NPR Sunday Puzzle

Yesterday’s puzzle from Programming Praxis asks us to solve a Sunday Puzzle from NPR:

Think of two familiar, unhyphenated, eight-letter words that contain the letters A, B, C, D, E and F, plus two others, in any order. What words are these?

It’s another in a long history of word games, my favorite sort of puzzle.

So how do we do it? Well, it turns out that the program is rather straight forward. You need something something that will loop through a file, reading in a line at a time. Racket’s (in-lines) makes this all sorts of easy. So we’ll start with that:

; find all words containing _chars_ with _length_ characters
(define (words-containing chars [length (length chars)])
  (with-input-from-file DICTIONARY
    (lambda ()
      (for/list ([word (in-lines)]
                 #:when (and (= length (string-length word))
                             (andmap (lambda (c) (string-contains? word c)) chars)))
        word))))

It’s generalized over the original problem to any required letters and any given word length. The only at all sneaky part here is using the #:when conditional to first check if the word is the proper length. If so, use andmap to make sure all of the letters are in the word in turn. The advantage of this is that andmap will short circuit, stopping as soon as we find a letter not in the word. It’ll only save us a bit of time, but over the length of of the entire dictionary, the savings can be impressive.

Likewise, the string-contains? function will short circuit, using a trick with call/cc to simulate return which I’ve used before:

; test if _str_ contains the character _char_
(define (string-contains? str char)
  (call/cc 
   (lambda (return)
     (for ([c (in-string str)])
       (when (eq? c char)
         (return #t)))
     #f)))

And that’s all there is to it. Using the same dictionary I’ve used before (wordsEn.txt), we’re ready to solve the actual problem:

> (define DICTIONARY "wordsEn.txt")
> (words-containing (string->list "abcdef") 8)
'("boldface" "feedback")

And that’s it. The only two common words with 8 letters, 6 of which are A through F are boldface and feedback.

If you’d like to see / download the entire code in one go, you can do so on my GitHub:
- NPR Sunday Puzzle source

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