Lesson 5 · 5 of 20

Reading input · Dart

Read what the user types in Dart with stdin.readLineSync() from dart:io and turn it into numbers with int.parse, double.parse and int.tryParse.

Dart · Reading input in DMarlon Studio: the example's code and its output in the terminal

To read from the keyboard you import the `dart:io` library with import 'dart:io'; on the first line. It brings stdin (input) and stdout (output). `stdout.write()` writes like print() but without a line break, which is exactly what you want for a question.

`stdin.readLineSync()` waits for the user to type a line and press Enter, and returns it as text. If there is nothing left to read it returns null, which is why the code says stdin.readLineSync() ?? '': the ?? operator uses an empty string in that case. Lesson 15 explains null and ?? properly.

Whatever is typed is always text, even digits. `int.parse()` turns '29' into the integer 29, which you can then add to, and `double.parse()` does the same for numbers with decimals such as '1.40'. With them the program works out a year of coffee: price times cups times 52 weeks.

If the text is not a number, int.parse raises an error and the program stops. `int.tryParse()` is more careful: it returns null instead of failing. The int? type allows that null, and the conditional operator picks what to print: here seven is not a number.

When it runs, the typing comes from input.txt, one line per question. The answers do not appear in the output because they come from the file, not the screen; in DMarlon Studio you can type them yourself in the console.

Code · main.dart

import 'dart:io';void main() {  // stdout.write prints without a new line, like a question.  stdout.write('What is your name? ');  // readLineSync() reads one line of text (or null if there is none).  String name = stdin.readLineSync() ?? '';  print('Hello, $name!');  // What is typed is always text: int.parse turns it into a number.  stdout.write('How old are you? ');  int age = int.parse(stdin.readLineSync() ?? '0');  print('In ten years you will be ${age + 10}.');  // double.parse for numbers with decimals.  stdout.write('Price of a coffee? ');  double price = double.parse(stdin.readLineSync() ?? '0');  stdout.write('Coffees a week? ');  int cups = int.parse(stdin.readLineSync() ?? '0');  double perYear = price * cups * 52;  print('$name, you spend ${perYear.toStringAsFixed(2)} euros a year on coffee.');  // tryParse gives null instead of failing on bad text.  stdout.write('Favourite number? ');  int? favourite = int.tryParse(stdin.readLineSync() ?? '');  print(favourite == null ? 'That is not a number.' : 'Doubled: ${favourite * 2}');}

Input (what gets typed) · input.txt

Lucia
29
1.40
5
seven

Output · dart run main.dart

What is your name? Hello, Lucia!
How old are you? In ten years you will be 39.
Price of a coffee? Coffees a week? Lucia, you spend 364.00 euros a year on coffee.
Favourite number? That is not a number.

Key points

import 'dart:io'; brings stdin and stdout.
stdin.readLineSync() reads one line (or null).
stdout.write() writes without a line break.
int.parse and double.parse turn text into numbers.
int.tryParse returns null instead of failing.

Exercise

Ask for the width and length of a room in metres (with decimals) and print its area with two decimals.