F# - Strings

In F#, the string type represents immutable text as a sequence of Unicode characters.

String Literals

String literals are delimited by the quotation mark (") character.

Some special characters are there for special uses like newline, tab, etc. They are encoded using backslash (\) character. The backslash character and the related character make the escape sequence. The following table shows the escape sequence supported by F#.

Character Escape sequence
Backspace \b
Newline \n
Carriage return \r
Tab \t
Backslash \\
Quotation mark \"
Apostrophe \'
Unicode character \uXXXX or \UXXXXXXXX (where X indicates a hexadecimal digit)

Ways of lgnoring the Escape Sequence

The following two ways makes the compiler ignore the escape sequence −

  • Using the @ symbol.
  • Enclosing the string in triple quotes.

When a string literal is preceded by the @ symbol, it is called a verbatim string. In that way, all escape sequences in the string are ignored, except that two quotation mark characters are interpreted as one quotation mark character.

When a string is enclosed by triple quotes, then also all escape sequences are ignored, including double quotation mark characters.

Example

The following example demonstrates this technique showing how to work with XML or other structures that include embedded quotation marks −

Live Demo 
// Using a verbatim string
let xmldata = @"<book author = ""Lewis, C.S"" title = ""Narnia"">"
printfn "%s" xmldata

When you compile and execute the program, it yields the following output −

<book author = "Lewis, C.S" title = "Narnia">

Basic Operators on Strings

The following table shows the basic operations on strings −

Value Description
collect : (char → string) → string → string Creates a new string whose characters are the results of applying a specified function to each of the characters of the input string and concatenating the resulting strings.
concat : string → seq<string> → string Returns a new string made by concatenating the given strings with a separator.
exists : (char → bool) → string → bool Tests if any character of the string satisfies the given predicate.
forall : (char → bool) → string → bool Tests if all characters in the string satisfy the given predicate.
init : int → (int → string) → string Creates a new string whose characters are the results of applying a specified function to each index and concatenating the resulting strings.
iter : (char → unit) → string → unit Applies a specified function to each character in the string.
iteri : (int → char → unit) → string → unit Applies a specified function to the index of each character in the string and the character itself.
length : string → int Returns the length of the string.
map : (char → char) → string → string Creates a new string whose characters are the results of applying a specified function to each of the characters of the input string.
mapi : (int → char → char) → string → string Creates a new string whose characters are the results of applying a specified function to each character and index of the input string.
replicate : int → string → string Returns a string by concatenating a specified number of instances of a string.

The following examples demonstrate the uses of some of the above functionalities −

Example 1

The String.collect function builds a new string whose characters are the results of applying a specified function to each of the characters of the input string and concatenating the resulting strings.

Live Demo 
let collectTesting inputS =
   String.collect (fun c -> sprintf "%c " c) inputS
printfn "%s" (collectTesting "Happy New Year!")

When you compile and execute the program, it yields the following output −

H a p p y N e w Y e a r !

Example 2

The String.concat function concatenates a given sequence of strings with a separator and returns a new string.

Live Demo 
let strings = [ "Tutorials Point"; "Coding Ground"; "Absolute Classes" ]
let ourProducts = String.concat "\n" strings
printfn "%s" ourProducts

When you compile and execute the program, it yields the following output −

Tutorials Point
Coding Ground
Absolute Classes

Example 3

The String.replicate method returns a string by concatenating a specified number of instances of a string.

Live Demo 
printfn "%s" <| String.replicate 10 "*! "

When you compile and execute the program, it yields the following output −

*! *! *! *! *! *! *! *! *! *!
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