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F# Sort List Examples: sortBy, Lambdas

Sort a list with the List.sort function. Use sortBy with a fun to customize the sort order.
Sort. A list has numbers: 10, 5, 7. They are not sorted. With a function call we can reorder them in ascending or descending order.
For more advanced sorts, we can provide a function to sortBy. This selects a key that is then sorted. Collections like arrays can be sorted with Seq.
First example. Here we introduce a string list containing the names of geometric shapes. We call List.sort with this list as the argument.

Result: The shapes are sorted in ascending order (as strings). A copy is sorted, so the original is untouched.

String
F# program that uses List.sort let shapes = ["triangle"; "square"; "ellipse"; "rectangle"] // Use List.sort to sort the string list. // ... A sorted copy is returned. let result = List.sort shapes // Print both lists. printfn "Unsorted: %A" shapes printfn " Sorted: %A" result Output Unsorted: ["triangle"; "square"; "ellipse"; "rectangle"] Sorted: ["ellipse"; "rectangle"; "square"; "triangle"]
List.sortBy. This function is more advanced. It lets us select a key to sort for each element. The lambda we pass to sortBy must return a value—this is sorted.

Result: SortBy returns a copy of the List that is sorted. As with sort, the original is not changed.

F# program that uses List.sortBy, sorts strings by length let values = ["aa"; "x"; "zzz"; "yy"; "eeee"] // Sort the string list by length in ascending (low to high) order. let result = List.sortBy (fun (x : string) -> x.Length) values // Print our results. List.iter(fun x -> printfn "%A" x) result Output "x" "aa" "yy" "zzz" "eeee"
Array.sort, sortInPlace. An array in F# is not the same as a list—an array is a low-level region of memory with elements. We must use special Array functions to sort.Arrays

Array.sort: This returns a copied array that is sorted. It does not modify (mutate) the original array's data.

Array.sortInPlace: This rearranges the elements in an existing array. No new memory region is allocated.

F# program that uses Array.sort, sortInPlace let offsets = [|10; 2; -2; 4; 40|] let copy = Array.sort offsets printfn "Array = %A" offsets printfn "Copy = %A" copy Array.sortInPlace offsets printfn "After sortInPlace = %A" offsets Output Array = [|10; 2; -2; 4; 40|] Copy = [|-2; 2; 4; 10; 40|] After sortInPlace = [|-2; 2; 4; 10; 40|]
Seq.sort, pipeline. We can sort collections with Seq.sort. With Seq.ofList, we treat a list as a sequence. Then we can use functions like where and sort in a pipeline.

Here: We use "where" to only keep elements containing a lowercase "A." Then we sort them alphabetically, and convert back to a list.

F# program that uses Seq.sort let animals = ["cat"; "bird"; "zebra"; "leopard"; "shark"] // Act on list as a sequence. // ... Use where to filter our list. // Use Seq.sort to sort the sequence. // Use Seq.toList to convert back to a list. let sortResult = Seq.ofList animals |> Seq.where (fun x-> x.Contains "a") |> Seq.sort |> Seq.toList // Print all our results. List.iter (fun (x : string) -> printfn "%A" x) sortResult Output "cat" "leopard" "shark" "zebra"
With this language, we have features that are well-suited to advanced sorting. We have lambdas and immutable collections (like lists). We invoke built-in sort methods.
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