readLines
- Kotlin course
- Intermediate
- Lesson 97/116
- 5 min
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]) // applereadLines 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.
Practice 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.
Starter code
import java.io.File
// File object for path meyveler.txt
// write "apple\nbanana\ncherry" (three lines)
// read with readLines, print size then each line
fun main() {
// write here
}Sample 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)
}Expected output
3 apple banana cherry
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Match the readLines concepts
| Term | Meaning |
|---|---|
readLines() |
Reads lines |
List<String> |
Return type |
lines.size |
Line count |
lines[0] |
First line |
2 Read the lines
val lines = f.____()val lines = f.readLines()Explanation: readLines returns lines as a List<String>.
3 What type does readLines() return?
- String
- Array<Byte>
- Map<String, Int>
- List<String> Correct answer
Explanation: readLines returns a List<String> with each line as an element.