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Theory: Increment and decrement

Assignment operations

You already know that Kotlin supports basic arithmetic operations such as addition and subtraction:

var a = 3
a = a + 1 // 4
a = a - 1 // 3

Besides this, there are assignment operations that combine arithmetic operations and assignments. The assignment operator = has several forms that combine it with an operation to avoid repeating the variable twice:

  • += assignment after addition: A += B equals A = A + B
  • -= assignment after subtraction: A -= B equals A = A - B
  • *= assignment after multiplication: A *= B equals A = A * B
  • /= assignment after division: A /= B equals A = A / B
  • %= assignment of the remainder after division: A %= B equals A = A % B

Let's take a look at assignment operations performed on a single variable. Here is something very basic:

var a = 3
a += 2 // 5
a -= 2 // 3
a *= 2 // 6
a /= 2 // 3
a %= 2 // 1

Here, we performed calculations with number 2 and our variable a and then assigned the values to it. As you can see, these operations made the code shorter and clearer.

  • Kotlin 에서도 일반 프로그래밍 언어에서 지원하는 Assignment Operations 을 지원한다. (대입 연산 + 산술 연산을 합친 것)

Using increment and decrement

Another common operation is increasing or decreasing a number by one. Of course, you can use += 1 or -= 1, but Kotlin provides an even better way to do this: increment and decrement operations. Let's look at an example:

var num = 3
num++  // 4, increment
num--  // 3, decrement

The code above is actually the same as below:

var num = 3
num += 1  // 4
num -= 1  // 3

As you can see, increment ++ does the same as +=1 but in a simpler way. The same is true for decrement --.

Note that this works only when you are increasing or decreasing a number by one.

This looks easy, but increment and decrement operations are more complicated than you might think. Both increment and decrement operators have two forms that are very important to distinguish: prefix and postfix.

  • 값 1만 증가시키거나 1만 감소시키고 싶다면 ++ 나 -- 연산자를 이용하면 된다. (이걸 더 추천한다. 더 이해하기가 쉬워서인듯.)
  • 이런 연산자에서 중요한 점은 숫자 앞에 붙는지 (prefix) 로 쓰는지, 숫자 뒤에 쓰는지 (postfix) 로 쓰는지가 중요하다.

Prefix form

Prefix form changes the value of a variable before it is used. Let's look at some examples.

Prefix increment returns the incremented value:

var a = 10
val b = ++a
println(a)  // a = 11
println(b)  // b = 11

First, the value of the variable a is increased by one, and then its value is assigned to the variable b. So, a and b are both 11.

Prefix decrement returns the following:

var a = 10
val b = --a
println(a)  // a = 9
println(b)  // b = 9

Here, you see the same thing happening: the value of the variable a is decreased by one, and then its value is assigned to the variable b.

  • Prefix 로 ++ 나 -- 를 사용한다면 실제 사용하기 전에 값이 1이 더해지거나 빼지거나 되어 있다는 점이다.

Postfix form

By contrast, postfix form changes the value of a variable after it is used. Let's look at the examples.

Postfix increment returns the value before incrementing by one:

var a = 10
val b = a++
println(a)  // a = 11
println(b)  // b = 10

First, the value of the variable a is assigned to the variable b, and then the value of the variable a is increased by one. This way, a is 11 and b is 10.

Similarly, postfix decrement returns the following:

var a = 10
val b = a--
println(a)  // a = 9
println(b)  // b = 10
  • Postfix 는 Prefix 와는 반대로 값을 사용하고나서 업데이트를 하는 식으로 사용된다.

Order of precedence

Some operations take precedence over others, that is, they are performed first. Take a look at the list of operations in decreasing order of priority:

  1. Parentheses ( (expr) );
  2. Postfix increment/decrement ( expr++, expr--);
  3. Unary plus/minus, prefix increment/decrement ( -expr, ++expr, --expr );
  4. Multiplication, division, and modulus ( *, /, % );
  5. Addition and subtraction ( +, - );
  6. Assignment operations ( =, +=, -=, *=, /=, %= ).

The priority of operations should be taken into account when executing a set of arithmetic expressions:

val a = 2
var b = 3
val c = a + 4 * --b  
println(c)   // this is 10

Decrement has a higher priority than multiplication and addition, so --b is calculated first. Like in arithmetic, parentheses can be used to increase the priority of operation. You can also use them for clarity:

var a = 5
val b = 9
val c = 3
val d = a++ + (b / 2) - c - 4
println(d)   // this is 2
  • 연산도 우선순위가 있다라는 것.
  • Postfix 가 2번째의 우선순위를 가지고 있는데 postfix 는 값을 반환하고 나서 업데이트 시키므로 연산식 안에서는 값을 증가시켜 놓지는 않는다. 이를 알아두자.
  • 반대로 Prefix 는 값을 업데이트 하는게 먼저이므로 업데이트 된 값을 사용한다.