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I'm trying to find the palindromes in the dictionary, but my output isn't giving me all of them.

I have this so far

Palindrome[] := DictionaryLookup[y_ /; (y == StringReverse[y])]
Palindrome[]

And my the output just it gives me {a,I}. What's wrong with my input? I don't see it.

Thanks.

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  • 1
    $\begingroup$ Two underscores after your y should help $\endgroup$
    – bobthechemist
    Oct 22, 2013 at 0:37
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    $\begingroup$ Palindrome[] := DictionaryLookup[y__ /; (y == StringReverse[y])] $\endgroup$
    – bobthechemist
    Oct 22, 2013 at 0:38
  • $\begingroup$ Ah okay thanks. Why did I need two underscores? I read the reference on wolfram, but I'm not sure how it applies here. $\endgroup$
    – jas
    Oct 22, 2013 at 0:51
  • $\begingroup$ A single underscore is for a single character, and double underscore is for one or more. $\endgroup$
    – bobthechemist
    Oct 22, 2013 at 0:55
  • $\begingroup$ So by character, you mean the double underscore is applied to the y and that would give me more than a one letter word? $\endgroup$
    – jas
    Oct 22, 2013 at 0:58

2 Answers 2

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As mentioned in the comments, to search for multi-letter palindromes, two underscores are needed as described in the help for string patterns:

Palindrome[] := DictionaryLookup[y__ /; (y == StringReverse[y])]

Now, Mathematica is probably not good enough to come up with lyrical palindromes.

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Mathematica 10.3 introduced the function PalindromeQ, which can be utilized here

palindrome[] := DictionaryLookup[y__ /; PalindromeQ[y]]

palindrome[]

{"a", "aha", "aka", "bib", "bob", "boob", "bub", "CFC", "civic", "dad", "deed", "deified",
 "did", "dud", "DVD", "eke", "ere", "eve", "ewe", "eye", "gag", "gig", "huh", "I", 
 "kayak", "kook", "level", "ma'am", "madam", "mam", "MGM", "minim", "mom", "mum", "nan", 
 "non", "noon", "nun", "oho", "pap", "peep", "pep", "pip", "poop", "pop", "pup", "radar", 
 "redder", "refer", "repaper", "reviver", "rotor", "sagas", "sees", "seres", "sexes", 
 "shahs", "sis", "solos", "SOS", "stats", "stets", "tat", "tenet", "TNT", "toot", "tot", 
 "tut", "wow", "WWW"}

Unfortunately this is much slower (≈ 10 times slower) than using

DictionaryLookup[y__ /; y === StringReverse[y]]

but can be also used with the option IgnoreCase -> True:

DictionaryLookup[y__ /; PalindromeQ[y, IgnoreCase -> True]]

{"a", "Abba", "Ada", "aha", "aka", "Ana", "Anna", "Ava", "bib", "bob", "Bob", "boob", 
 "bub", "CFC", "civic", "dad", "deed", "deified", "did", "dud", "DVD", "Ede", "eke", 
 "ere", "eve", "Eve", "ewe", "eye", "gag", "gig", "Hannah", "huh", "I", "kayak", "kook", 
 "Laval", "level", "ma'am", "madam", "Malayalam", "mam", "MGM", "minim", "mom", "mum", 
 "nan", "Nan", "non", "noon", "nun", "oho", "Ono", "Otto", "pap", "peep", "pep", "pip", 
 "poop", "pop", "pup", "radar", "redder", "refer", "repaper", "reviver", "rotor", "sagas",
 "Salas", "sees", "seres", "sexes", "shahs", "sis", "solos", "SOS", "stats", "stets", 
 "tat", "tenet", "Tet", "Tevet", "TNT", "toot", "tot", "tut", "Tut", "wow", "WWW"}

However, for this using

DictionaryLookup[y__ /; ToLowerCase@y === StringReverse[ToLowerCase@y]]

is much faster.

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  • $\begingroup$ interesting. why does the official implementation perform worse than the user-defined one? $\endgroup$
    – sunt05
    Oct 16, 2015 at 5:18
  • $\begingroup$ @sunt05 PalindromeQ has additional overheads, especially for strings. $\endgroup$
    – Karsten7
    Oct 16, 2015 at 5:24
  • $\begingroup$ @sunt05 and it seems, many new functions, at least at first, perform worse, than what could be user-defined in previous versions. One example is ContainsAll and related functions. $\endgroup$
    – LLlAMnYP
    Oct 16, 2015 at 11:19

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