Vectors

## Vectors

Goals: This laboratory exercise reviews basic elements of vectors within Scheme.

A vector is a collection of elements which are indexed by integers from 0 through n-1, where n is the length of the vector. Furthermore, in Scheme, vectors have the property that any element within a vector may be accessed directly. As an example, consider a list of 10 names and a vector of the same names:

In order to find the seventh name (MacKay) in the list, we must start at the first element and move element-by-element to get the desired item. Within the vector, however, we simply specify the appropriate index (since indexing starts at 0, the seventh item is in position 6).

Representation of Vectors in Scheme: Just as a string is a sequence of characters enclosed in double quotes ("), a vector is a sequence of elements beginning with #( and ending with ). Thus, the following are valid vectors with four components:
```
#(2 3/4 -7 3.14179)
#(3 #\a "fred" henry)
```
Chez Scheme also allows a notation where the number of components in a vector is placed between the opening number sign # and the left parenthesis (, as shown below:
```
#4(2 3/4 -7 3.14179)
#4(3 #\a "fred" henry)
```
The vector? predicate tests whether or not an element is a vector. Some examples are:
```
(vector? #(2 3/4 -7 3.14179)) ==> #t
(vector? #4(3 #\a "fred" henry)) ==> #t
(vector? "fred") ==> #f
```
Constructing Vectors: In addition to explicitly writing out a vector (e.g., the vector literals above), vectors can be constructed with either of two constructor procedures, vector and make-vector.

vector makes a vector out of the elements that follow, as shown in these examples:

```
(vector 2 3/4 -7 3.14179)
(vector 3 #\a "fred" 'henry)
```
make-vector constructs a vector of a specified length. If a second argument is given, then each component of the new vector contains a copy of the value of the second argument. For example, (make-vector 5) produces a vector with five components, and (make-vector 5 7) generates a vector of 5 elements, each of which contains the number 7. Thus, (make-vector 5 7) produces the vector #5(7 7 7 7 7) .

Accessor Procedures: Once a vector is produced, the vector-length procedure returns its length (its number of components), and vector-ref returns the element in a given position. Some examples follow.

```
(vector-length #(3 #\a "fred" henry)) ==> 4
(vector-length #()) ==> 0
(vector-ref #(3 #\a "fred" henry) 0) ==> 3
(vector-ref #(3 #\a "fred" henry) 2) ==> "fred"
(vector-ref #(3 #\a "fred" henry) 6) ==> error:  invalid index
```
Remember that components of a vector are indexed, beginning with index 0. Thus, 3 is element 0 in the vector #(3 #\a "fred" henry)

Conversion Procedures The procedures vector->list and list->vector allow us to convert vectors to lists and conversely. Some examples are:

```
(vector->list #(3 #\a "fred" henry)) ==> (3 #\a "fred" henry)
(list->vector '(3 #\a "fred" henry)) ==> #4(3 #\a "fred" henry)
```
These operations are summarized in the following table:

Procedure Sample Call Result of Example Comment
vector?(vector? #('a 'b 'c)) #t Return true (#t) if element is a vector
vector(vector 'a 'b 'c) #('a 'b 'c) Make a vector of the parameters
make-vector(make-vector 4) #(__ __ __ __) Make an uninitialized vector of the given length
make-vector(make-vector 4 5) #(5 5 5 5) Make vector with length, with all components specified
vector-length(vector-length #('a 'b 'c)) 3 Return length of given vector
vector-ref(vector-ref #('a 'b 'c) 1) 'b Return given component of vector
vector->list(vector->list #('a 'b 'c)) ('a 'b 'c) Converts vector to corresponding list
list->vector(list->vector '(a b c)) #3(a b c) Converts list to corresponding vector
Now consider the procedure view as follows:

```
(define view
(lambda (vec)
;Pre-condition:  vec is a vector
;Post-condition:  vec is printed at the keyboard
(let ((highest-index (sub1 (vector-length vec))))
(display "#(")
(let loop ((i 0))
(display (vector-ref vec i))
(if (< i highest-index)
(begin
(display " ")

(display ")")))

```
1. Run view on several vector examples (such as those appearing earlier in this lab) to check what this procedure does.

2. Write a paragraph explaining how view works.

3. As already noted, Chez Scheme prints a number after the initial # symbol in a symbol, indicating how many elements are present in a vector. Modify view, so that it also prints this length.

4. Write a procedure vector-find that searches a vector for a given item. If the item is found, vector-find should return the location of the first occurrence of the item. If the vector is not present, vector-find should return -1.

```(vector-find 4 '#(6 2 4 10 -18)) ===> 2
(vector-find 5 '#(6 2 4 10 -18)) ===> -1
(vector-find 'a '#(a b)) ===> 0
(vector-find 'a '#()) ===> -1
```
5. Define a Scheme procedure named `vector-of` that does for vectors what the `list-of` procedure (from the lab on procedure abstraction) does for lists: Given any predicate, `pred?`, `vector-of` should return a predicate that takes a vector as argument and determines whether each of its elements satisfies `pred?`.

```((vector-of even?) '#(6 2 4 10 -18)) ===> #t
((vector-of positive?) '#(6 2 4 10 -18)) ===> #f
((vector-of symbol?) '#(a b)) ===> #t
((vector-of real?) '#()) ===> #t
```

Modifying Vectors: In the lab on sorting and mutation, Scheme's procedure set! was introduced as a means to change the value of a variable. Similarly, procedure vector-set! changes the value stored in a designated location within a vector. The form for vector-set! is:

```
(vector-set! vect index new-value)
```
That is, within vector vect, the element at the given index is changed to the specified new-value.

As the following examples indicate, procedure vector-set! itself does not return a new value -- rather it changes parameter vect.

1. Type in the following sequence, and explain what is returned at each step:
```
(define x #(3 #\a "fred" 0.31894 12/5))
x
(vector-set! x 3 "susan")
x
(vector-set! x 1 3.141592)
x
(define f
(lambda (y)
(vector-set! y 4 15)))
(f x)
x
(vector-set! #(1 2 3 4 5 6) 3 10)
(define y #(1 2 3 4 5 6))
(vector-set! y 3 10)
y
```
Again, it is important to emphasize that vector-set! need not return a useful value. (Standard Scheme does not specify what is value returned.) Instead, the vector parameter to vector-set! changes. Further, this change even occurs when vector-set! is applied within another function -- x changes in the call (f x), since f in turn calls vector-set!.

Similarly, procedure vector-fill! changes all components of a given vector to a specified value, as illustrated in the following sequence:

```
(define x #(3 #\a "fred" 0.31894 12/5))
x
(vector-fill! x "henry")
x
(vector-set! x 3 "terry")
x
```
In this example, note that Chez Scheme uses a shorthand notation when printing a vector, all of whose components are identical. In particular, Chez Scheme prints
```
#5("henry")
```
to represent a vector of five components, with each component being the string "henry".

If this notation #5("henry") seems confusing, Chez Scheme can provide a more conventional output of vectors. First, type

```
(print-vector-length #f)
```
In subsequent lines, vectors will be printed showing each component.
1. After using the print-vector-length procedure as shown, try the above sequence (with vector-fill!) to see this alternative output.

To restore the previous format for printing vectors, type
```
(print-vector-length #t)
```
Convention: In writing Scheme procedures, it is common to use an exclamation point (!) at the end of a procedure name, when that procedure changes a parameter.

1. Review the examples for vector-fill!, and write a couple of sentences explaining the result of each Scheme command.

2. Define a Scheme procedure named `vector-iota` that takes a natural number `n` as argument and returns a vector of length `n` containing all of the natural numbers less than `n`, in ascending order.
```(vector-iota 9) ===> #9(0 1 2 3 4 5 6 7 8)
(vector-iota 2) ===> #2(0 1)
(vector-iota 0) ===> #0()
```

This document is available on the World Wide Web as

```http://www.math.grin.edu/~walker/courses/153.sp00/lab-vectors.html
```

created April 3, 1997
last revised January 11, 2000