Strings

Introduction

In this chapter we will introduce strings and the functions available to manipulate them.

Strings in xtlang are immutable. Whenever you call a function that manipulates a string it will return a new string leaving the original string unchanged:

($ (let ((str1 (String "hello world"))
         (str2 (substring str 0 5)))
        (begin
            (println str1)  ;; outputs 'hello world'
            (println str2)))) ;; outputs 'hello'

The String Type

Confusingly xtlang has two string types so as to maintain compatibility with the ‘C language’. These types are:

• i8* - or an array of characters.
• String* - an object for storing and manipulating strings.

Unless you’re writing libraries that are interfacing directly with C code (this will be discussed in the chapter on C interopability), you should never be using i8* strings directly. However you will sometimes use them without realizing as the string literal type "string" has the type i8*:

• "hello":i8*
• (String "hello"):String*

Hopefully this state of affairs will be improved in the future, but for the moment we’re stuck with this.

Creating Strings

Strings are creating using the String function:

($ (String "My First String))

(bind-val my-string String "My String")

;; you can also create a new string from an old one. This creates a copy of the original string.
($ (String (String "hello")))

You can also use Str as an alias for String if you prefer.

Strings can be created from other types using the `toString function:

($ (toString 4))

(bind-val my-string-num2 String* (toString 4))

String Length

To get the length of a string use length:

($ (length (String "hello"))) ;; 5

String Comparisons

Equality

To compare two strings use equal:

($ (equal (String "hello") (String "bob"))) ;; #f
($ (equal (String "hello") (String "hello"))) ;; #t

Levenshtein Distance

The Levenstein Distance is the minimum number of single-character edits (insertions, deletions or substitutions) required to change one word into the other. xt_lang offers the levenshtein for this.

To get the length of a string use equal:

($ (levenshtein (String "hello") (String "hello"))) ;; 0
($ (levenshtein (String "hello") (String "hedo"))) ;; 2

String Manipulation

XTlang gives you a number of functions for manipulating and accessing parts of strings.

Trimming Whitespace

You can remove whitespace using the trim, ltrim and rtrim functions. ltrim removes white space at the beginning of a string, rtrim removes white space at the end of a string and trim removes whitespace at both ends of a string:

($ (ltrim (String "  hello world  "))) ;; 'hello world  '
($ (rtrim (String "  hello world  "))) ;; '  hello world'
($ (trim  (String "  hello world  "))) ;; 'hello world'

TODO Write rtrim and ltrim…. :)

Substrings

You can create substrings from existing strings using substring:

;; Returns "rum"
($ (substring (String "Scrumptious") 2 5))

substring takes 3 parameters:

1. The original string.
2. start position for new substring
3. end position for new substring.

If the start, or end, positions lie outside the string then the function will return null.

NOTE: This may change in the future to something safer.

Combining Strings

You can combine strings using cat and cat2:

;; returns "hello world"
($ (cat2 (String "hello") (String " world")))

;; returns "one".
($ (cat2 null (String "one")))

;; returns "hello my world"
($ (cat (String "hello ") (String "my ") (String "world")))

cat2 takes two parameters, while cat generalizes to as many strings as you need to cobmine. If you try to combine a null with a string then cat and cat2 will simply return the string.

Replacing Substrings

The functions replace and replace_all can be used to replace parts of a string with a different string. replace replaces the first occurrence that it finds, and then returns the string. replace_all replaces every occurrence of the string.

;; returns (String "hello your face is my name")
($ (replace "hello your name is my name" "name" "face"))

;; returns (String "hello your face is my face")
($ (replace_all "hello your name is my name" "name" "face"))

;; returns (String "hello your name is my name")
($ (replace_all "hello your name is my name" "mouth" "face"))

replace and replace_all can be used with any combination of String and String literal that you like:

($ (replace (String "hello world") "hello" "goodbye"))

($ (replace "hello world" (String "hello") "goodbye"))