F# create random string of letters and numbers [closed] - f#

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I have an issue with the below code. I am attempting to create a random string of letters and numbers. The below code works, however only creates 1 random string per session, so if I call the randomNum again it still hold the first random string created.
let randomNum(len : int) =
let rand = new System.Random()
let mutable str = ""
let chars = "ABCDEFGHIJKLMNOPQRSTUVWUXYZ0123456789"
let mutable count = 0
for i in 1 .. len do
count <- (rand.Next() % 36)
str <- String.concat "" [str; chars.[count].ToString()]
str
Does anyone know of an easy way to create a random string each time the randomNum is called.
Thanks

Answer 1

You can do it using a more idiomatic syntax:
let randomStr =
let chars = "ABCDEFGHIJKLMNOPQRSTUVWUXYZ0123456789"
let charsLen = chars.Length
let random = System.Random()
fun len ->
let randomChars = [|for i in 0..len -> chars.[random.Next(charsLen)]|]
new System.String(randomChars)
We keep the chars array, it's length and the Random generator in a cloture and we return a function which takes the desired length to build the string from.
Calling the function:
let randomString10 = randomStr(10)

Answer 2

Here's a version which doesn't use mutability:
module String =
let ofChars arr =
arr |> Array.fold (fun acc elem -> sprintf "%s%c" acc elem) ""
let randomString len =
let rand = new System.Random()
let chars = Array.append [|'A'..'Z'|] [|'0'..'9'|]
|> String.ofChars
Array.init len (fun _ -> chars.[rand.Next(chars.Length)])
|> String.ofChars
(Performance could be improved further by not using sprintf in ofChars.)

Answer 3

Actually it's the
new System.Random()
I have just moved it to the top of my code
let rand = new System.Random()
let randomNum(len : int) =
let mutable str = ""
let chars = "ABCDEFGHIJKLMNOPQRSTUVWUXYZ0123456789"
let mutable count = 0
for i in 1 .. len do
count <- (rand.Next() % 36)
str <- String.concat "" [str; chars.[count].ToString()]
str

Answer 4

Figured it out. I was calling the randomNum function from a different page and storing it as a variable
let randomtitlevertical = Functions.randomNum(10)
let randomtitlevaluevertical = randomtitlevertical
This only ran the randomNum function once per seassion. If I have the following instead
let randomtitlevertical = Functions.randomNum(10)
Then it will run the randomNum fucntion once per project.