Add Chars concept

concepts/chars/.meta/config.json

@@ -0,0 +1,7 @@
+{
+  "authors": [
+    "colinleach"
+  ],
+  "contributors": [],
+  "blurb": "Kotlin was designed from the beginning to use a large subset of Unicode characters, to represent a wide range of writing systems."
+}

concepts/chars/about.md

@@ -0,0 +1,143 @@
+# About chars
+
+This is potentially a _big_ subject!
+It is possible to write a long book about it, and several people have done so (search Amazon for "unicode book" to see some examples).
+
+## A very brief history
+
+Handling characters in computers was much simpler in earlier decades, when programmers assumed that English was the only important language.
+So: 26 letters, upper and lower case, 10 digits, several punctuation marks, plus a code (0x07) to ring a bell, and it all fitted into 7 bits: the [ASCII][wiki-ascii] character set.
+
+Naturally, people started asking what about à, ä and Ł, then other people started asking about ऄ, ஹ and ญ, and young people wanted emojis 😱.
+What to do?
+
+To cut a long story short, many smart and patient people had to serve on committees for years, working out the details of the [Unicode][web-unicode] character set, and of encodings such as [UTF-8][wiki-utf-8], and lots of software needed a _very_ complicated rewrite.
+Also, lots of new bugs were introduced.
+
+To prevent _everything_ breaking, the Unicode/UTF-8 design ensures that the first 127 codes are identical to ASCII _(even the bell)_.
+
+## Characters in Kotlin
+
+Languages designed after about 2005 have the huge advantage that a reasonably stable Unicode standard already existed.
+
+Kotlin (first released in 2011) was able to assume that users would use a variety of (human) languages, and would need Unicode to express them.
+
+~~~~exercism/advanced
+[Characters][ref-char] in Kotlin are 16-bit (UTF-16) [`codepoints`][wiki-codepoint], the same as a JVM `char`.
+This is enough to express most written alphabets, but not the entire range of emojis.
+
+The full Unicode standard uses up to six bytes (48 bits) per character (called a [`grapheme`][wiki-grapheme]).
+
+Kotlin `Strings` support this full standard by using multiple codepoints per character, when necessary.
+For example, 😱 would be `\uD83D` and `\uDE31`.
+
+Unfortunately, Java has no built-in grapheme support, and for compatibility neither does Kotlin.
+
+[wiki-codepoint]: https://en.wikipedia.org/wiki/Code_point
+[wiki-grapheme]: https://en.wikipedia.org/wiki/Grapheme
+[ref-char]: https://kotlinlang.org/docs/characters.html
+~~~~
+
+
+Character literals are written in single-quotes, and are distinct from strings written in double quotes.
+This is probably obvious to people from the C/C++ world, but potentially confusing to Python and JavaScript programmers.
+
+```kotlin
+val a = 'a'
+a::class.qualifiedName  // => kotlin.Char
+a.code  // => 97
+
+val jha = 'झ'  // Devanagari alphabet
+jha.code  // => 2333
+
+val heart = '❤'  // heart emoji
+heart.code  // => 10084
+
+Char.MAX_VALUE.code // => 65535 (64k, the largest code point allowed)
+
+val not_char = 'abc' // => Too many characters in a character literal.
+```
+
+Converting between `Char` and `Int` is straightforward:
+
+```kotlin
+a.code  // => 97
+Char(97) // => 'a'
+```
+
+The compiler allows _some_ forms of integer arithmetic on `Char`s:
+
+```kotlin
+'a' + 5     // => 'f'
+'c' - 'a'   // => 2
+'c' + 'a'   // => error!
+
+'f' + ('A' - 'a')  // => 'F' (same as 'f'.uppercase()
+
+'f'.dec() // => 'e' (decrement)
+'f'.inc() // => 'g' (increment)
+```
+
+## Some functions for `Char`
+
+As always, there are far too [many functions][ref-char-lib] to discuss here, so this is just a selection.
+
+- For appropriate alphabets, change case with [`uppercase()`][ref-uppercase] and [`lowercase()`][ref-lowercase].
+- Test case with [`isUpperCase()`][ref-isuppercase] and [`isLowerCase()`][ref-islowercase].
+- Test character type with:
+    - [`isLetter()`][ref-isletter], covers many alphabets
+    - [`isDigit()`][ref-isdigit], in range 0..9
+    - `isLetterOrDigit()`, combines the previous two
+    - [`isWhitespace()`][ref-iswhitespace], any whitespace character
+
+```kotlin
+'झ'.isLetter()      // => true
+'A'.isLowerCase()   // => false
+'4'.isDigit()       // => true
+'\t'.isWhitespace() // => true  (tab character)
+```
+
+Also, [regular expressions][ref-regex] (which will be the subject of a later Concept) allow powerful search and manipulation.
+
+## Char List and String interconversions
+
+To convert from a `String` to a `List` of `Char`s, we can use `toList()`.
+
+To convert a `List` of `Char`s to a `String`, there is the [`joinToString()`][ref-jointostring] function, which takes a separator (often the empty string) as argument.
+
+```kotlin
+val kt = "kotlin".toList()  // => [k, o, t, l, i, n]
+kt.joinToString("")   // => "kotlin"
+kt.joinToString("_")  // => "k_o_t_l_i_n"
+```
+
+Note that `joinToString()` operates on a List or Array.
+To _cast_ a single `Char` to a 1-character string, use `toString()`.
+
+```kotlin
+'a'.toString() // => "a"
+```
+
+To check if a character is present in a `String`, or a `Char` list or array, we have `in`, which maps to the [`contains()`][ref-contains] function.
+
+```kotlin
+val clist = "kotlin".toList()  // => [k, o, t, l, i, n]
+'t' in clist     // => true
+'t' in "kotlin"  // => true
+```
+
+[ref-char]: https://kotlinlang.org/docs/characters.html
+[ref-char-lib]: https://kotlinlang.org/api/core/kotlin-stdlib/kotlin/-char/
+[wiki-ascii]: https://en.wikipedia.org/wiki/ASCII
+[web-unicode]: https://home.unicode.org/
+[wiki-utf-8]: https://en.wikipedia.org/wiki/UTF-8
+[ref-uppercase]: https://kotlinlang.org/api/core/kotlin-stdlib/kotlin.text/uppercase.html
+[ref-lowercase]: https://kotlinlang.org/api/core/kotlin-stdlib/kotlin.text/lowercase.html
+[ref-isuppercase]: https://kotlinlang.org/api/core/kotlin-stdlib/kotlin.text/is-upper-case.html
+[ref-islowercase]: https://kotlinlang.org/api/core/kotlin-stdlib/kotlin.text/is-lower-case.html
+[ref-isletter]: https://kotlinlang.org/api/core/kotlin-stdlib/kotlin.text/is-letter.html
+[ref-isdigit]: https://kotlinlang.org/api/core/kotlin-stdlib/kotlin.text/is-digit.html
+[ref-iswhitespace]: https://kotlinlang.org/api/core/kotlin-stdlib/kotlin.text/is-whitespace.html
+[ref-jointostring]: https://kotlinlang.org/api/core/kotlin-stdlib/kotlin.collections/join-to-string.html
+[ref-contains]: https://kotlinlang.org/api/core/kotlin-stdlib/kotlin.text/contains.html
+[ref-regex]: https://kotlinlang.org/api/core/kotlin-stdlib/kotlin.text/-regex/

concepts/chars/introduction.md

@@ -0,0 +1,125 @@
+# Introduction
+
+This is potentially a _big_ subject!
+It is possible to write a long book about it, and several people have done so (search Amazon for "unicode book" to see some examples).
+
+## A very brief history
+
+Handling characters in computers was much simpler in earlier decades, when programmers assumed that English was the only important language.
+So: 26 letters, upper and lower case, 10 digits, several punctuation marks, plus a code (0x07) to ring a bell, and it all fitted into 7 bits: the [ASCII][wiki-ascii] character set.
+
+Naturally, people started asking what about à, ä and Ł, then other people started asking about ऄ, ஹ and ญ, and young people wanted emojis 😱.
+What to do?
+
+To cut a long story short, many smart and patient people had to serve on committees for years, working out the details of the [Unicode][web-unicode] character set, and of encodings such as [UTF-8][wiki-utf-8], and lots of software needed a _very_ complicated rewrite.
+Also, lots of new bugs were introduced.
+
+To prevent _everything_ breaking, the Unicode/UTF-8 design ensures that the first 127 codes are identical to ASCII _(even the bell)_.
+
+## Characters in Kotlin
+
+Languages designed after about 2005 have the huge advantage that a reasonably stable Unicode standard already existed.
+
+Kotlin (first released in 2011) was able to assume that users would use a variety of (human) languages, and would need Unicode to express them.
+
+Character literals are written in single-quotes, and are distinct from strings written in double quotes.
+This is probably obvious to people from the C/C++ world, but potentially confusing to Python and JavaScript programmers.
+
+```kotlin
+val a = 'a'
+a::class.qualifiedName  // => kotlin.Char
+a.code  // => 97
+
+val jha = 'झ'  // Devanagari alphabet
+jha.code  // => 2333
+
+val heart = '❤'  // heart emoji
+heart.code  // => 10084
+
+Char.MAX_VALUE.code  // => 65535 (64k, the largest code point allowed)
+```
+
+Converting between `Char` and `Int` is simple:
+
+```kotlin
+a.code  // => 97  (a.toInt() is deprecated)
+Char(97) // => 'a'
+```
+
+The compiler allows _some_ forms of integer arithmetic on `Char`s:
+
+```kotlin
+'a' + 5       // => 'f'
+'c' - 'a'   // => 2
+'c' + 'a'   // => error!
+
+'f' + ('A' - 'a')  // => 'F' (same as 'f'.uppercase()
+
+'f'.dec() // => 'e' (decrement)
+'f'.inc() // => 'g' (increment)
+```
+
+## Some functions for `Char`
+
+As always, there are far too [many functions][ref-char-lib] to discuss here, so this is just a selection.
+
+- For appropriate alphabets, change case with [`uppercase()`][ref-uppercase] and [`lowercase()`][ref-lowercase].
+- Test case with [`isUpperCase()`][ref-isuppercase] and [`isLowerCase()`][ref-islowercase].
+- Test character type with:
+    - [`isLetter()`][ref-isletter], covers many alphabets
+    - [`isDigit()`][ref-isdigit], in range 0..9
+    - `isLetterOrDigit()`, combines the previous two
+    - [`isWhitespace()`][ref-iswhitespace], any whitespace character
+
+```kotlin
+'झ'.isLetter()      // => true
+'A'.isLowerCase()   // => false
+'4'.isDigit()       // => true
+'\t'.isWhitespace() // => true  (tab character)
+```
+
+## Char List and String interconversions
+
+For String to Char List, we can use `toList()`.
+
+For Char List to String, there is the [`joinToString()`][ref-jointostring] function, which takes a separator (often the empty string) as argument.
+
+```kotlin
+val kt = "kotlin".toList()  // => [k, o, t, l, i, n]
+kt.joinToString("")   // => "kotlin"
+kt.joinToString("_")  // => "k_o_t_l_i_n"
+```
+
+Note that `joinToString()` operates on a List or Array.
+To _cast_ a single `Char` to a 1-character string, use `toString()`.
+
+```kotlin
+'a'.toString() // => "a"
+```
+
+To check if a character is present in a `String`, or a `Char` list or array, we have `in`, which maps to the [`contains()`][ref-contains] function.
+
+```kotlin
+val clist = "kotlin".toList()  // => [k, o, t, l, i, n]
+'t' in clist     // => true
+'t' in "kotlin"  // => true
+```
+
+
+[ref-char]: https://kotlinlang.org/docs/characters.html
+[ref-char-lib]: https://kotlinlang.org/api/core/kotlin-stdlib/kotlin/-char/
+[wiki-ascii]: https://en.wikipedia.org/wiki/ASCII
+[web-unicode]: https://home.unicode.org/
+[wiki-utf-8]: https://en.wikipedia.org/wiki/UTF-8
+[ref-uppercase]: https://kotlinlang.org/api/core/kotlin-stdlib/kotlin.text/uppercase.html
+[ref-lowercase]: https://kotlinlang.org/api/core/kotlin-stdlib/kotlin.text/lowercase.html
+[ref-isuppercase]: https://kotlinlang.org/api/core/kotlin-stdlib/kotlin.text/is-upper-case.html
+[ref-islowercase]: https://kotlinlang.org/api/core/kotlin-stdlib/kotlin.text/is-lower-case.html
+[ref-isletter]: https://kotlinlang.org/api/core/kotlin-stdlib/kotlin.text/is-letter.html
+[ref-isdigit]: https://kotlinlang.org/api/core/kotlin-stdlib/kotlin.text/is-digit.html
+[ref-iswhitespace]: https://kotlinlang.org/api/core/kotlin-stdlib/kotlin.text/is-whitespace.html
+[ref-jointostring]: https://kotlinlang.org/api/core/kotlin-stdlib/kotlin.collections/join-to-string.html
+[ref-contains]: https://kotlinlang.org/api/core/kotlin-stdlib/kotlin.text/contains.html
+[ref-regex]: https://kotlinlang.org/api/core/kotlin-stdlib/kotlin.text/-regex/
+
+

concepts/chars/links.json

@@ -0,0 +1,10 @@
+[
+  {
+    "url": "https://kotlinlang.org/docs/characters.html",
+    "description": "Kotlin introduction to characters."
+  },
+  {
+    "url": "https://kotlinlang.org/api/core/kotlin-stdlib/kotlin/-char/",
+    "description": "Char type and functions"
+  }
+]

config.json

@@ -1387,6 +1387,11 @@
       "uuid": "168827c0-4867-449a-ad22-611c87314c48",
       "slug": "conditionals",
       "name": "Conditionals"
+    },
+    {
+      "uuid": "660e1acd-3072-4bbc-ae44-bbc204da5eab",
+      "slug": "chars",
+      "name": "Chars"
     }
   ],
   "key_features": [