String basics

As you know, Kotlin has a lot of data types such as String, Int, and Double. In this topic, we will focus on the String type. This is a very useful type as even a simple "Hello world!" program uses strings. A string is a sequence of zero or more characters enclosed by double quotes e.g. "John", "". You will surely use strings on a regular basis in your future projects. In this topic, we will look at some important basics of working with strings.

The length of a string

You will often need to get the length of a String. The length of the string is determined by the number of characters that are enclosed between the double quotes. To do this, simply use .length which gives a value of the Int type:

val language = "Kotlin"
println(language.length) // 6

val empty = ""
println(empty.length) // 0

Concatenating strings

Another common operation with String is concatenation. It is used to construct one string out of other strings.

Two strings can be concatenated with the +:

val str1 = "ab"
val str2 = "cde"
val result = str1 + str2 // "abcde"

When we concatenate two strings, a new string is created.

val one = "1"
val two = "2"
val twelve = one + two 
println(one)      // 1, no changes
println(two)      // 2, no changes
println(twelve)   // 12

You can concatenate multiple strings in the same expression:

val firstName = "John"
val lastName = "Smith"
val fullName = firstName + " " + lastName

Appending values to strings

The + also works for appending values of different types to a String. The value is automatically converted to a String and then concatenated to the target String:

val stringPlusBoolean = "abc" + 10 + true
println(stringPlusBoolean) // abc10true

val code = "123" + 456 + "789"
println(code) // 123456789

Take a look at the example below. It wouldn't work because the first operand is a number:

val errorString = 10 + "abc" // an error here!

Let's consider a different situation:

val stringAndNumbers = "abc" + 11 + 22
println(stringAndNumbers) // abc1122

Why did that work? First, it appends 11 to the string "abc", and then it appends 22 to the string "abc11".

You may concatenate a character with a String to get a new String:

val charPlusString = 'a' + "bc"
println(charPlusString) // abc
val stringPlusChar = "de" + 'f'
println(stringPlusChar) // def

Also, you may append any value to the result, because characters plus a string gives you a String:

val charPlusStringPlusInt = 'a' + "bc" + 123
println(charPlusStringPlusInt) // abc123

We will discuss how to work with characters later, but for now, just remember that you can concatenate characters and strings to get a String value. If you are interested in how characters operate with integers, look at this topic.

Repeating the string

If you need to repeat one string two or more times, then hold your loops: Kotlin provides the repeat function:

print("Hello".repeat(4)) // HelloHelloHelloHello

Now, imagine your friend senior developer gave you a secret on how to become the best programmer:

Let's try to transform this piece of paper to a code snippet that prints your schedule for every day of the week:

println("Eat. Sleep. Code.\n".repeat(7)) // \n gives you a line feed (new line)

And here you have your week schedule:

Eat. Sleep. Code.
Eat. Sleep. Code.
Eat. Sleep. Code.
Eat. Sleep. Code.
Eat. Sleep. Code.
Eat. Sleep. Code.
Eat. Sleep. Code.

No time for procrastination!

Raw string

Sometimes you need some special symbols like tabs or quote marks in your string. You can do it with the help of escape sequences. For example:

// prints 'H' is the first letter of "Hello world!" string.
println("\'H\' is the first letter of \"Hello world!\" string.")

This looks a little heavy. If you need to write a fairly large text with newlines and special characters, it can be difficult to read.

For these cases, you can use a raw string. A raw string can contain newlines and any other characters. You just need to wrap the text in triple quotes ("""):

val largeString = """
    This is the house that Jack built.
      
    This is the malt that lay in the house that Jack built.
       
    This is the rat that ate the malt
    That lay in the house that Jack built.
       
    This is the cat
    That killed the rat that ate the malt
    That lay in the house that Jack built.
""".trimIndent() // removes the first and the last lines and trims indents
print(largeString)

This text prints:

This is the house that Jack built.

This is the malt that lay in the house that Jack built.

This is the rat that ate the malt
That lay in the house that Jack built.

This is the cat
That killed the rat that ate the malt
That lay in the house that Jack built.

As you can see, we also used the function .trimIndent(). It cuts off all the lines of the common minimal indent and removes the first and last lines if they are empty. For example:

val unevenString = """
        123
         456
          789""".trimIndent()
print(unevenString)
println()

val rawString = """123
         456
          789
""".trimIndent()
print(rawString )

This code prints:

123
 456
  789

123
         456
          789

Conclusion

Now you know the most basic String operations! These functions are pretty simple but absolutely crucial in programming. There are actually many other useful String operations, but you will learn about them a little later. Good luck with your tasks!