3.9 Vectors (original) (raw)
3.9 Vectors🔗ℹ
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 viavector->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:
'#("Three" "Blind" "Mice")