💬 Lesson 2: User Input & Interactivity

⏱ 25-35 min 📊 Beginner 🏗️ Project-Based 🔗 Official Source

🔄

Quick Review: What We've Learned

Before diving into user input, let's quickly review what you already know!

What function starts every Dart program?

🎯

Today's Mission: "Smart Assistant CLI"

We'll build an interactive command-line assistant that:

🤝 Greets users and remembers their name
🧮 Performs calculations on demand
🎲 Generates random numbers and facts
📝 Takes notes and displays them
🔄 Runs until the user decides to quit

Step 1

Reading User Input with stdin

🎤 How Programs "Listen" to Users

Think of your program as a conversation. You need two things:

📢

stdout.write()

Your program "speaks" - asks questions, shows prompts

stdout.write('What is your name? ');

👂

stdin.readLineSync()

Your program "listens" - waits for the user to type and press Enter

String? name = stdin.readLineSync();

🎮 Try It: Input Simulator

What is your name?

📤 Program Response:

Waiting for input...

🔧 Live Code Editor

import 'dart:io';

void main() {
  stdout.write('What is your name? ');
  String? name = stdin.readLineSync();
  print('Hello, $name! 👋');
}

🔍 What this does:

import 'dart:io'; - Brings in input/output tools
stdout.write() - Asks a question (no new line)
stdin.readLineSync() - Waits for you to type
String? - The ? means it might be empty/null

Step 2

Understanding Null Safety

🛡️ The Null Safety Shield

Dart protects you from "null errors" - one of the most common bugs in programming!

✅ Safe: Using tryParse

int? age = int.tryParse('25');
Result: age = 25 (not null!)

⚠️ Handled: Null-Aware Operator

String name = stdin.readLineSync() ?? 'Guest';

If input is empty, uses 'Guest' instead

❌ Dangerous: Direct Parse

int age = int.parse(input);

CRASHES if input isn't a number!

🎮 Null Safety Playground

Try converting this to a number:

int.parse('abc'): 💥 Throws error!

int.tryParse('abc'): ✅ Returns null safely

Step 3

Converting Text to Numbers

🔄 The Number Factory

Users type text, but math needs numbers. Let's see how conversion works!

Input String

→

int.tryParse()

⚙️

→

Output Number

Type: int

✏️ Interactive Parser Tester

Test Input:

Success! 3.14 × 2 = 6.28

Step 4

Building Interactive Menus

🏗️ Build Your Own Menu

Menu Logic: while + switch

Start Loop

↓

Display Menu

↓

Get User Choice

↓

switch(choice)

↓

Execute Option

↓

Still running?

🎮 Live Menu Simulator

🏆

Build: Smart Assistant CLI

📋 Your Mission

Combine everything to build a complete interactive assistant!

Persistent menu that loops until exit

Greeting that remembers user's name

Calculator with error handling

Random number/joke generator

🧩 Code Assembly Area

Drag and drop code blocks to build the complete program!

                                    import 'dart:io';
                                    import 'dart:math';
                                

                                    void main() {
                                      bool running = true;
                                

                                    while (running) {
                                      print('\n=== ASSISTANT ===');
                                      print('1. Greet');
                                      print('2. Calculate');
                                      print('3. Random');
                                      print('4. Exit');
                                

                                    String? choice = stdin.readLineSync();
                                    switch (choice) {
                                      case '1': // Greet logic
                                      case '2': // Calculate logic
                                      case '3': // Random logic
                                      case '4': running = false;
                                    }
                                

📦 Drop code blocks here to assemble your program

🎮 Challenge: Input Master Quiz

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Streak: 🔥 0

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