Strings & Numbers

Real-world analogy

What is it?

Strings hold text and have powerful methods for searching, transforming, and combining text. String interpolation ($variable) lets you embed values inside strings. Numbers include int (whole) and double (decimal) with full arithmetic support. Dart also provides dart:math for advanced math operations.

Real-world relevance

String manipulation is everywhere: displaying user names, formatting prices, parsing API responses, validating email addresses. Number operations handle everything from shopping cart totals to game scores to GPS coordinates. These are the building blocks of every app.

Key points

• String Interpolation with $ — Use $variableName inside a string to insert a variable's value. For expressions, use ${expression}. This is much cleaner than joining strings with the + operator. Dart developers use interpolation everywhere.
• Multiline Strings — Use triple quotes (single or double) to write strings that span multiple lines. This is great for long text, JSON templates, or formatted output. The line breaks are preserved in the output.
• String Methods — Strings have many built-in methods: toUpperCase(), toLowerCase(), trim(), split(), contains(), startsWith(), endsWith(), replaceAll(), substring(), padLeft(), and more. These let you transform and search text easily.
• Substring and IndexOf — substring() extracts part of a string by position. indexOf() finds where a character or word first appears. Together, they let you locate and extract specific parts of text, like finding a username in an email.
• Arithmetic Operators — Dart supports +, -, *, / (division returns double), ~/ (integer division), and % (remainder/modulo). These work on int and double. Integer division (~/) is unique to Dart -- it divides and rounds down to a whole number.
• Number Parsing and Conversion — Convert strings to numbers with int.parse() and double.parse(). Convert numbers to strings with .toString() and .toStringAsFixed(). Use tryParse() for safe parsing that returns null instead of crashing on bad input.
• Math Operations — The dart:math library provides advanced math: sqrt(), pow(), min(), max(), and the Random class. Import it with 'import dart:math;' to access these tools.
• Number Properties — Numbers have useful properties: isEven, isOdd, isNegative, isNaN, isFinite, isInfinite. These return true or false and help you check number characteristics without writing comparison logic yourself.
• String Comparison and Equality — Compare strings with == (equal) and != (not equal). Use compareTo() for alphabetical ordering. String comparison in Dart is case-sensitive, so 'Hello' and 'hello' are NOT equal.

Code example

import 'dart:math';

void main() {
  // String interpolation
  var name = 'Flutter';
  var version = 3;
  print('Welcome to $name $version!');
  print('Name has ${name.length} letters');

  // String methods
  var email = 'User@Example.COM';
  print(email.toLowerCase());     // user@example.com
  print(email.contains('@'));     // true
  print(email.split('@'));        // [User, Example.COM]

  // Multiline string
  var message = '''
Dear $name,
You are version $version.
Keep being awesome!''';
  print(message);

  // Number arithmetic
  int apples = 10;
  int friends = 3;
  print('Each gets: ${apples ~/ friends} apples');  // 3
  print('Leftover: ${apples % friends}');            // 1

  // Double for precision
  double price = 29.99;
  double tax = 0.08;
  double total = price + (price * tax);
  print('Total: $${total.toStringAsFixed(2)}');     // $32.39

  // Parsing strings to numbers
  var input = '42';
  var number = int.parse(input);
  print('Double it: ${number * 2}');                 // 84

  // Random numbers
  var rng = Random();
  var dice = rng.nextInt(6) + 1;  // 1-6
  print('You rolled a $dice!');
}

Line-by-line walkthrough

1. 1. Importing the dart:math library for math functions
2. 2.
3. 3. The main function starts
4. 4. A comment for string interpolation section
5. 5. Creating a String variable 'name' set to 'Flutter'
6. 6. Creating an int variable 'version' set to 3
7. 7. Printing a message with both variables embedded using $ syntax
8. 8. Using ${} with an expression to print the name's length
9. 9.
10. 10. A comment for string methods
11. 11. An email string with mixed case for demonstration
12. 12. toLowerCase() converts the entire string to lowercase
13. 13. contains() checks if '@' exists in the string -- returns true
14. 14. split('@') breaks the string into a list at the @ symbol
15. 15.
16. 16. A comment for multiline strings
17. 17. Triple quotes begin a multiline string
18. 18. The $name variable is interpolated inside the multiline string
19. 19. The $version variable is also embedded
20. 20. The closing triple quotes end the multiline string
21. 21. Printing the multiline message
22. 22.
23. 23. A comment for number arithmetic
24. 24. Declaring apples as int 10
25. 25. Declaring friends as int 3
26. 26. Integer division ~/ gives 3 (10 divided by 3, no decimal)
27. 27. The % modulo operator gives remainder 1 (10 mod 3)
28. 28.
29. 29. A comment for doubles
30. 30. A price as a double
31. 31. A tax rate as a double
32. 32. Calculating total by adding price plus price times tax
33. 33. Printing the total formatted to 2 decimal places with a dollar sign
34. 34.
35. 35. A comment for parsing
36. 36. A string variable that looks like a number
37. 37. int.parse() converts the string '42' to the integer 42
38. 38. Multiplying the parsed number by 2 and printing the result
39. 39.
40. 40. A comment for random numbers
41. 41. Creating a Random number generator
42. 42. Generating a random integer from 1 to 6 (like a dice roll)
43. 43. Printing the dice result

Spot the bug

void main() {
  var price = '19.99';
  var quantity = 3;
  var total = price * quantity;
  print('Total: $total');
}

Explain like I'm 5

Fun fact

Hands-on challenge

More resources

• Dart Strings (Dart Official)
• Dart Numbers (Dart Official)
• dart:math Library (Dart API)