Kenhuang Academy
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      • Strings and Interpolation

      String Basics

      Strings in Dart represent sequences of UTF-16 code units. You can create strings using single or double quotes.

      String single = 'Single quotes';
String double = "Double quotes";

      String Interpolation

      String interpolation allows you to embed expressions inside strings using $ or ${}.

      Simple Interpolation

      var name = 'Alice';
var age = 25;

print('My name is $name');  // My name is Alice
print('I am $age years old');  // I am 25 years old

      Expression Interpolation

      Use ${} for expressions:

      var price = 10;
var quantity = 3;

print('Total: \$${price * quantity}');  // Total: $30
print('Next year I will be ${age + 1}');  // Next year I will be 26

      Multi-line Strings

      Triple Quotes

      String multiline = '''
This is a
multi-line
string
''';

String another = """
Another way to
create multi-line
strings
""";

      String Concatenation

      String part1 = 'Hello, ';
String part2 = 'World!';
String combined = part1 + part2;  // Hello, World!

// Adjacent string literals
String message = 'This is '
    'a long string '
    'split across lines';

      Raw Strings

      Raw strings treat backslashes as literal characters:

      String path = r'C:\Users\Documents\file.txt';
print(path);  // C:\Users\Documents\file.txt

String regex = r'\d+';  // Regular expression pattern

      Escape Sequences

      String newline = 'Line 1\nLine 2';
String tab = 'Column1\tColumn2';
String quote = 'She said, "Hello"';
String singleQuote = 'It\'s a nice day';
String backslash = 'Path: C:\\folder\\file';

      Common String Operations

      Length

      String text = 'Hello';
print(text.length);  // 5

      Case Conversion

      String name = 'Alice';
print(name.toUpperCase());  // ALICE
print(name.toLowerCase());  // alice

      Trimming

      String padded = '  Hello  ';
print(padded.trim());       // 'Hello'
print(padded.trimLeft());   // 'Hello  '
print(padded.trimRight());  // '  Hello'

      Checking Content

      String text = 'Hello, World!';

print(text.contains('World'));     // true
print(text.startsWith('Hello'));   // true
print(text.endsWith('!'));         // true
print(text.isEmpty);               // false
print(text.isNotEmpty);            // true

      Substring

      String text = 'Hello, World!';

print(text.substring(0, 5));    // Hello
print(text.substring(7));       // World!

      Splitting and Joining

      String csv = 'apple,banana,orange';
List<String> fruits = csv.split(',');
print(fruits);  // [apple, banana, orange]

String joined = fruits.join(' - ');
print(joined);  // apple - banana - orange

      Replacing

      String text = 'Hello, World!';

print(text.replaceAll('o', '0'));        // Hell0, W0rld!
print(text.replaceFirst('o', '0'));      // Hell0, World!
print(text.replaceRange(7, 12, 'Dart')); // Hello, Dart!

      Index Operations

      String text = 'Hello';

print(text[0]);              // H
print(text.indexOf('l'));    // 2
print(text.lastIndexOf('l')); // 3

      Padding

      String num = '42';

print(num.padLeft(5, '0'));   // 00042
print(num.padRight(5, '0'));  // 42000

      String Comparison

      String str1 = 'apple';
String str2 = 'banana';

print(str1 == str2);           // false
print(str1.compareTo(str2));   // negative (str1 < str2)

      String Buffer

      For efficient string concatenation in loops:

      StringBuffer buffer = StringBuffer();

for (int i = 0; i < 5; i++) {
  buffer.write('Number $i ');
}

String result = buffer.toString();
print(result);  // Number 0 Number 1 Number 2 Number 3 Number 4

      Unicode and Runes

      String emoji = '😀';
print(emoji.length);        // 2 (UTF-16 code units)
print(emoji.runes.length);  // 1 (actual character)

// Iterate over characters
for (var rune in emoji.runes) {
  print(String.fromCharCode(rune));
}

      Regular Expressions

      String text = 'My email is alice@example.com';
RegExp emailRegex = RegExp(r'\b[\w\.-]+@[\w\.-]+\.\w+\b');

if (emailRegex.hasMatch(text)) {
  String? email = emailRegex.stringMatch(text);
  print('Found email: $email');  // Found email: alice@example.com
}

// Replace with regex
String censored = text.replaceAll(emailRegex, '[EMAIL]');
print(censored);  // My email is [EMAIL]

      Best Practices

      1. Use string interpolation instead of concatenation
      2. Use raw strings for paths and regex patterns
      3. Use StringBuffer for building strings in loops
      4. Prefer single quotes for simple strings
      5. Use triple quotes for multi-line strings

      Complete Example

      void main() {
  // Basic strings
  String firstName = 'Alice';
  String lastName = 'Johnson';
  
  // Interpolation
  String fullName = '$firstName $lastName';
  print('Full name: $fullName');
  
  // Multi-line
  String address = '''
  123 Main Street
  New York, NY 10001
  USA
  ''';
  print('Address:$address');
  
  // Operations
  String email = 'ALICE@EXAMPLE.COM';
  print('Lowercase: ${email.toLowerCase()}');
  print('Contains @: ${email.contains('@')}');
  
  // Splitting
  List<String> parts = email.split('@');
  print('Username: ${parts[0]}');
  print('Domain: ${parts[1]}');
  
  // Building strings
  StringBuffer message = StringBuffer();
  message.write('Hello, ');
  message.write(firstName);
  message.write('! ');
  message.write('Welcome to Dart.');
  print(message.toString());
}

      Next Steps