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Theory: Characters

In this topic, you will learn how to work with characters in Kotlin. This is a pretty fundamental concept, and being familiar with it will help you better understand how to work with texts.

What is a character

Each character is just a symbol enclosed in single quotes. The Char type represents letters (both uppercase and lowercase), digits, and other symbols:

val lowerCaseLetter: Char = 'a'
val upperCaseLetter: Char = 'Q'
val number: Char = '1'
val space: Char = ' '
val dollar: Char = '$'

This type can represent any symbol including hieroglyphs, as well as some special symbols.

A character can also be represented by its hexadecimal code in the Unicode table. The code starts with \u:

val ch = '\u0040' // it represents '@'
println(ch) // @

Although we use a sequence of characters to represent such code, the code itself represents exactly one character.

For example, Latin capital letters have hexadecimal codes from '\u0041' to '\u005A', and Latin lowercase letters have codes ranging from '\u0061' to '\u007A'.

You can also convert numbers to characters and vice versa. Let's take a look at how it works:

val ch = 'a'
println(ch.toInt())   // 97
val num = 97
println(num.toChar()) // a
  • Char 는 하나의 symbol 을 나타낼 때 주로 쓰인다.
  • '\u' 와 16진수 숫자로도 Char 를 나타낼 수 있다. 이때는 Unicode Table 에 있는 숫자와 매핑된다.
  • Char 의 각 Symbol 은 Unicode 의 문자와 매핑되고 이 문자를 가리키는 숫자가 있다. 그래서 문자와 숫자간의 서로 변환이 된다.

How to read characters

Kotlin does not provide a convenient function to read a Char. However, you can do it another way: if you need to read one Char in a whole line, use this construction:

val ch: Char = readLine()!!.first()

You just read a String and get its first element, which will be a Char. Strings are composed of characters, but we will discuss this in more detail a little later.

  • Kotlin 에서는 Char 만을 읽는 편리한 함수는 제공해주지 않지만 그와 유사한게 있다.
  • 이는 readLine()!!.first() 를 통해서 첫 글자만 읽어오는 함수다.
  • Kotlin 에서 String 은 Character 의 연속적인 집합으로 구성되어있는 관계다.

Retrieving subsequent characters

There are two operators for adding (+) and subtracting (-) integer numbers in order to get the next and previous characters according to the Unicode order

val ch1 = 'b'
val ch2 = ch1 + 1 // 'c'
val ch3 = ch2 - 2 // 'a'

Remember that you cannot add a symbol to a number as it will cause an error.

val ch = 'a'
val ch1 = 1 + ch // Error

You also cannot sum up two characters:

val charsSum = 'a' + 'b' // Error

It is possible to use the increment (++) and decrement (--) operators in their prefix and postfix forms. The assignment operator combined with + or - also works for characters, as well as += and -=.

  • Character 에 숫자를 더해서 다음 Unicode 문자를 가져오는 건 가능하다. (이때 +,- 연산자 뿐 아니라 ++, --, +=, -= 같은 연산자 모두 가능)
  • Character + 숫자 해서 Unicode 문자를 가져오는 건 가능하지만, 숫자 + 문자 는 안됨. (직관에 맞지 않아서 그런)

Escape sequences

There are some special characters starting with a backslash , which are known as escape or control sequences. Most of them do not have corresponding symbols, and you cannot find them on your keyboard.

'\n' is the newline character; '\t' is the tab character; '\r' is the carriage return character; '\' is the backslash character itself; ''' is the single quote mark; '"' is the double quote mark.

Here are a few examples:

print('\t') // makes a tab
print('a')  // prints 'a'
print('\n') // goes to a new line
print('c')  // prints 'c'

The above code will print:

  a
c
  • \ 로 시작하는 특별한 의미를 가진 special character 들이 있다. 이것들을 escape or control sequence 라고 한다.

Relational operations with characters

You can compare characters using relational operations (==, <, >, <=, >=, and !=) according to their position in the Unicode table.

println('a' < 'c')  // true
 println('x' >= 'z') // false
 
 println('D' == 'D') // true
 println('Q' != 'q') // true because capital and small letters are not the same
 
 println('A' < 'a')  // true because capital Latin letters are placed before small ones

Using relational operations and codes, we can check whether a Char is a digit: all ten digits have codes from '\u0030' to '\u0039'.

Here is a program that does it:

fun main() {
    val ch = readLine()!!.first()
    val isDigit = ch >= '\u0030' && ch <= '\u0039'

    println(isDigit)
}

If the input is a digit '0', '1', '2', ..., '9' (without quotes), the program prints true. Otherwise, it prints false.

  • Character 문자들도 관계 연산자들 (<,>,=,!= 등) 을 사용할 수 있다. 이것들을 사용할 땐 Unicode Table 에서 매핑하는 숫자들 기반으로 사용한다.

Processing characters

There's a variety of useful functions to work with characters. You can use these functions instead of dealing with character codes.

  • isDigit() returns true if the given character represents a digit ('1', '2', etc); otherwise, false;
  • isLetter() returns true if the given character represents a letter ('a', 'B', 'm', etc); otherwise, false;
  • isLetterOrDigit() returns true if the given character represents a letter or a digit; otherwise, false;
  • isWhitespace() returns true if the given character represents a whitespace (' ', or '\t', or '\n'); otherwise, false;
  • isUpperCase() returns true if the given character is an uppercase character; otherwise, false;
  • isLowerCase() returns true if the given character is a lowercase character; otherwise, false;
  • toUpperCase() returns the uppercase form of the given character (before Kotlin 1.5; you shouldn't use it nowadays);
  • uppercaseChar() returns the uppercase form of the given character (since Kotlin 1.5);
  • uppercase() returns a String with the uppercase form of the given character (since Kotlin 1.5);
  • toLowerCase() returns the lowercase form of the given character (before Kotlin 1.5; you shouldn't use it nowadays);
  • lowercaseChar() returns the lowercase form of the given character (since Kotlin 1.5);
  • lowercase() returns a String with the lowercase form of the given character (since Kotlin 1.5).

Let's take a look at some examples of the functions listed above:

val one = '1'

val isDigit = one.isDigit()   // true
val isLetter = one.isLetter() // false

val capital = 'A'
val small = 'e'

val isLetterOrDigit = capital.isLetterOrDigit() // true

val isUpperCase = capital.isUpperCase() // true
val isLowerCase = capital.isLowerCase() // false

val capitalEString = small.uppercase() // "E"
val capitalE = small.uppercaseChar()   // 'E'
  • Character 를 다루는 유용한 메소드가 있다.
  • isDigit(), isLetter(), isWhitespace(), isLetterOrDigit()
  • uppercaseChar(), uppercase()
  • lowercaseChar(), lowercase()