string_templates.md

Theory: String templates

Sometimes you need to put values of variables in a text. Lucky for you, Kotlin can help you with that. Meet the string templates. In this topic, we will discuss their purpose.

Suppose we need to display a message about today's temperature in Celsius:

Now, the temperature in ... is ... degrees Celsius.

Instead of ... we need to display certain values.

If we have two variables city and temp, we can build the resulting string with concatenations:

val city = "Paris"
val temp = "24"

println("Now, the temperature in " + city + " is " + temp + " degrees Celsius.")

It is a simple solution. But perfect? No. It's rather awkward.

Kotlin provides a more convenient way to do the same thing using string templates. How do they work? To add a variable value to a string, write the dollar sign $ before a variable name:

val city = "Paris"
val temp = "24"

println("Now, the temperature in $city is $temp degrees Celsius.")

The code becomes more readable. Both code snippets print the same message:

Now, the temperature in Paris is 24 degrees Celsius.

If we do not want to print a string, we can create a variable:

val value = "55"
val currency = "dollars"
val price = "$value $currency" // "55 dollars"

We recommend using string templates to build strings with variable values. Try using it instead of string concatenation.

• String 사이에 변수를 끼워넣고 싶을 때 자바에서는 '+' 와 같은 연산을 통해 문자열을 이어주는 식으로 사용한다.
• 이 방법은 읽기에 약간 불편하다 코틀린은 $변수이름 이 방식을 통해서 변수 사이에 데이터를 출력하는게 가능하다. 읽는데 많은 도움을 준다.

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Templates for expressions

You can use string templates to put the result of an arbitrary expression in a string. To do that, include the entire expression in curly braces {...} after the dollar sign $. For example:

val language = "Kotlin"
println("$language has ${language.length} letters in the name")

It prints:

Kotlin has 6 letters in the name

Here {language.length} is an expression that will be evaluated. If you skip the braces, it will output something different:

Kotlin has Kotlin.length letters in the name

So, always use curly braces for expressions in string templates to avoid these mistakes. Do not add them if you want to output only a variable value even though it does work.

• String template 안에서 단순히 변수 출력이 아니라 표현식을 사용하고 싶다면 ${} (괄호까지 포함된 식) 을 이용하면 된다.

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Idiom

Idioms are templates for clear and readable code. These patterns clarify the code for other people, so it is a good idea to use them. All idioms are endorsed by the community, so using them will bring you closer to the genuine Kotlin-style code. You can find an exhaustive list of idioms at Kotlin docs.

The idiom we'll look at is the string interpolation:

val language = "Kotlin"
println("Have a nice $language!")        // nice code
println("Have a nice " + language + "!") // bad practice

• 관용구를 사용하자.