Our local Second Grade class sends back a variety of word puzzles to exercise the kids' verbal skills. A favorite is a (usually seasonal) word, from which a child is asked "How many different words can you make from the letters in this word?"

So, for example, the word "bread" will yield: red, are, ear, dare, dear, read, bar, etc. There are more (at least 17 common words, actually, but we'll get to that). We have fairly verbal kids, who now and then come up with more words than the "official" target. No wonder ...

---

Word: a ad red are ear era dare dear read bad be bed bar bare bear beard bread 31 letter combos (25 - 1) 17 dictionary words found.

How to Solve It

How would you get a list of all of the right answers? First, you have to figure out how to ennumerate the complete list of letter subsets found in the "parent" word, and then you have to figure out what words (if any) can be formed by rearranging the letters in any one subset. To generate an initial list of all possible letter subsets from the "parent" word, we can employ a clever hack involving binary numbers.

"There are 10 kinds of people in this world: those who can handle binary arithmetic and those who can't."

Any word you are going to find involves using some subset of the letters in the "parent" word. Said another way, in any given subset, a letter from the parent word is used (1) or it isn't (0). That means that any given subset of letters can be represented as a string of 1s and 0s according to whether the letter in the parent word was used or not. Consider the word "east":

#	east	binary	subset
1	east	0001	t
2	east	0010	s
3	east	0011	st
4	east	0100	a
5	east	0101	at
6	east	0110	as
7	east	0111	ast
8	east	1000	e
9	east	1001	et
10	east	1010	es
11	east	1011	est
12	east	1100	ea
13	east	1101	eat
14	east	1110	eas
15	east	1111	east

Map the letters you use to a "1" and any that you don't use to a "0", and the letter combo you use suddenly has a unique numeric "signature".

For a word N characters long, there must be 2^N - 1 possible combinations of letters (you subtract one since one of the combos is all 0s ... no letters used ... so it can't ever help).

For a four letter word like "east", that's 2⁴ - 1 = 16 - 1 = 15 combos, and the binary form of each number from 1 to 15 can be used as a map for which letters are included in the subset.

Many "parent" words will have repeating letters, so some of these binary letter subsets will boil down to the same set of letters (just originating from different places in the "parent" word), so we steal a trick from the Jumble Solver, and sort the letters in each subset, and only keep the list of unique combos.

Finally, we recognize each subset may represent more than one word, depending on how the letters are arranged (for example, the subset of "e", "a", and "t" can generate words "eat", "ate", and "tea"). But this is just the Jumble problem we've solved elsewhere, so we use that code to get the set of matches from the dictionary.

The dictionary I use contains over 100,000 words, but also includes a flag for the 5,000 most common English words, and we restrict our search to this subset, which is still pretty expansive for the K-12 crowd. (Although, maybe not ... in our "bread" example, this restriction misses "bead" and (gasp) "bard" ... so maybe it is a little too restrictive!)

The latest challenge to come home from the 2^nd Grade was to find the 39 words hidden in "ornaments". Using this script, we can find 105! Time to start talking about extra credit ...