For. In F# a loop continues forward—we implement loops with for and while. Loops are powerful, but functional-based designs are usually emphasized in this language.
In this language, declarative constructs are often preferred. But for getting things done, loops are often a good choice—and they can be rewritten later if needed.
For example. This program shows 2 for-loops, and both of the constructs loop over the same values. In programs, either form can be used.
Start The first for-loop uses an iteration variable called "i" and goes from 0 through 3 inclusive.
Next This loop uses range syntax. The two periods are part of an inclusive range of 0, 1, 2 and 3.
// Loop from 0 through 3 inclusive.// ... The variable i is incremented by 1 each time.
for i = 0 to 3 do
printfn "For, to: %A" i
// This is the same loop but with a range.
for i in 0 .. 3 do
printfn "For, in: %A" iFor, to: 0
For, to: 1
For, to: 2
For, to: 3
For, in: 0
For, in: 1
For, in: 2
For, in: 3
For-in, list. A for-in loop enumerates over each element in a collection (like a list). Here we create a list of ints called "codes."
Note The for-in loop accesses all elements in the list. It provides no indexes, so the syntax is simpler.
let codes = [3; 12; 34; 3]
// Loop over ints from list.
for code in codes do
// If code is 3 then write a special word.
if (code = 3) then
printfn "Three"// Write value of int from list.
printfn "%A" codeThree
3
12
34
Three
3
Downto. With this keyword we create a decrementing loop. After each iteration, the iteration variable (in this program "i") is reduced by 1.
// Specify a decrement for-loop with downto.
for i = 10 downto 6 do
printfn "%A" i10
9
8
7
6
While. The most versatile loop in F# is the while-loop. Here we can increment, decrement, or loop forever. We must be careful not to make mistakes.
Info The iteration variable used as a counter in a while-loop should be marked as mutable so we can change it.
Tip We decrement the variable "x" by assigning it the current value minus 1.
// Use an iteration variable that starts at 4.// ... The variable must be mutable.
let mutable x = 4
// Continue looping while x is positive.
while x >= 0 do
// Write value.
printfn "%A" x
// Decrement x.
x <- x - 14
3
2
1
0
A review. In functional programs, recursion and method-based iteration is often preferred. But loops are still wonderful. In F# we can use iteration and increment variables.
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This page was last updated on Dec 22, 2023 (edit link).