What is infix in kotlin

Infix Notation : Kotlin

Ever imagined calling a public function of a class without dot and parentheses of the parameter in Kotlin. Kotlin provides infix notation with which we can call a function with the class object without using a dot and parentheses across the parameter. Using infix function provides more readability to a function similar to other operators like in , is , as in Kotlin.

infix fun Int.add(b : Int) : Int = this + b
val x = 10.add(20)
val y = 10 add 20 // infix call 

• They must be member functions or extension functions.
• They must have a single parameter.
• The parameter must not accept a variable number of arguments and must have no default value.

What if I use an infix function with other operators.

• 1 add 2 + 3 is equivalent to 1 add (2 + 3)
• 0 until n * 2 is equivalent to 0 until (n * 2)
• xs union ys as Set is equivalent to xs union (ys as Set )

• a && b xor c is equivalent to a && (b xor c)
• a xor b in c is equivalent to (a xor b) in c

But infix functions always require both the receiver and the parameter to be specified. When you are calling a method on the current receiver using the infix notation, you need to use this explicitly. Unlike a regular method call, this cannot be omitted. This is required to ensure unambiguous parsing.

class StringCollection  
infix fun add(s: String) // perform some action here 
>
 
fun build() this add "abc" // Works fine, calls the infix function 
add("abc") // Works fine, note this is not an infix call 
add "abc" // error: the receiver must be specified 
>
> 

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Functions

Function parameters are defined using Pascal notation — name: type. Parameters are separated using commas, and each parameter must be explicitly typed:

You can use a trailing comma when you declare function parameters:

Default arguments

Function parameters can have default values, which are used when you skip the corresponding argument. This reduces the number of overloads:

A default value is set by appending = to the type.

Overriding methods always use the base method’s default parameter values. When overriding a method that has default parameter values, the default parameter values must be omitted from the signature:

If a default parameter precedes a parameter with no default value, the default value can only be used by calling the function with named arguments:

If the last argument after default parameters is a lambda, you can pass it either as a named argument or outside the parentheses:

fun foo( bar: Int = 0, baz: Int = 1, qux: () -> Unit, ) < /*. */ >foo(1) < println("hello") >// Uses the default value baz = 1 foo(qux = < println("hello") >) // Uses both default values bar = 0 and baz = 1 foo < println("hello") >// Uses both default values bar = 0 and baz = 1

Named arguments

You can name one or more of a function’s arguments when calling it. This can be helpful when a function has many arguments and it’s difficult to associate a value with an argument, especially if it’s a boolean or null value.

When you use named arguments in a function call, you can freely change the order that they are listed in. If you want to use their default values, you can just leave these arguments out altogether.

Consider the reformat() function, which has 4 arguments with default values.

fun reformat( str: String, normalizeCase: Boolean = true, upperCaseFirstLetter: Boolean = true, divideByCamelHumps: Boolean = false, wordSeparator: Char = ‘ ‘, ) < /*. */ >

When calling this function, you don’t have to name all its arguments:

You can skip all the ones with default values:

You are also able to skip specific arguments with default values, rather than omitting them all. However, after the first skipped argument, you must name all subsequent arguments:

You can pass a variable number of arguments ( vararg ) with names using the spread operator:

When calling Java functions on the JVM, you can’t use the named argument syntax because Java bytecode does not always preserve the names of function parameters.

Unit-returning functions

If a function does not return a useful value, its return type is Unit . Unit is a type with only one value — Unit . This value does not have to be returned explicitly:

The Unit return type declaration is also optional. The above code is equivalent to:

Single-expression functions

When the function body consists of a single expression, the curly braces can be omitted and the body specified after an = symbol:

Explicitly declaring the return type is optional when this can be inferred by the compiler:

Explicit return types

Functions with block body must always specify return types explicitly, unless it’s intended for them to return Unit , in which case specifying the return type is optional.

Kotlin does not infer return types for functions with block bodies because such functions may have complex control flow in the body, and the return type will be non-obvious to the reader (and sometimes even for the compiler).

Variable number of arguments (varargs)

You can mark a parameter of a function (usually the last one) with the vararg modifier:

In this case, you can pass a variable number of arguments to the function:

Inside a function, a vararg -parameter of type T is visible as an array of T , as in the example above, where the ts variable has type Array .

Only one parameter can be marked as vararg . If a vararg parameter is not the last one in the list, values for the subsequent parameters can be passed using named argument syntax, or, if the parameter has a function type, by passing a lambda outside the parentheses.

When you call a vararg -function, you can pass arguments individually, for example asList(1, 2, 3) . If you already have an array and want to pass its contents to the function, use the spread operator (prefix the array with * ):

If you want to pass a primitive type array into vararg , you need to convert it to a regular (typed) array using the toTypedArray() function:

val a = intArrayOf(1, 2, 3) // IntArray is a primitive type array val list = asList(-1, 0, *a.toTypedArray(), 4)

Infix notation

Functions marked with the infix keyword can also be called using the infix notation (omitting the dot and the parentheses for the call). Infix functions must meet the following requirements:

• They must be member functions or extension functions.
• They must have a single parameter.
• The parameter must not accept variable number of arguments and must have no default value.

infix fun Int.shl(x: Int): Int < . >// calling the function using the infix notation 1 shl 2 // is the same as 1.shl(2)

Infix function calls have lower precedence than arithmetic operators, type casts, and the rangeTo operator. The following expressions are equivalent:

• 1 shl 2 + 3 is equivalent to 1 shl (2 + 3)
• 0 until n * 2 is equivalent to 0 until (n * 2)
• xs union ys as Set is equivalent to xs union (ys as Set)

On the other hand, an infix function call’s precedence is higher than that of the boolean operators && and || , is — and in -checks, and some other operators. These expressions are equivalent as well:

Note that infix functions always require both the receiver and the parameter to be specified. When you’re calling a method on the current receiver using the infix notation, use this explicitly. This is required to ensure unambiguous parsing.

Function scope

Kotlin functions can be declared at the top level in a file, meaning you do not need to create a class to hold a function, which you are required to do in languages such as Java, C#, and Scala (top level definition is available since Scala 3). In addition to top level functions, Kotlin functions can also be declared locally as member functions and extension functions.

Local functions

Kotlin supports local functions, which are functions inside other functions:

A local function can access local variables of outer functions (the closure). In the case above, visited can be a local variable:

Member functions

A member function is a function that is defined inside a class or object:

Member functions are called with dot notation:

For more information on classes and overriding members see Classes and Inheritance.

Generic functions

Functions can have generic parameters, which are specified using angle brackets before the function name:

For more information on generic functions, see Generics.

Tail recursive functions

Kotlin supports a style of functional programming known as tail recursion. For some algorithms that would normally use loops, you can use a recursive function instead without the risk of stack overflow. When a function is marked with the tailrec modifier and meets the required formal conditions, the compiler optimizes out the recursion, leaving behind a fast and efficient loop based version instead:

val eps = 1E-10 // «good enough», could be 10^-15 tailrec fun findFixPoint(x: Double = 1.0): Double = if (Math.abs(x — Math.cos(x)) < eps) x else findFixPoint(Math.cos(x))

This code calculates the fixpoint of cosine, which is a mathematical constant. It simply calls Math.cos repeatedly starting at 1.0 until the result no longer changes, yielding a result of 0.7390851332151611 for the specified eps precision. The resulting code is equivalent to this more traditional style:

To be eligible for the tailrec modifier, a function must call itself as the last operation it performs. You cannot use tail recursion when there is more code after the recursive call, within try / catch / finally blocks, or on open functions. Currently, tail recursion is supported by Kotlin for the JVM and Kotlin/Native.

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What is infix in kotlin

Инфиксная нотация представляет помещение оператора или функции перед операндами или аргументами. Для определения инфиксной функции вначале ее определения указывается ключевое слово infix :

infix fun название_функции(параметр: тип_параметра): тип_возвращаемого_значения< // действия функции >

Инфиксная функция должна принимать только один параметр. При этом параметр не должен иметь значение по умолчанию и не должен представлять неопределенный набор значений.

Есть два способа определения инфиксной функции: либо внутри класса, либо как функции расширения.

Определим вначале внутри класса:

fun main() < val acc = Account(1000) acc put 150 // равноценно вызову acc.put(150) acc.printSum() // 1300 >class Account(var sum: Int) < infix fun put(amount: Int)< sum = sum + amount >fun printSum() = println(sum) >

Здесь определен класс Account — класс банковского счета, который через конструктор принимает начальную сумму на счете. С помощью инфиксной функции put() определяем добавление на счет суммы, переданной через параметр функции.

Вызов функции выглядит следующим образом:

Первый параметр (здесь переменная acc ) представляет объект, который вызывает функцию. А второй параметр — данные, которые непосредственно будут передаваться инфиксной функции через ее параметр. То есть данный вызов фактически аналогичен вызову:

Также инфиксная функция может определяться как функция расширения. Например, перепишем выше использованную функцию put() в виде функции расширения:

fun main() < val acc = Account(1000) acc put 150 acc.put(150) acc.printSum() // 1300 >infix fun Account.put(amount: Int) < this.sum = this.sum + amount >class Account(var sum: Int)

Стоит отметить, что функция расширения в отличие от функции внутри класса имеет доступ только тем свойствам, которые являются публичными.

Однако использование функций расширений позволяет добавить инфиксные функции к уже существующим типам. Например, определим инфиксную функцию для подсчета частоты символа в строке:

fun main() < val hello = "hello world" val lCount = hello wordCount 'l' val oCount = hello wordCount 'o' println(lCount) // 3 println(oCount) // 2 >infix fun String.wordCount(c: Char) : Int < var count = 0 for(n in this)< if(n == c) count++ >return count >

Здесь функция wordCount проходит по всем символам строки и подсчитывает, сколько раз встречается символ, передаваемый через параметр функции. Результат возвращается функцией. Затем мы можем применить инфиксную нотацию:

val lCount = hello wordCount 'l'

Поскольку функция возвращает результат типа Int, то мы можем получить этот результат в переменную.

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