2.4.8.3 Sharpsign Left-Parenthesis

#( and ) are used to notate a simple vector.

If an unsigned decimal integer appears between the # and (, it specifies explicitly the length of the vector. The consequences are undefined if the number of objects specified before the closing ) exceeds the unsigned decimal integer. If the number of objects supplied before the closing ) is less than the unsigned decimal integer but greater than zero, the last object is used to fill all remaining elements of the vector.

[Editorial Note by Barmar: This should say "signals...".] The consequences are undefined if the unsigned decimal integer is non-zero and number of objects supplied before the closing ) is zero. For example,

 #(a b c c c c)
 #6(a b c c c c)
 #6(a b c)
 #6(a b c c)

all mean the same thing: a vector of length 6 with elements a, b, and four occurrences of c. Other examples follow:

 #(a b c)               ;A vector of length 3
 #(2 3 5 7 11 13 17 19 23 29 31 37 41 43 47)
                        ;A vector containing the primes below 50
 #()                    ;An empty vector

The notation #() denotes an empty vector, as does #0().

For information on how the Lisp printer prints vectors, see Printing Strings, Printing Bit Vectors, or Printing Other Vectors.