Convert fun match printfn ROT13 String IndexOf String Replace String Split String Substring

Strings

In the F# language we use .NET string methods like IndexOf and ToLower. We call these methods within functional constructs.

With String.map we can apply a mapping to all characters in a string. A lambda expression is used. We can chain string manipulations together in a pipeline.

First example

This example adds three string functions. In uppercase() it calls ToUpper. In addGreeting and addComma it adds some surrounding text to a string.

Info In the "let result" statement we pipeline the three methods together with a name argument (with value "visitor").

Result The 3 functions handle the string in order and the resulting text has been processed as expected.

// Three string manipulation functions.
let uppercase (x : string) = x.ToUpper()
let addGreeting (x : string) = "Hi " + x
let addComma (x : string) = x + ","

// Used to compute result.
let name = "visitor"

// Apply three string functions with pipeline operator on the name.
let result =
    name
    |> uppercase
    |> addGreeting
    |> addComma

// Write result.
printfn "%A" result
"Hi VISITOR,"

`String.map`

In F# we find a String.map function that changes each character in a string according to a function argument. We use upperMap here to uppercase only the letters "a" and "z."

Start We use pattern-matching in the upperMap function to determine how to modify the letter.

Tip With String.map we have an effective way to translate strings. For something like a ROT13 transformation this is ideal.

let value = "abcxyz"

// This function only uppercases 2 letters.
let upperMap = fun c ->
    match c with
    | 'a' -> 'A'
    | 'x' -> 'X'
    | _ -> c

// Apply our upperMap function with String.map.
let result = String.map upperMap value
printfn "%A" result
"AbcXyz"

Methods

With F# we can invoke string methods from the .NET Framework. Some of the top methods are Split, IndexOf, Replace. These help us make programs that actually do things.

`Parse`, convert `int`

With Int32.TryParse, we can convert a string to an int. No custom parsing logic is needed. The F# language requires special syntax here.

Ciphers, ROT13

With the ROT13 cipher, we rotate (shift) characters 13 places. We learn character-based manipulation of strings. With F# we employ the String.map function.

We use strings in almost all programs, even functional F# ones. With the heavily-tested, fast string methods from .NET, we have a powerful string solution.