User Input in Dart

Reading from stdin

In Dart, user input from the command line is read using stdin.readLineSync() from the dart:io library. This reads one line of text entered by the user.

import 'dart:io';

void main() {
  stdout.write('Enter your name: ');  // write without newline
  String? name = stdin.readLineSync();
  print('Hello, $name!');
}

Reading a String

import 'dart:io';

void main() {
  print('What is your favourite language?');
  String? lang = stdin.readLineSync();
  print('Great choice: $lang!');
}

Reading a Number

User input is always a String. You must parse it to use it as a number:

import 'dart:io';

void main() {
  stdout.write('Enter your age: ');
  String? input = stdin.readLineSync();
  int age = int.parse(input!);   // parse to int

  if (age >= 18) {
    print('You are an adult.');
  } else {
    print('You are a minor.');
  }
}

Null Safety with Input

Since readLineSync() returns String?, handle null values safely:

import 'dart:io';

void main() {
  stdout.write('Enter a number: ');
  String? input = stdin.readLineSync();

  // Safe approach with null check
  if (input == null || input.isEmpty) {
    print('No input provided.');
    return;
  }

  int? number = int.tryParse(input);
  if (number == null) {
    print('Invalid number!');
  } else {
    print('Double: $${number * 2}');
  }
}

Full Example

import 'dart:io';

void main() {
  // Get student info
  stdout.write('Enter student name: ');
  String name = stdin.readLineSync() ?? 'Unknown';

  stdout.write('Enter score (0-100): ');
  String? scoreInput = stdin.readLineSync();
  double score = double.tryParse(scoreInput ?? '') ?? 0.0;

  // Calculate grade
  String grade;
  if (score >= 90) grade = 'A';
  else if (score >= 80) grade = 'B';
  else if (score >= 70) grade = 'C';
  else if (score >= 60) grade = 'D';
  else grade = 'F';

  print('\n--- Result ---');
  print('Student: $name');
  print('Score: $score');
  print('Grade: $grade');
}

Enter student name: Alice
Enter score (0-100): 88

--- Result ---
Student: Alice
Score: 88.0
Grade: B

⚠️ Common Mistakes

• Forgetting import 'dart:io'; — stdin is not available without it.
• Not parsing the input before arithmetic: int age = stdin.readLineSync(); is a type error.
• Using input! (null assertion) without checking if input is null — causes a runtime error.

🧠 Quiz

Summary

Read user input in Dart using stdin.readLineSync() from dart:io. It returns a nullable String?, so always handle potential null values. Parse to numbers with int.parse() or safe int.tryParse(). Use stdout.write() for prompts without newlines.