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### 6.3.6 Vectors

Vectors are heterogenous structures whose elements are indexed by integers. A vector typically occupies less space than a list of the same length, and the average time required to access a randomly chosen element is typically less for the vector than for the list.

The length of a vector is the number of elements that it contains. This number is a non-negative integer that is fixed when the vector is created. The valid indexes of a vector are the exact non-negative integers less than the length of the vector. The first element in a vector is indexed by zero, and the last element is indexed by one less than the length of the vector.

Vectors are written using the notation #(obj …,). For example, a vector of length 3 containing the number zero in element 0, the list ‘(2 2 2 2)’ in element 1, and the string ‘"Anna"’ in element 2 can be written as following:

```#(0 (2 2 2 2) "Anna")
```

Note that this is the external representation of a vector, not an expression evaluating to a vector. Like list constants, vector constants must be quoted:

```'#(0 (2 2 2 2) "Anna")
⇒  #(0 (2 2 2 2) "Anna")
```
procedure: vector? obj

Returns #t if obj is a vector, otherwise returns #f.

procedure: make-vector k
procedure: make-vector k fill

Returns a newly allocated vector of k elements. If a second argument is given, then each element is initialized to fill. Otherwise the initial contents of each element is unspecified.

library procedure: vector obj …,

Returns a newly allocated vector whose elements contain the given arguments. Analogous to ‘list’.

```(vector 'a 'b 'c)                      ⇒  #(a b c)
```
procedure: vector-length vector

Returns the number of elements in vector as an exact integer.

procedure: vector-ref vector k

k must be a valid index of vector. ‘Vector-ref’ returns the contents of element k of vector.

```(vector-ref '#(1 1 2 3 5 8 13 21)
5)
⇒  8
(vector-ref '#(1 1 2 3 5 8 13 21)
(let ((i (round (* 2 (acos -1)))))
(if (inexact? i)
(inexact->exact i)
i)))
⇒ 13
```
procedure: vector-set! vector k obj

k must be a valid index of vector. ‘Vector-set!’ stores obj in element k of vector. The value returned by ‘vector-set!’ is unspecified.

```(let ((vec (vector 0 '(2 2 2 2) "Anna")))
(vector-set! vec 1 '("Sue" "Sue"))
vec)
⇒  #(0 ("Sue" "Sue") "Anna")

(vector-set! '#(0 1 2) 1 "doe")
⇒  error  ; constant vector
```
library procedure: vector->list vector
library procedure: list->vector list

Vector->list’ returns a newly allocated list of the objects contained in the elements of vector. ‘List->vector’ returns a newly created vector initialized to the elements of the list list.

```(vector->list '#(dah dah didah))
⇒  (dah dah didah)
(list->vector '(dididit dah))
⇒  #(dididit dah)
```
library procedure: vector-fill! vector fill

Stores fill in every element of vector. The value returned by ‘vector-fill!’ is unspecified.

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