#### 3.9Vectors

A vector is a fixed-length array of arbitrary values. Unlike a list, a vector supports constant-time access and update of its elements.

A vector prints similar to a list—as a parenthesized sequence of its elements—but a vector is prefixed with # after ', or it uses vector if one of its elements cannot be expressed with quote.

For a vector as an expression, an optional length can be supplied. Also, a vector as an expression implicitly quotes the forms for its content, which means that identifiers and parenthesized forms in a vector constant represent symbols and lists.

Reading Vectors in The Racket Reference documents the fine points of the syntax of vectors.

Examples:
 > #("a" "b" "c") '#("a" "b" "c") > #(name (that tune)) '#(name (that tune)) > #4(baldwin bruce) '#(baldwin bruce bruce bruce) > (vector-ref #("a" "b" "c") 1) "b" > (vector-ref #(name (that tune)) 1) '(that tune)

Like strings, a vector is either mutable or immutable, and vectors written directly as expressions are immutable.

Vectors can be converted to lists and vice versa via vector->list and list->vector; such conversions are particularly useful in combination with predefined procedures on lists. When allocating extra lists seems too expensive, consider using looping forms like for/fold, which recognize vectors as well as lists.

Example:
 > (list->vector (map string-titlecase (vector->list #("three" "blind" "mice"))))

'#("Three" "Blind" "Mice")

Vectors in The Racket Reference provides more on vectors and vector procedures.