Dart Lesson 13 of 102 2 min read

Reading User Input in Dart with stdin

Read text and numbers typed by the user in a Dart console program using stdin.readLineSync, with safe parsing and validation.

So far your programs have only talked. To listen, a console program reads from standard input using the dart:io library.

dart:io does not work in DartPad or in the browser. Save these examples to a file and run them with dart run file.dart.

Reading a line of text #

import 'dart:io';

void main() {
  stdout.write('What is your name? ');
  String? name = stdin.readLineSync();

  print('Nice to meet you, $name!');
}

What is your name? Priya
Nice to meet you, Priya!

Two things to notice:

• stdout.write prints without a new line, so the cursor stays next to the question.
• readLineSync returns String?. The ? means the result can be null, which happens when there is no more input.

Dealing with the possible null #

Give a fallback with ?? so the rest of your code has a real String.

import 'dart:io';

void main() {
  stdout.write('City: ');
  final city = stdin.readLineSync()?.trim() ?? '';

  if (city.isEmpty) {
    print('You did not type anything.');
  } else {
    print('$city sounds lovely.');
  }
}

Reading a number #

Input always arrives as text. Convert it with tryParse and check the result.

import 'dart:io';

void main() {
  stdout.write('Enter your age: ');
  final age = int.tryParse(stdin.readLineSync() ?? '');

  if (age == null) {
    print('That is not a whole number.');
  } else {
    print('In 10 years you will be ${age + 10}.');
  }
}

Enter your age: 24
In 10 years you will be 34.

Asking until the answer is valid #

import 'dart:io';

int askForNumber(String question) {
  while (true) {
    stdout.write(question);
    final value = int.tryParse(stdin.readLineSync() ?? '');
    if (value != null) return value;
    print('Please type a whole number.');
  }
}

void main() {
  final a = askForNumber('First number: ');
  final b = askForNumber('Second number: ');
  print('$a + $b = ${a + b}');
}

First number: ten
Please type a whole number.
First number: 10
Second number: 5
10 + 5 = 15

Reading several values on one line #

import 'dart:io';

void main() {
  stdout.write('Enter numbers separated by spaces: ');
  final line = stdin.readLineSync() ?? '';

  final numbers = line
      .split(' ')
      .map(int.tryParse)
      .whereType<int>()
      .toList();

  print('Sum: ${numbers.fold(0, (a, b) => a + b)}');
}

Try it yourself #

Write a tip calculator. Ask for the bill amount and the tip percentage, then print the tip and the total with two decimal places. Make sure it does not crash when the user types letters.