Question

let tag:String = "+1"
for str in readFile do
    let feature = str.Split [|' '; '\t'|]
    if feature.[8] = "0" then
        tag = "-1"
    else 
        tag = "+1"

    printf "\n%s %s\n" feature.[8] tag 

The code changes tries to change the value of tag to “-1” if feature.[8] is 0, or “+1” otherwise. However the tag variable value remains “+1” throughout, irrespective of whatever the value feature.[8] is.

How does one deal with simple value changes based on conditional statement in F#?

Answer 1

@John Palmer has your answer, but I’ll add a bit to it…

Note that the reason why your code compiles but does not work as you expect is because the = operator used in the context of tag = "-1" and tag = "+1" is the equality operator. So those expressions are valid but return a bool value. However, you should be receiving the following warning:

This expression should have type ‘unit’, but has type ‘bool’. Use
‘ignore’ to discard the result of the expression, or ‘let’ to bind the
result to a name.

It would serve you well to heed that warning in your F# coding adventures.

Also note that you can write your algorithm in a purely functional way (without mutable variables) using Seq.fold (among other alternative functional approaches):

let tag =
    readFile 
    |> Seq.fold 
        //we use the wild card match _ here because don't need the 
        //tag state from the previous call 
        (fun _ (str:string) ->
            let feature = str.Split [|' '; '\t'|]
            //return "-1" or "+1" from the if / then expression,
            //which will become the state value in the next call
            //to this function (though we don't use it)
            if feature.[8] = "0" then
                "-1"
            else 
                "+1")
        ("+1") //the initial value of your "tag"