F# for and while loops
last modified July 17, 2023
In this article, we show how to work with for and while loops in F#.
Loops are used to repeatedly execute a block of code. There are two kinds of loops: for and while.
F# for in loop
With for in loop, we go through a sequnce of values one by one.
let vals = seq { 1..5 }
for e in vals do
printfn "%d" e
printfn "--------------------"
let len = Seq.length (vals) - 1
for idx in 0..len do
printfn "%d" (Seq.item idx vals)
In the program, we loop over a sequence of integers.
let vals = seq { 1..5 }
With seq, we define a sequence of five integers.
for e in vals do
printfn "%d" e
We loop over the sequence and print each number.
let len = Seq.length (vals) - 1
for idx in 0..len do
printfn "%d" (Seq.item idx vals)
In this code, we loop over a range of numbers. The values of the range are used as indexes to the sequence.
λ dotnet fsi main.fsx 1 2 3 4 5 -------------------- 1 2 3 4 5
F# for to/downto
With to and downto keywords, we can iterate over
a range of values.
for e = 1 to 5 do
printfn "%d" e
for e = 5 downto 1 do
printfn "%d" e
The program loops over a range of 1..5 and 5..1 values.
λ dotnet fsi main.fsx 1 2 3 4 5 5 4 3 2 1
F# execute n times
A common task in programming is to execute something n times. We can do it with a for loop and a range.
for _ in 0..5 do
printfn "%s" "falcon"
In the program, we print the word "falcon" six times. Since we do not need
the values of the range, we use the _ operator.
λ dotnet fsi main.fsx falcon falcon falcon falcon falcon falcon
F# for in loop with range
Range operators can be used in a for loop.
for e in 1..2..10 do
printfn "%d" e
for e in 10..-2..0 do
printfn "%d" e
We can create ascending and descending ranges of values with different steps.
λ dotnet fsi main.fsx 1 3 5 7 9 10 8 6 4 2 0
F# nested for loops
For loops can be nested.
for i in [1; 2; 3; 4; 5; 6; 7; 6; 5; 4; 3; 2; 1] do
for _ in 1..i do
printf "*"
printf "\n"
With two for loops, we create a rectangle using * characters. For each cycle of an outer loop, the inner loop is executed n times.
λ dotnet fsi main.fsx * ** *** **** ***** ****** ******* ****** ***** **** *** ** *
F# for loop with functions
For loops are in fact expressions.
open System
let rand1 () =
Random(DateTime.Now.Millisecond).NextInt64(1, 10)
let rand2 () =
Random(DateTime.Now.Millisecond).NextInt64(10, 20)
for e in rand1 () .. rand2 () do
printfn "%d" e
printfn "--------------------------------"
for e = int (rand1 ()) to int (rand2 ()) do
printfn "%d" e
In the example, we use the computed random values in a for range.
let rand1 () =
Random(DateTime.Now.Millisecond).NextInt64(1, 10)
With random, we generate a random number between 1 and 10.
for e in rand1 () .. rand2 () do
printfn "%d" e
The returned values of the two functions are used to generate a random range of values.
λ dotnet fsi main.fsx 2 3 4 5 6 7 8 9 10 11 -------------------------------- 6 7 8 9 10 11 12 13 14 15
F# while loop
A while loop is a control flow statement that allows code to be executed repeatedly based on a given boolean condition.
let vals = [ 1; 2; 3; 4; 5 ]
let mutable i = 0
while i < vals.Length do
printfn "%d" val[i]
i <- i + 1
In this while loop, we loop over the list elements in a classic imperative way.
F# funcional iteration
The for and while loops are used to create imperative loops. Functional languages often prefer to iterate over sequences with functions.
let vals = [ 1; 2; 3; 4; 5 ] List.iter (printfn "%d") vals
For instance, F# has List.iter to go over the elements of a list.
In this article we have worked with for and while loops in F#.