Question

F# function

Problem:

given a list of items e.g.:

["5";"10";"2";"53";"4"]

and a Search Index, I require a function such that it compares the current given index against its neighbor, returning the largest index

Example:

• Given Index 1 will return Index value 2 (because 10 is greater than 5).
• Given Index 4 will return Index 4 (because 53 is greater than 4)

Currently this is my function. It does not compile:

let GetMaxNode (x:Array) Idx = if x.[Idx] > x.[Idx+1] then  Idx else If  x.[Idx] < x.[Idx+1] then  Idx+1

The errors I’m getting for all the x’ are:

The field, constructor or member 'Item' is not defined (FS0039)

And also the second If:

The value or constructor 'If' is not defined (FS0039)

I suspect I’m still thinking in a procedural way, I was thinking about using pattern matching, however I was not confident enough with the syntax to try it.

Please can you also explain the answer as well, as I’m trying to learn F#, just the solution will not help me much.

Answer 1

Here’s some code based on yours:

let GetMaxNode (x:_[]) idx = 
    if x.[idx] > x.[idx+1] then  
        idx 
    elif x.[idx] < x.[idx+1] then  
        idx+1 
    else
        idx // same, return this one

The main changes are

• to declare an array type, say <typename> []. In this case, we don’t care about the type, so I use _ as a “don’t care, please go infer the right thing for me” type variable.
• “else if” is spelled elif in F#
• need an else case for if equal