The challenge is to create one program that will accept as arguments (arrays of arrays) or on standard input (line-by-line, separated by commas) a collection of families and names, like so:


As well as two other arguments (two more arrays of pairs, as arrays) or on standard input (a blank line after the preceding input, followed by two comma-separated lists on two lines with an = separating pairings; see example) that represent the two previous years' Christmas lists:


The function/program must return/output (in any format) a way that each person is assigned somebody to give a present to, following these rules:

  1. A person cannot give a present to him or herself.
  2. A person cannot give a present to anybody in his or her own family.
  3. A person cannot be assigned the same person he or she was assigned in any of the two previous years.

If such a matching is impossible for every person, output/return null or nothing or something appropriate.

This is a code golf, so shortest code wins, but I'll also give a bounty to the code that runs fastest (on my computer, at least, though results shouldn't vary much).

  • 1
    \$\begingroup\$ Are the gift assignments symmetric? The text doesn't say so, but using = to represent the assignments suggests it... \$\endgroup\$ Jul 7, 2012 at 6:05
  • 1
    \$\begingroup\$ Shouldn't the number of people be guaranteed to be even? \$\endgroup\$ Jul 7, 2012 at 6:13
  • \$\begingroup\$ @KeithRandall: No, they aren't symmetric. \$\endgroup\$
    – Ry-
    Jul 7, 2012 at 13:35
  • \$\begingroup\$ @PeterTaylor: No, because they aren't symmetric. (Dan=Marge,Marge=Carolyn,Carolyn=Don,Don=Bob,Bob=Dan) \$\endgroup\$
    – Ry-
    Jul 7, 2012 at 13:37
  • 2
    \$\begingroup\$ @userunknown What, we can't prepare early for christmas? I like to buy my christmas cards in January in the sales. :-) Seriously though, I think -1 for being unseasonal is a little harsh. \$\endgroup\$
    – Gareth
    Jul 7, 2012 at 16:31

1 Answer 1


GolfScript, 90 characters


The program does a simple recursive search on the names. It actually calculates all possible lists but saves them to a variable for output, so only the last one is returned.

The input/output is on the interpreter stack (first families then two previous years` assignments), e.g.

[["John" "Anna"]["Mark" "James"]["Elizabeth" "Daniel" "Isabelle"]["Ryan" "Ruth"]]
[["Ruth" "John"] ["Ryan" "Daniel"] ["Isabelle" "Mark"] ["Daniel" "Anna"] ["Elizabeth" "James"] ["James" "Elizabeth"] ["Mark" "Ryan"] ["Anna" "Isabelle"] ["John" "Ruth"]]
[["Ruth" "Anna"] ["Ryan" "Elizabeth"] ["Isabelle" "John"] ["Daniel" "James"] ["Elizabeth" "Mark"] ["James" "Daniel"] ["Mark" "Isabelle"] ["Anna" "Ruth"] ["John" "Ryan"]]

+:q;:f[]*:^[]:r\{.,{(^3${1=}%-f {2$\?)},~-q{(3$={-.}*;}/{.[[2$\]]4$|3$c;}%;;;}{;:r}if;}:c~r


prints the output

[["Ruth" "Elizabeth"] ["Ryan" "John"] ["Isabelle" "James"] ["Daniel" "Mark"] ["Elizabeth" "Anna"] ["James" "Ruth"] ["Mark" "Daniel"] ["Anna" "Ryan"] ["John" "Isabelle"]]

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