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### 8.2 Selecting Vector Components

— procedure: vector? object

Returns `#t` if object is a vector; otherwise returns `#f`.

— procedure: vector-length vector

Returns the number of elements in vector.

— procedure: vector-ref vector k

Returns the contents of element k of vector. K must be a valid index of vector.

```          (vector-ref '#(1 1 2 3 5 8 13 21) 5)    =>  8
```
— procedure: vector-set! vector k object

Stores object in element k of vector and returns an unspecified value. K must be a valid index of vector.

```          (let ((vec (vector 0 '(2 2 2 2) "Anna")))
(vector-set! vec 1 '("Sue" "Sue"))
vec)
=>  #(0 ("Sue" "Sue") "Anna")
```
— procedure: vector-first vector
— procedure: vector-second vector
— procedure: vector-third vector
— procedure: vector-fourth vector
— procedure: vector-fifth vector
— procedure: vector-sixth vector
— procedure: vector-seventh vector
— procedure: vector-eighth vector

These procedures access the first several elements of vector in the obvious way. It is an error if the implicit index of one of these procedurs is not a valid index of vector.

— procedure: vector-binary-search vector key<? unwrap-key key

Searches vector for an element with a key matching key, returning the element if one is found or `#f` if none. The search operation takes time proportional to the logarithm of the length of vector. Unwrap-key must be a procedure that maps each element of vector to a key. Key<? must be a procedure that implements a total ordering on the keys of the elements.

```          (define (translate number)
(vector-binary-search '#((1 . i)
(2 . ii)
(3 . iii)
(6 . vi))
< car number))
(translate 2)  =>  (2 . ii)
(translate 4)  =>  #F
```