F# Array ExamplesInstantiate arrays of strings and ints. Use ranges, accesses elements and stores values in arrays.
Arrays. In the park we find an array of animals: a cat, a dog, a bird. In F# we can store elements (like strings) in an array. This is a low-level, efficient type.
With list, a special F# type, we have a linked list. But an array stores its elements together in memory. It has no head or tail, but we can quickly loop over it.
This example creates an array from a range of numbers (enclosed in vertical bars). The array has 4 elements. The Length property returns 4.
First: The first element in an array is at index 0. We use a period before the index brackets.
Last: The last element in a nonempty array is equal to the length minus 1. We sometimes need to check for an empty array.
F# program that uses array
// Create an array of 4 ints.
let array = [|0 .. 3|]
printfn "%A" array
// Access the Length property.
let length = array.Length
printfn "Length = %A" length
// Access the first and last elements with indexes.
let first = array.
printfn "First = %A" first
let last = array.[array.Length - 1]
printfn "Last = %A" last
[|0; 1; 2; 3|]
Length = 4
First = 0
Last = 3
Arrays can be used in many ways. We call Array.create to construct an array of 4 elements, all with the string "empty." We then use a for-loop over each index in the array.
Assign: We assign elements in the array with an index. We must use the left-pointer operator.
Printfn: This call inserts a newline at the end. It prints all an array's elements. No looping is needed.Printfn
F# program that uses Array.create, loop
// Create an array of 4 strings with the string empty.
let sizes = Array.create 4 "empty"
// Assign all values to full.
for i = 0 to sizes.Length - 1 do
sizes.[i] <- "full"
// Assign first value to special string.
sizes. <- "start"
// Display result.
printfn "%A" sizes
[|"start"; "full"; "full"; "full"|]
All elements. Here we create a string array with 3 strings in it. We specify all elements in the initialization statement. No range syntax is needed.
F# program that specifies all elements
// Create an array with all elements specified.
let values = [|"cat"; "bird"; "fish"|]
// Loop over elements and display them.
for v in values do
printfn "%A" v
We can find elements in an array with Array.tryFind and tryFindIndex. These functions do not throw exceptions, so they are safer than find and findIndex.
TryFind: We pass a lambda that returns true if the value is equal to or greater than 9. We use IsSome on the optional int returned.
IsSome: This tests the optional int (similar to a Nullable in C#) for a valid internal value. We access that value.
TryFindIndex: Here we locate the index where an element is equal to 15. In our array, this is located at index 2 (the third element).
Get: We use Array.get with two arguments: the array and the index of the element we want to get. This returns the element value.
F# program that uses Array.tryFind
let values = [|5; 10; 15|]
// Find an element greater than or equal to 9.
let result = Array.tryFind (fun x -> x >= 9) values
// See if option has a value.
if result.IsSome then do
printfn "Greater or equal to 9: %A" result.Value
// Find index where value is 15.
let resultIndex = Array.tryFindIndex (fun x -> x = 15) values
// See if option has a value.
if resultIndex.IsSome then do
// Print value and get the element.
printfn "Index with value equal to 15: %A" resultIndex.Value
printfn "%A" (Array.get values resultIndex.Value)
Greater or equal to 9: 10
Index with value equal to 15: 2
With Array.get we get an element at an index from an array. We pass two arguments to Array.get: the array name, and the element index.
Tip: We can read F# function calls as sentences. Here "Array.get volumes 2" is like an English command.
F# program that uses Array.get, gets elements
let volumes = [|20 .. 30|]
// Get second element from the array (at index 1).
let elementTwo = Array.get volumes 1
printfn "Element two: %A" elementTwo
// Get third element from the array (at index 2).
printfn "Element three: %A" (Array.get volumes 2)
Element two: 21
Element three: 22
Array.append. We combine two arrays with Array.append. This function takes two arguments: the first array and the second. It returns those arrays combined together.
F# program that uses append
let array1 = [|10; 20; 30|]
let array2 = [|40; 50; 60|]
// Append the second array to the first.
let merged = Array.append array1 array2
// Print lengths of the arrays.
printfn "%d + %d = %d" array1.Length array2.Length merged.Length
// Print the merged array.
printfn "%A" merged
3 + 3 = 6
[|10; 20; 30; 40; 50; 60|]
F# has the ability to do solve puzzles and analyze complex data. Here we use allPairs. This takes 2 arrays, and returns all possible pairs of elements of the 2 arrays.
So: It takes each element from the first array, and creates a pair with each element from the second array.
Tip: If you need algorithm that handles all possible pairings from 2 arrays, use allPairs.
F# program that uses allPairs
let array1 = [|1; 2; 3|]
let array2 = [|1; 2; 3|]
// Use allPairs to get all possible pairs from 2 arrays.
let pairs = Array.allPairs array1 array2
printfn "%A" pairs
[|(1, 1); (1, 2); (1, 3); (2, 1); (2, 2); (2, 3); (3, 1); (3, 2); (3, 3)|]
An array does not have functions like map on it. Instead, we can use it as a sequence by calling Seq.ofArray. We can then use Seq.toArray to convert back to an array if needed.Convert: Seq.ofArray
A summary. Arrays are a fundamental data type. They are everywhere. Even in a functional, high-level language like F# they are critical. They store data in an efficient way.
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