# Vectors

Summary: In this laboratory, you will explore various aspects of the Vector data type that Scheme provides as an alternative to lists.

Contents:

## Exercises

### Exercise 0: Preparation

b. Start DrScheme.

c. Tell DrScheme not to print the lengths of vectors by entering `(print-vector-length #f)`.

### Exercise 1: Create a Simple Vector

a. In DrScheme's interaction window, type in a vector literal that denotes a vector containing just the two elements 3.14159 and 2.71828. How does DrScheme display the value of this vector?

b. Create a vector that contains the same two values by using the `vector` procedure.

c. Create a vector that contains the same two values by using the `make-vector` and `vector-set!` procedures.

### Exercise 2: Modifying Lists and Vectors

Consider the following code.

```> (define aardvark (list 1 2 3 4))
> (define baboon aardvark)
> (define aardvark (cons 5 (cdr aardvark)))
> (define chinchilla (vector 1 2 3 4))
> (define dingo chinchilla)
> (vector-set! chinchilla 0 5)
```

a. What do you expect the output of the following commands to be?

```> (list-ref aardvark 0)
_____
> (list-ref baboon 0)
_____
```

d. What do you expect the output of the following commands to be?

```> (vector-ref chinchilla 0)
_____
> (vector-ref dingo 0)
_____
```

### Exercise 3: Specifying Vector Length

a. Tell DrScheme to print the length of vectors by entering `(print-vector-length #t)`.

b. Enter each of the following vector expressions in DrScheme; consider the result (perhaps by examining individual elements with `vector-ref`); and indicate what vector has been created.

• `#4(0)`
• `#4(1)`
• `#4(1 2)`
• `#2(1 2 3 4)`
• `(make-vector 4 0)`

c. Tell DrScheme not to print the lengths of vectors and reenter each expression. Do your results differ? What do the differences suggest?

### Exercise 4: Summing Vectors

Write a procedure, `(vector-sum numbers)`, which takes one argument, a vector of numbers, and returns the sum of the elements of that vector.

You can use `vector->list` from the reading as a pattern for `vector-sum` -- only a few judicious changes are needed. However, you should not use `vector->list` as a helper.

### Exercise 5: Filling Vectors

In the reading on vectors, we saw that it was possible to implement `list->vector` and `vector->list` by using more primitive operations (particularly `vector-set!` and `vector-length`).

Write your own version of `vector-fill!`. Remember that `vector-fill!` takes two parameters, a vector and a value, and puts that value in every position of the vector.

### Exercise 6: Rotating Vectors

Write a procedure, `(rotate! vec)` that rotates the elements in vec. That is, `rotate!` puts the initial element of vec at the end, the element at position 1 in position 0, the element at position 2 in position 1, and so on and so forth.

## If You Have Extra Time

### Extra 1: Reversing Vectors

a. Write a procedure, `(reverse-vector vec)`, that creates a new vector whose elements appear in the reverse order of the elements in vec.

b. Write a procedure, `(reverse-vector! vec)`, that reverses vec in place. That is, instead of producing a new vector, it rearranges the elements within vec.

### Extra 2: Rotating Vectors, Revisited

Write a procedure, `(rotate! vec amt)`, that rotates the values in vec by amt positions. That is, the first amt values in vec move to the end, the value in position amt moves to position 0, the value in position amt+1 moves to position 1, and so on and so forth.

## Notes

### Notes on Exercise 5: Filling Vectors

Just as in the case of `list->vector`, you will probably want to define a helper procedure that fills only part of the vector. Your termination condition will certainly be different and should probably involve the length of the vector.

## History

Disclaimer: I usually create these pages on the fly, which means that I rarely proofread them and they may contain bad grammar and incorrect details. It also means that I tend to update them regularly (see the history for more details). Feel free to contact me with any suggestions for changes.

This document was generated by Siteweaver on Tue Dec 9 13:59:24 2003.
The source to the document was last modified on Mon Oct 27 11:12:19 2003.
This document may be found at `http://www.cs.grinnell.edu/~rebelsky/Courses/CS151/2003F/Labs/vectors.html`.

Samuel A. Rebelsky, rebelsky@grinnell.edu