Roughly cover the materials in Chapters 7 and 8.

Strings

• String is the data type for text.
  • A sequence of Char
  • "Text" and "t" are String.
  • 'T' and 't' are Char.
• Length: .length
• Template:
  • $val
  • $var
  • ${expression}
• str[i]: Access the i-th Char in a String type variable/value str.
  • Kotlin is zero-based.

fun main() {
    val str = "NCTU"
    println("The string $str has length ${str.length}")
    println(str[0])
    println(str[1])
    println(str[2])
    println(str[3])
    // intended to crash the program
    // with StringIndexOutOfBoundsException
    println(str[4])
}

• Escape sequences
  • To represent special characters
    • Consider a Char to represent '
      • ''' does not work!
      • Use '\''
    • Consider a String to represent ""
      • """" does not work!
      • Use "\"\""
    • Consider a Char to represent \
      • '\' does not work!
      • Use '\\'
    • Consider a String to represent $
      • "$" does not work!
      • Use "\$"
  • \': Single quote '
  • \": Double quote
  • \\: Backslash
  • \$: Dollar sign
  • \t: Tab
  • \b: Backspace
  • \n: Newline
  • \r: Carriage return
  • \uXXXX: Unicode, see Wikipedia.
• Kotlin Document for String

Substring Operations

substring

• Give a slice of the string.
• Argument:
  • start: Int
  • start: Int, end: Int
  • range: IntRange

fun main() {
    val str = "National Chiao Tung University"
    // Zero-based: drop the first character N
    println(str.substring(1))
    // [start, end): start at 't', end at 'i' and 'i' is not included
    println(str.substring(2, 3))
    // Should be ""
    println(str.substring(4, 4))
    // Should be "o"
    println(str.substring(4..4))
    // Does not work: IntProgression is not IntRange
    // println(str.substring(10 downTo 0))
}

Index

• indexOf
  • Argument: char: Char
  • Return the index of the first occurence
• lastIndexOf
  • Argument: char: Char
  • Return the index of the last occurence
• indexOfFirst
  • Argument: predicate: (Char) -> Boolean
  • Return the index of the first occurence which satisfies the predicate
• indexOfLast
  • Argument: predicate: (Char) -> Boolean
  • Return the index of the last occurence which satisfies the predicate

fun main() {
    val str = "National Chiao Tung University"
    println(str.indexOf(' '))
    println(str.lastIndexOf('l'))
    // The first character in lower case and at least 'm'
    println(str.indexOfFirst{'m' <= it && it <= 'z'})
    // The last character in upper case.
    println(str.indexOfLast{'A' <= it && it <= 'Z'})
}

split

• Split the string into a list of substrings.
• List access: use [i] to access the i-th element.
• Destructure: if you know the number of pieces before compile, you may declare more than one variable / value in a line!

fun main() {
    val str = "National Chiao Tung University"
    val listOfPieces = str.split(" ")
    println("String \"$str\" has ${listOfPieces.size} words.")
    println("The first is ${listOfPieces[0]}")
    println("The last is ${listOfPieces.last()}")
    // "destructuring" declaration
    // will crash if listOfPieces.size < 4
    val (N, C, T, U) = listOfPieces
    println("N is $N")
    println("C is $C")
    println("T is $T")
    println("U is $U")
}

replace

• Replace all occurences
• Usage
  • (oldChar: Char, newChar: Char)
  • (oldString: String, newString: String)
  • (regex: Regex, replacement: String)
  • (regex: Regex, transform: (MatchResult) -> CharSequence)

fun main() {
    var str = "National Chiao Tung University"
    // String is immutable, replace won't change str
    // Use assignment to change str
    str = str.replace('N', 'n')
    println(str)
    str = str.replace("Chiao", "Yang")
    println(str)
    str = str.replace("Tung", "Chiao")
    println(str)
    str = str.replace(Regex("[aeiou]"), " ")
    println(str)
    str = str.replace(Regex("[lv]")){
        when(it.value) {
            "l" -> "1"
            "t" -> "7"
            "v" -> "\\/"
            else -> it.value
        }
    }
    println(str)
}

Type Conversion

• toInt()
• toIntOrNull()
• toDouble()
• toDoubleOrNull()
• Note: toLowerCase() is not a type conversion.

Practice

• Task 1: From leet to 1337
  • 請參考 Wikipedia，寫一個函數 leet(str: String): String，將字串中正常的英文字母，轉換成非英文字母的 leet 寫法。只要換出來沒英文就可以。
• Task 2: Word count
  • 寫一個函數 fun wordCount(str: String): Int ，計算 str 之中有多少個英文字(Word)，相同的英文字如果出現數次，按出現次數計算。保證 str 中只有英文字母、空白(' ')、逗號(',')、句號('.')。
• Task 3: English number
  • 寫一個函數 fun number(str: String): Int?，將英文字 str 換算成對應的整數值，如 one 將換成 1， zero 換成 0。只需要處理零到九十九萬九千九百九十九之間的整數即可。輸入可能含有大寫字母，函數必須無視大小寫。如果 str 並非一個英文數字，請回傳 null 。

Numbers

Numeric Types

• Integers

  • Binary numbers
    • Decimal:
      ${1243}_{10}=1\times {10}^3+2\times {10}^2+4\times {10}^1+3\times {10}^0$
    • Binary:
      ${1011}_2=1\times 2^3+0\times 2^2+1\times 2^1+1\times 2^0={11}_{10}$
  • Two's complement
  • Byte
    • 8-bit: -128 to 127
  • Short
    • 16-bit: -32768 to 32767
  • Int
    • 32-bit: -2147483648 to 2147483647
  • Long
    • 64-bit: -9223372036854775808 to 9223372036854775807

• Floating point numbers

  • Scientific notation
    • Decimal:
      $a\times {10}^b$
      • $a$: significand
      • $b$: exponent
      • Normalized:
        $1\le a<10$
      • Significant figures of
        $1.2304\times {10}^8$ is
        $5$.
      • We cannot express
        $\frac{1}{3}$ by scientific notation, since you don't have enough space for infinitely many
        $3$.
    • Binary:
      $f\times 2^e$
      • Normalized:
        $1\le f<2$
      • Binary fraction number
        • $1.0011=1+2^{-3}+2^{-4}=1+\frac{1}{8}+\frac{1}{16}=\frac{19}{16}=1.1875$
        • ${0.1}_{10}=\frac{1}{10}=0.0{\overline{0011}}_2=1.{\overline{1001}}_2\times 2^{-4}$
  • Float: 32-bit (Single precision)
    • Exponent:
      $-126\le e\le 127$
    • Significant figures: 24 bit (~7.22 digits)
  • Double: 64-bit (Double precision)
    • Exponent:
      $-1022\le e\le 1023$
    • Significant figures: 53 bit (~15.95 digits)

Conversion

• Convert String to number
  • With exception: .toByte(), .toShort(), toInt(), toLong(), toFloat(), toDouble().
  • Without exception: .toByteOrNull(), .toShortOrNull(), …
• Convert Int to Double
  • Not working
    • Integer division: 1/2 is 0
  • Working
    • Adds 0.0
    • Times 1.0
    • .toDouble()
• Convert Double to Int
  • .toInt()
    • Try 1.9.toInt() and -1.9.toInt()
  • .roundToInt()
    • import kotlin.math.roundToInt before use
    • Try
      • 1.5.roundToInt()
      • 2.5.roundToInt()
      • -0.5.roundToInt()
      • -1.5.roundToInt()
• Convert numbers to String
  • Use .toString()
    • Integers: radix=2 means binary, and radix=10 means decimal.
      • Default radix is 10
  • Use string template "$val"
  • Use formatting string
    • Example:
      • "%.3f".format(0.12345678)
      • "%.4f".format(9.87654321)
    • See the doucument for more.