stdin/stdout
- Dart course
- Advanced
- Lesson 57/83
- 5 min
Learn I/O with stdin and stdout.
stdin and stdout
stdin and stdout are the input/output objects in Dart's dart:io library. stdin.readLineSync() reads a line from the user; stdout.writeln('msg') prints a line to the screen.
Example: stdout.write('Name: '); final name = stdin.readLineSync(); stdout.writeln('Hello $name');. This asks the user for a name and prints a greeting. It is used for interactive input in CLI applications. readLineSync is synchronous; it waits until a line arrives.
Interactive input
A CLI tool talks to the user: it asks a question, waits for an answer, then acts. You print the question with stdout.write('Question: ') and read the answer with stdin.readLineSync(). The answer comes as a String? so you can give a default with ??.
Example: final name = stdin.readLineSync() ?? 'Guest';. If the user enters nothing or input ends, 'Guest' is used. This makes CLI tools resilient; unexpected input becomes a default instead of a crash.
Practice Print with stdout
Add import 'dart:io';. Define a function readName that calls stdout.write('Name: ') and returns stdin.readLineSync() ?? 'Guest'. In main set final name = 'Ayşe'; and print with stdout.writeln('Hello $name').
Starter code
// import the dart:io library
// define readName (write output + read input)
void main() {
// print output with a fixed name
}Sample solution
import 'dart:io';
String readName() {
stdout.write('Name: ');
return stdin.readLineSync() ?? 'Guest';
}
void main() {
final name = 'Ayşe';
stdout.writeln('Hello $name');
}Expected output
Hello Ayşe
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Match the stdin/stdout parts
| Term | Meaning |
|---|---|
stdin |
Input |
stdout |
Output |
readLineSync |
Reads a line |
writeln |
Writes a line |
2 What does stdin.readLineSync return?
- double
- bool
- String? Correct answer
- int
Explanation: stdin.readLineSync returns String?.
3 The readLineSync call
String readName() {
stdout.write('Name: ');
return stdin.____() ?? 'Guest';
}String readName() {
stdout.write('Name: ');
return stdin.readLineSync() ?? 'Guest';
}Explanation: stdin.readLineSync() reads a line from the user.