Arrays and seqs in F#

Arrays index and mutate; seqs are lazy views over anything.

// Arrays are mutable and indexed; seqs are lazy views over anything.
let readings = [| 33.1; 32.7; 32.9; 31.8 |]
readings.[0] <- 33.4
printfn "First reading now %.1f" readings.[0]
printfn "Array average: %.2f" (Array.average readings)

// A seq computes on demand; the pipeline decides how much to pull.
let cooling =
    readings
    |> Seq.pairwise
    |> Seq.filter (fun (a, b) -> b < a)
    |> Seq.length
printfn "Cooling steps: %d" cooling
printfn "As a list: %A" (readings |> Array.toList |> List.truncate 2)

How it works

1. .[0] <- writes in place — arrays are the mutable collection.
2. Seq.pairwise walks neighbors without materializing.
3. Conversions like Array.toList cross between the families.

Keywords and builtins used here

• fun
• let

The run, in numbers

Lines

14

Characters to type

499

Tokens

104

Three-star pace

65 tpm

At the three-star pace of 65 tokens a minute, this run takes about 96 seconds.

Type this snippet

Step 3 of 3 in Collections, step 18 of 27 in Language basics.

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