Section9.47

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## 9.47 List

A list is a finite collection of elements in a specified order that can contain duplicates. A list is a convenient structure to work with because it is easy to add or remove elements and the length need not be constant. There are many different kinds of lists in Axiom, but the default types (and those used most often) are created by the List constructor. For example, there are objects of type List Integer, List Float and List Polynomial Fraction Integer. Indeed, you can even have List List List Boolean (that is, lists of lists of lists of Boolean values). You can have lists of any type of Axiom object.

### 9.47.1 Creating Lists

The easiest way to create a list with, for example, the elements 2, 4, 5, 6 is to enclose the elements with square brackets and separate the elements with commas.

The spaces after the commas are optional, but they do improve the readability.

[2, 4, 5, 6]

[2, 4, 5, 6]

Type: List PositiveInteger

To create a list with the single element 1, you can use either [1] or the operation listlistList.

[1]

[1]

Type: List PositiveInteger
list(1)

[1]

Type: List PositiveInteger

Once created, two lists k and m can be concatenated by issuing append(k,m). appendappendList does not physically join the lists, but rather produces a new list with the elements coming from the two arguments.

append([1,2,3],[5,6,7])

[1, 2, 3, 5, 6, 7]

Type: List PositiveInteger

Use consconsList to append an element onto the front of a list.

cons(10,[9,8,7])

[10, 9, 8, 7]

Type: List PositiveInteger

### 9.47.2 Accessing List Elements

To determine whether a list has any elements, use the operation empty? empty? List.

empty? [x+1]

false

Type: Boolean

Alternatively, equality with the list constant nilnilList can be tested.

([] = nil)@Boolean

true

Type: Boolean

We'll use this in some of the following examples.

k := [4,3,7,3,8,5,9,2]

[4, 3, 7, 3, 8, 5, 9, 2]

Type: List PositiveInteger

Each of the next four expressions extracts the firstfirstList element of k.

first k

4

Type: PositiveInteger
k.first

4

Type: PositiveInteger
k.1

4

Type: PositiveInteger
k(1)

4

Type: PositiveInteger

The last two forms generalize to k.i and k(i), respectively, where 1≤i≤n and n equals the length of k.

This length is calculated by #.

n := #k

8

Type: PositiveInteger

Performing an operation such as k.i is sometimes referred to as indexing into k or elting into k. The latter phrase comes about because the name of the operation that extracts elements is called elteltList. That is, k.3 is just alternative syntax for elt(k,3). It is important to remember that list indices begin with 1. If we issue k := [1,3,2,9,5] then k.4 returns 9. It is an error to use an index that is not in the range from 1 to the length of the list.

The last element of a list is extracted by any of the following three expressions.

last k

2

Type: PositiveInteger
k.last

2

Type: PositiveInteger

This form computes the index of the last element and then extracts the element from the list.

k.( #k)

2

Type: PositiveInteger

### 9.47.3 Changing List Elements

We'll use this in some of the following examples.

k := [4,3,7,3,8,5,9,2]

[4, 3, 7, 3, 8, 5, 9, 2]

Type: List PositiveInteger

List elements are reset by using the k.i form on the left-hand side of an assignment. This expression resets the first element of k to 999.

k.1 := 999

999

Type: PositiveInteger

As with indexing into a list, it is an error to use an index that is not within the proper bounds. Here you see that k was modified.

k

[999, 3, 7, 3, 8, 5, 9, 2]

Type: List PositiveInteger

The operation that performs the assignment of an element to a particular position in a list is called seteltseteltList. This operation is destructive in that it changes the list. In the above example, the assignment returned the value 999 and k was modified. For this reason, lists are called mutable objects: it is possible to change part of a list (mutate it) rather than always returning a new list reflecting the intended modifications.

Moreover, since lists can share structure, changes to one list can sometimes affect others.

k := [1,2]

[1, 2]

Type: List PositiveInteger
m := cons(0,k)

[0, 1, 2]

Type: List Integer

Change the second element of m.

m.2 := 99

99

Type: PositiveInteger

See, m was altered.

m

[0, 99, 2]

Type: List Integer

But what about k? It changed too!

k

[99, 2]

Type: List PositiveInteger

### 9.47.4 Other Functions

An operation that is used frequently in list processing is that which returns all elements in a list after the first element.

k := [1,2,3]

[1, 2, 3]

Type: List PositiveInteger

Use the restrestList operation to do this.

rest k

[2, 3]

Type: List PositiveInteger

To remove duplicate elements in a list k, use removeDuplicatesremoveDuplicatesList.

removeDuplicates [4,3,4,3,5,3,4]

[4, 3, 5]

Type: List PositiveInteger

To get a list with elements in the order opposite to those in a list k, use reversereverseList.

reverse [1,2,3,4,5,6]

[6, 5, 4, 3, 2, 1]

Type: List PositiveInteger

To test whether an element is in a list, use member? member? List: member?(a,k) returns true or false depending on whether a is in k or not.

member?(1/2,[3/4,5/6,1/2])

true

Type: Boolean
member?(1/12,[3/4,5/6,1/2])

false

Type: Boolean

As an exercise, the reader should determine how to get a list containing all but the last of the elements in a given non-empty list k.reverse(rest(reverse(k))) works.

### 9.47.5 Dot, Dot

Certain lists are used so often that Axiom provides an easy way of constructing them. If n and m are integers, then expand [n..m] creates a list containing n, n+1, ... m. If n > m then the list is empty. It is actually permissible to leave off the m in the dot-dot construction (see below).

The dot-dot notation can be used more than once in a list construction and with specific elements being given. Items separated by dots are called segments.

[1..3,10,20..23]

[1..3, 10..10, 20..23]

Type: List Segment PositiveInteger

Segments can be expanded into the range of items between the endpoints by using expandexpandSegment.

expand [1..3,10,20..23]

[1, 2, 3, 10, 20, 21, 22, 23]

Type: List Integer

What happens if we leave off a number on the right-hand side of ..?

expand [1..]

[1, 2, 3, 4, 5, 6, 7, 8, 9, 10,…]

Type: Stream Integer

What is created in this case is a Stream which is a generalization of a list. See StreamXmpPage for more information.

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