F# Looping Sample Code

csharp_coding_challenge.cs

int sum = 0;
for (int number = 1; number < 21; number++)
{
  if (number % 3 == 0)
  {
    sum = sum + number;
  }
}
Console.WriteLine($"The sum is {sum}");

csharp_coding_challenge_factored.cs

int AddNewConditionally(int last, int number)
{
    if(number % 3 == 0)
        return last + number;
    else
        return last;
}

int sum = 0;
for (int number = 1; number < 21; number++)
{
    sum = AddNewConditionally(sum, number);
}
Console.WriteLine($"The sum is {sum}");

csharp_for_to_foreach.cs

foreach (var number in Enumerable.Range(1,20))
{
    sum = AddNewConditionally(sum, number);
}

fsharp_fold.fs

let addNewConditionally last number =
    if number % 3 = 0 then last + number else last

// Start with "0"
let startingValue = 0

let valuesToProcess = { 1 .. 20 }

// Fold instead of looping
let sum = Seq.fold addNewConditionally startingValue valuesToProcess

printfn "The sum is %d" sum

fsharp_fold_final.fs

let addNewConditionally last number =
    if number % 3 = 0 then last + number else last

let sum = { 1 .. 20 } |> Seq.fold addNewConditionally 0

printfn "The sum is %d" sum

fsharp_fold_v2.fs

let buildListConditionally l number =
    if number % 3 = 0 then (string number)::l else l

let values = { 1 .. 20 } |> Seq.fold buildListConditionally []

printfn "The values are %A" values
printfn "The first value is \"%s\"" (List.head values)

fsharp_loop_with_steps.fs

let addNewConditionally last number =
    if number % 3 = 0 then last + number else last

// Start with "0"
let startingValue = 0

let valuesToProcess = { 1 .. 20 }

// Make a value we can increment as we loop
let mutable runningValue = startingValue
for number in valuesToProcess do
    // Adjust this each step
    runningValue <- addNewConditionally runningValue number

// Get the result at the end
let sum = runningValue

printfn "The sum is %d" sum

fsharp_ported_with_loop.fs

let addNewConditionally last number =
    if number % 3 = 0 then last + number else last

let mutable sum = 0
for number in { 1..20 } do
    sum <- addNewConditionally sum number

printfn "The sum is %d" sum

Seq.fold

Seq.fold : ('State -> 'T -> 'State) -> 'State -> seq<'T> -> 'State