Two Palindrome Exercises

October 6, 2017

We’ve done the first task previously. One approach converts the number to a string, then compares, but it’s easy to strip off the digits of the number, one by one, comparing at the end:

(define (number-palindrome? n)
  (let loop ((x n) (z 0))
    (if (zero? x) (= n z)
      (let ((q (quotient x 10)) (r (remainder x 10)))
        (loop q (+ (* z 10) r))))))

And here are some examples:

> (number-palindrome? 123454321)
> (number-palindrome? 123456789)

For the second task, we convert the input strings to a list of characters and compare the list to its reverse; mappend is like map, but uses append instead of cons to assemble its output, thereby splitting multi-letter words into their individual letters:

(define (string-palindrome? xs)
  (let ((chars (mappend string->list xs)))
    (equal? chars (reverse chars))))

And here are some examples:

> (define x ‘(“a” “bcd” “ef” “g” “f” “ed” “c” “ba”))
> (string-palindrome? x)
> (string-palindrome? (cdr x))

You can run the program at


Pages: 1 2

5 Responses to “Two Palindrome Exercises”

  1. chaw said

    We can avoid separate calls to quotient and remainder by using floor/ which returns both.

  2. John Cowan said

    With SRFI 13, we can eliminate converting strings to lists and back, and just say:

    (let ((str (apply string-append xs)
      (string=? str (string->reverse str)))

    That said, the successors of SRFI 13 do not support string-reverse, because it is only useful in artificial examples like these. In a Unicode world, “Hägar the Horrible”, when expressed in decomposed form (as here). reverses not to “elbirroH eht ragäH” but to “elibrroH eht rag̈aH”, with the umlaut on the “g” rather than the first “a”.

  3. Globules said

    A Haskell version.

    import Data.Bool (bool)
    import Data.List (unfoldr)
    import Data.Tuple (swap)
    import Numeric (showInt)
    -- Is a number a base-10 palindrome?  (We only consider non-negative numbers
    -- because the showInt library function only supports them.)
    isNumPalindrome :: Integral a => a -> Bool
    isNumPalindrome n | n < 0     = False
                      | otherwise = isPalindrome (showInt n "")
    -- Is the result of concatenating a list of lists a palindrome?
    isConcatPalindrome :: Eq a => [[a]] -> Bool
    isConcatPalindrome = isPalindrome . concat
    -- Is a list a palindrome?
    isPalindrome :: Eq a => [a] -> Bool
    isPalindrome xs = xs == reverse xs
    -- An alternate way of checking whether a number is a palindrome, by explicitly
    -- converting the number to a list of digits, rather than using the showInt
    -- library function to convert it to a list of characters.
    -- The list of a number's digits, in base b.  The least significant digits are
    -- first.  The empty list represents 0.
    toDigits :: Integral a => a -> a -> [a]
    toDigits b = unfoldr step
      where step 0 = Nothing
            step i = Just . swap $ i `quotRem` b
    -- Is a number a base-10 palindrome?  We allow negative numbers: -n is a
    -- palindrome if n is.
    isNumPalindrome' :: Integral a => a -> Bool
    isNumPalindrome' = isPalindrome . toDigits 10
    test :: (Eq a, Show a) => (a -> Bool) -> a -> IO ()
    test palFn x = putStrLn $ "Is " ++ show x ++ " a palindrome?  " ++
                   bool "No" "Yes" (palFn x)
    main :: IO ()
    main = do
      test isNumPalindrome 0
      test isNumPalindrome 123
      test isNumPalindrome 12321
      test isConcatPalindrome ([] :: [String])
      test isConcatPalindrome ["a", "bc"]
      test isConcatPalindrome ["a", "bcd", "ef", "g", "f", "ed", "c", "ba"]
      test isNumPalindrome' (-12)
      test isNumPalindrome' (-121)
      test isNumPalindrome' 121
    $ ./palin2 
    Is 0 a palindrome?  Yes
    Is 123 a palindrome?  No
    Is 12321 a palindrome?  Yes
    Is [] a palindrome?  Yes
    Is ["a","bc"] a palindrome?  No
    Is ["a","bcd","ef","g","f","ed","c","ba"] a palindrome?  Yes
    Is -12 a palindrome?  No
    Is -121 a palindrome?  Yes
    Is 121 a palindrome?  Yes
  4. Steve said
            Klong 20170905
            f1::{[a]; a::,/:~x; a=|a}
            f2::{[a]; a::0;{x>0}{[r]; a::(r::x!10)+a*10; (x-r):%10}:~x; x=a}
            f1(["a" "bc" "def" "g" "h"])
            f1(["a" "bc" "def" "g" "h" "h" "g" "f" "e" "d" "c" "b" "a"])
            f1(["a" "bc" "def" "g" "h" "g" "f" "e" "d" "c" "b" "a"])
  5. lijigang said

    (defun palindromic-p (str)
    (equal (string-reverse str) str))

    (defun palindromic-integer-p (n)
    (palindromic-p (int-to-string n)))

    (defun palindromic-list-p (list)
    (palindromic-p (apply #’concat list)))

    ;; (palindromic-integer-p 123454321)

    ;; t
    ;; (palindromic-integer-p 1234544321)

    ;; nil

    ;; (palindromic-list-p ‘(“a” “bcd” “ef” “g” “f” “ed” “c” “ba”))

    ;; t
    ;; (palindromic-list-p ‘(“a” “bcd” “ef” “xg” “f” “ed” “c” “ba”))

    ;; nil

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