readLines

Learn to read lines with File.readLines.

readLines: List of lines

readLines() reads the file line by line and returns each line as a String inside a List<String>. Line break characters (\n) are stripped. This makes it easy to process lines one by one.

val f = File("meyveler.txt")
f.writeText("apple\nbanana\ncherry")
val lines = f.readLines()
println(lines.size)   // 3
println(lines[0])     // apple

readLines loads all lines into memory; it suits small and medium files. For very large files prefer useLines for stream-based reading. The returned list is immutable; to modify it use toMutableList().

Looping over lines

You can loop over the list returned by readLines with a for loop to visit each line. This is a common pattern to process each line separately (for example numbering or filtering). Use forEachIndexed to also get the index.

val lines = f.readLines()
for ((i, line) in lines.withIndex()) {
    println("${i + 1}: $line")
}

Here each line is printed with its number in front. The withIndex() method turns the list into index-value pairs. This pattern is very useful when processing log files or reports.

Exercise Read lines and print them

Create val f = File("meyveler.txt"). Write three lines with f.writeText("apple\nbanana\ncherry"). Read the lines with val lines = f.readLines(). Print the count with println(lines.size), then loop with for (line in lines) and println each line. Add the import java.io.File import.

Show example solution

fun main() {
    val f = File("meyveler.txt")
    f.writeText("apple\nbanana\ncherry")
    val lines = f.readLines()
    println(lines.size)
    for (line in lines) println(line)
}

Test yourself

Match the readLines concepts

readLines()

Reads lines
List<String>

Return type
lines.size

Line count
lines[0]

First line

Read the lines

val lines = f.____()

Show answer

val lines = f.readLines()

What type does readLines() return?
- String
- Array<Byte>
- Map<String, Int>
- List<String>
Show answer

List<String> — readLines returns a List<String> with each line as an element.

Next lesson: appendText

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