Strings, templates and escaping

Look at your phone

Open any app and count the things on screen that are text. A greeting with your name in it. A score. A price. A date. A button label. An error message.

Nearly all of them were built — assembled out of a fixed part the programmer wrote and a changing part the app worked out a moment ago. "Hello, " never changes. "Ada" does.

The String is the type that holds text, and this lesson is about building good ones without tying yourself in knots.

Making text

Text goes between double quotes. Always double, never single — single quotes mean Char, exactly one character.

1val greeting = "Hello"
2val empty = ""
3val spaced = "  Ada  "

Two strings can be glued together with +. This is called concatenation:

1val first = "Ada"
2val last = "Lovelace"
3println(first + " " + last)   // Ada Lovelace

That works, and for two pieces it is fine. For four pieces it turns into a hunt for missing spaces, which is why Kotlin gives you something better.

Templates: the dollar sign

Put a $ in front of a name inside a string and Kotlin drops that value straight in:

1val name = "Ada"
2val age = 36
3println("Hello, $name!")
4println("You are $age.")

This is a string template. It prints Hello, Ada! and You are 36.

For anything more than a plain name — a sum, a function call, a lookup — wrap it in curly brackets:

1val age = 36
2println("Next year: ${age + 1}")
3println("Letters: ${name.length}")

The rule is short: $name for a bare name, ${anything} for everything else. When in doubt use the brackets; they are never wrong.

Escaping: getting awkward characters in

Here is a problem. A string is ended by a double quote. So how do you put a double quote inside one?

println("She said "hello" quietly.")

Kotlin reads that as the string "She said ", then some rubbish. The fix is a backslash, which means "do not treat the next character normally":

println("She said \"hello\" quietly.")

A backslash plus a character is an escape sequence. There are five worth memorising:

\n is the useful one. A single println can produce several lines:

println("Line one\nLine two")

And \$ matters the moment you print a price:

1val cost = 5
2println("Cost: \$cost")   // Cost: $5

The first \$ is a real dollar sign. The second $ opens the template gap for cost. Read it slowly once and it stops looking like a cat walked over the keyboard.

When there is a lot of text

For several lines of fixed text, three quote marks in a row let you press Enter instead of writing \n everywhere. Nothing is escaped inside — quote marks are just quote marks:

1val rules = """
2  Roll the dice.
3  Highest score wins.
4  Say "well played" afterwards.
5""".trimIndent()
6println(rules)

trimIndent() removes the leading spaces you added to keep the code tidy, so the output lines up on the left.

Comparing text

Use == to ask whether two strings hold the same characters:

1val a = "ada"
2println(a == "ada")    // true
3println(a == "Ada")    // false

Capitals count. If you want to ignore them, compare in one case:

println(a.uppercase() == "ADA")   // true

Putting it together