F#10 : Lists

Any serious programming you do in any language will always involve lists. As such you will be pleased to know that F# has very very good support for Lists, by way of its List module. A list in F# is an ordered, immutable series of elements of the same type.

Creating Lists

In F# there are several ways in which you can create Lists, shown below are a few examples.

Create an empty list
Create a simple list containing 1 to 10
Create a simple list creating odd numbers between 1 and 10, which is achieved using a step operator
Using a for loop to create a list

let prettyPrint desc list =
    printfn desc
    printfn "%A" list
        

//empty list
let listEmpty = []
//simple list
let list1 = [1 .. 10 ]
//simple list, with step value
let list2 = [1 ..2..10 ]
//using for loops to create lists
let list3 = [for i in 1 .. 10 -> i*2 ]



prettyPrint "let listEmpty = []" listEmpty
prettyPrint "let list1 = [ 1 .. 10 ]" list1
prettyPrint "let list2 = [ 1 .. 2..10 ]" list2
prettyPrint "[ for i in 1 .. 10 -> i*2 ]" list3

Which when run will give the following results:

List Comprehensions

The last example in the one above showed how to use a for loop to create a list, which is very cool, but there is something even more cool and powerful within the F# toolbox, which are “List Comprehensions”.

“List Comprehensions” are a powerful technique that allow you to create lists by using pretty much and of the standard F#, which includes functions/loops/conditions etc.

Lets continue to have a look at example of how we might construct lists using list comprehensions.

let is2 x = match x with
    | 2 -> "YES"
    | _ -> "NO"



//yield directly
let list1 = [
    yield 1;
    yield 2;
    yield 3;
]

//yield numbers between 1 and 20 where we use the
//Math.Pow function to return a new number
let list2 = [for i in 1.0 .. 20.0 do
                yield Math.Pow(i,2.0)
            ]
    
//yield only numbers between 1 and 20 that 
//can be divided by 5
let list3 = [
                for i in 1 .. 20 do
                    if i % 5 = 0 then
                        yield i
            ]


//yields YES/NO strings depending on
//whether source int = 2 or not
let list4 = [for i in 1 .. 5 ->
                is2 i
            ]

Which prints this when run

Some Useful List Operators

Cons Operator

We can use the cons operator “::” to append values to an existing list, so suppose we had this list

let list1 = [1;2;3;4]
let list2 = 42 :: list1

Which gives this result

Concat Operator

Another quite useful operator is the “@” operator, which allows you to concatenate lists that have the same type. So for example if we had this

let list1 = [1;2;3;4]
let list2 = [5;6;7;8]
let list3 = list1 @ list2

We would get the following results

The List Module

I don’t think I am overplaying things when I say that the List module is a pivotal module in the F# landscape. I fact the MSDN documentation for the List module is extremely good when compared to other areas in F#. As such I don’t think I will be able to add much value to some of the examples found on MSDN, but I will include a few here for your convenience, but for more information you should examine MSDN : http://msdn.microsoft.com/en-us/library/dd233224.aspx

Properties

A list has a number of useful properties which are quite useful, which are shown in the table below:

Property	Type	Description
Head	‘T	The first element
Empty	‘T list	A static property that returns an empty list of the appropriate type
IsEmpty	bool	true if the list has no elements
Item	‘T	The element at the specified index (zero-based)
Length	int	The number of elements
Tail	‘T list	The list without the first element

Where we can see example usage of these as follows:

let list1 = [ 1; 2; 3 ]

// Properties
printfn "list1.IsEmpty is %b" (list1.IsEmpty)
printfn "list1.Length is %d" (list1.Length)
printfn "list1.Head is %d" (list1.Head)
printfn "list1.Tail.Head is %d" (list1.Tail.Head)
printfn "list1.Tail.Tail.Head is %d" (list1.Tail.Tail.Head)
printfn "list1.Item(1) is %d" (list1.Item(1))

Which when run, will give the following results:

There are also many (too many for one blog, but we will go through a few, for the rest MSDN is actually very on) functions that are available in the F# List module. In fact the MSDN page for the List module is excellent, and has an example for each list module function, as such I would urge you to peruse MSDN for a look at what you can do.

The MSDN page is here : http://msdn.microsoft.com/en-us/library/ee353738.aspx

We will however look at a few examples here, for fun like (note I have taken most of these examples straight from MSDN)

Filter

Returns a new collection containing only the elements of the collection for which the given predicate returns true.Here is a trivial example that only picks even numbers from a list to produce a new list

let evenOnlyList = List.filter (fun x -> x % 2 = 0) [1; 2; 3; 4; 5; 6]