Dart Stdin
last modified April 4, 2025
The Stdin class in Dart provides functionality to read input from
the standard input stream (typically the console). It's part of Dart's
dart:io library and is essential for console applications.
Stdin offers synchronous and asynchronous methods for reading input. It can read bytes, lines, or transform input using various encodings.
Basic Definition
Stdin represents the standard input stream of a Dart process.
It's accessed through stdin property of the dart:io
library's top-level functions.
Key features include line reading, byte reading, encoding support, and asynchronous operations. It's crucial for interactive console applications.
Reading a Single Line
This example shows how to read a single line of text from the console.
import 'dart:io';
void main() {
stdout.write('Enter your name: ');
String name = stdin.readLineSync() ?? 'Unknown';
print('Hello, $name!');
}
We use readLineSync() to read a line synchronously. The method
returns null if EOF is reached, so we provide a default value. stdout.write
prints without a newline.
$ dart main.dart Enter your name: John Hello, John!
Reading Numbers
This example demonstrates reading and parsing numeric input.
import 'dart:io';
void main() {
stdout.write('Enter first number: ');
var num1 = int.tryParse(stdin.readLineSync() ?? '0') ?? 0;
stdout.write('Enter second number: ');
var num2 = int.tryParse(stdin.readLineSync() ?? '0') ?? 0;
print('Sum: ${num1 + num2}');
}
We read strings and convert them to integers. tryParse handles
invalid input by returning null, so we provide a fallback value. This makes
the program more robust against bad input.
$ dart main.dart Enter first number: 12 Enter second number: 7 Sum: 19
Asynchronous Line Reading
This example shows how to read input asynchronously.
import 'dart:io';
Future<void> main() async {
stdout.write('Enter your favorite color: ');
var color = await stdin.readLine();
stdout.write('Enter your age: ');
var age = await stdin.readLine();
print('$age-year-old who loves $color');
}
We use readLine() which returns a Future<String>.
The await keyword pauses execution until input is received. This
approach is non-blocking and better for complex applications.
$ dart main.dart Enter your favorite color: blue Enter your age: 25 25-year-old who loves blue
Reading Raw Bytes
This example demonstrates reading raw bytes from standard input.
import 'dart:io';
void main() {
stdout.writeln('Press any key to continue...');
var byte = stdin.readByteSync();
print('You pressed: $byte (${String.fromCharCode(byte)})');
}
readByteSync() reads a single byte from input. We convert the
byte to its character representation. This is useful for low-level input
processing or single-key responses.
$ dart main.dart Press any key to continue... a You pressed: 97 (a)
Reading Until EOF
This example shows how to read multiple lines until end-of-file.
import 'dart:io';
void main() {
print('Enter multiple lines (Ctrl+D to end):');
var lines = <String>[];
while (true) {
var line = stdin.readLineSync();
if (line == null) break;
lines.add(line);
}
print('You entered ${lines.length} lines:');
lines.forEach(print);
}
We read lines in a loop until readLineSync() returns null (EOF).
The collected lines are stored in a list. This pattern is useful for processing
piped input or multi-line user input.
$ dart main.dart Enter multiple lines (Ctrl+D to end): hello world dart You entered 3 lines: hello world dart
Best Practices
- Error handling: Always validate and sanitize user input
- Async preference: Use async methods in larger applications
- Encoding: Specify encoding when working with non-ASCII text
- Prompting: Always provide clear input prompts
Source
This tutorial covered Dart's Stdin class with practical examples showing basic input reading, numeric parsing, async operations, and byte-level access.
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