "abcdefghijklmnopqrstuvwxyz";
const findMissingLetter = (arr) => {
  if(arr.length===0)
    return -1;
  let start = alph.indexOf(arr[0]);
  for(let i=0; i<arr.length; i++) {
    if(arr[i] !== alph[start+i]) {
      return alph[start+i]; // found the first mismatch
    }
  }
  return -1 // return -1 if there is no missing char
}

console.log(findMissingLetter([]),"-1?");
console.log(findMissingLetter(['a']),"-1?");
console.log(findMissingLetter(['b']),"-1?");
console.log(findMissingLetter(['a','b']),"-1?");
console.log(findMissingLetter(['a','c']),"b?");
console.log(findMissingLetter(['b','c','e','f']),"d?");

hey I just noticed that I forgot to tell you that this function also has to take in array like ['b','c','e','f'], and return the next missing character. — twominds, Jun 03 '21 at 23:02
@twominds ok, "start" added, see below the horizontal line - this way the single-character test cases do not make much sense, but left them there for consistency. — tevemadar, Jun 03 '21 at 23:22
Thanks again! I realize that my original question had nothing to do with all cases, so I appreciate you going above and beyond! — twominds, Jun 03 '21 at 23:26

Khorne07 · Answer 2 · 2021-06-04T14:10:06.070

1

The reason is that forEach ignores return or any shortcircuit statement. You must instead of trying return the value from the foreach, just save it into another variable and return that variable after the forEach is done.

const alph = "abcdefghijklmnopqrstuvwxyz";
  const findMissingLetter = (arr) => {
      let missingLetter
      arr.forEach((letter, index) => {
        if(letter !== alph[index]) missingLetter ??= alph[index]
        else missingLetter = -1
      })
    return missingLetter
}

console.log(findMissingLetter(['a','c']))

edited Jun 04 '21 at 14:10

answered Jun 03 '21 at 23:03

Khorne07

199
3
12

Thanks. I was using my cellphone and stackoverflow doesn't allowed me to submit code from there – Khorne07 Jun 03 '21 at 23:19
Appreciated! actually, your answer is my questions exact solution, but @tevemadar just went above and beyond to rework the entire code so that it works with all cases. Thanks regardless Khorne07! – twominds Jun 03 '21 at 23:25
Yes I saw. He did a good job. Thanks for your vote anyway. I'm glad to help. – Khorne07 Jun 03 '21 at 23:36

Find Missing Character in a Character Array (javascript)

2 Answers2