# JS to CLJS # 4

I’ll regularly take a JavaScript problem and work it out in ClojureScript. This will hopefully take away some of the Magic™ of functional programming and show how to solve real problems. I promise you won’t have to learn what a monad is.

### Problem

Write a function that takes an array of consecutive (increasing) letters as input and returns the missing letter.

- You will always get an valid array.
- The array will be always exactly one letter be missing.
- The length of the array will always be at least 2.
- The array will always contain letters in only one case.

### Solution

### Thoughts

The main breakthrough is realizing that the fundamental problem is comparing one character to the next character. A nice cljs trick is to use `partition`

to get the data structure that better represents our problem:

- With our problem-aligned data structure, we work our way through the list, comparing the character code of the first letter to that of the second.
- If the characters are in order, we return
`nil`

. - If we’ve found our missing letter, we return it.
`keep`

returns a sequence of all non-nil values, so the value of`keep`

will be the sequence of missing letters.- Since the problem states there will only be one missing letter, we return the
`first`

one (`keep`

is lazy; when we find our first value, cljs will stop processing the rest of the list).

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