#( 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 section Printing Strings, section Printing Bit Vectors, or section Printing Other Vectors.
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