Type Conversions

Learn how to convert between different data types in Dart

Introduction

Type conversion (also called type casting) is the process of converting a value from one data type to another. In Dart, you’ll often need to convert between types, such as turning a String into an int, or an int into a double.

Dart performs explicit type conversion - you must explicitly convert types using built-in methods. This prevents accidental type-related bugs.

Common Type Conversions

String → int

Parse text to integer

int → String

Convert number to text

int → double

Convert integer to decimal

double → int

Convert decimal to integer

String → double

Parse text to decimal

double → String

Convert decimal to text

String to int Conversion

Use int.parse() to convert a String containing a number into an integer:

void main() {
  String variable = '50';
  
  // Convert String to int
  int numero = int.parse(variable);
  
  print(numero);        // 50 (as integer)
  print(numero + 10);   // 60 (can now do math)
}

int.parse() will throw an error if the string doesn’t contain a valid integer. For example, int.parse('hello') will crash your program.

Safe Parsing with tryParse

For safer parsing that won’t crash your program, use int.tryParse():

void main() {
  String validNumber = '42';
  String invalidNumber = 'hello';
  
  int? result1 = int.tryParse(validNumber);    // 42
  int? result2 = int.tryParse(invalidNumber);  // null
  
  print(result1);  // 42
  print(result2);  // null
}

int to String Conversion

Use .toString() to convert any number to a String:

void main() {
  int numero = 50;
  
  // Convert int to String
  String valor = numero.toString();
  
  print(valor);           // '50' (as String)
  print(valor + ' km');   // '50 km' (string concatenation)
}

Working with Large Numbers

void main() {
  int numeroUno = 123;
  int numeroDos = 6524200889711234;
  
  print(numeroDos);                    // 6524200889711234
  print(numeroDos.toString());         // '6524200889711234'
}

int to double Conversion

Use .toDouble() to convert an integer to a double (decimal number):

void main() {
  int numeroUno = 123;
  
  // Convert int to double
  double dobleUno = numeroUno.toDouble();
  
  print(numeroUno);   // 123
  print(dobleUno);    // 123.0
}

Converting to double is useful when you need decimal precision in calculations, even if your starting value is a whole number.

double to int Conversion

Use .toInt() to convert a double to an integer. This truncates the decimal part (doesn’t round):

void main() {
  double dobleDos = 1234523456243563567.86348623487623465;
  
  // Convert double to int (truncates decimal)
  int entero = dobleDos.toInt();
  
  print(dobleDos);  // 1.2345234562435636e+18
  print(entero);    // 1234523456243563520
}

.toInt() truncates (cuts off) the decimal part, it doesn’t round. For example:

• 3.9.toInt() returns 3 (not 4)
• 3.1.toInt() returns 3
• -3.9.toInt() returns -3

Rounding Doubles to Integers

If you want to round instead of truncate, use these methods:

void main() {
  double number = 3.7;
  
  print(number.toInt());       // 3 (truncate)
  print(number.round());       // 4 (round to nearest)
  print(number.floor());       // 3 (round down)
  print(number.ceil());        // 4 (round up)
}

String to double Conversion

Use double.parse() to convert a String to a double:

void main() {
  String precio = '19.99';
  
  // Convert String to double
  double valor = double.parse(precio);
  
  print(valor);          // 19.99
  print(valor * 2);      // 39.98
}

Complete Conversion Example

Here’s a comprehensive example showing all the common conversions:

void main() {
  // String to int
  String variable = '50';
  int numero = int.parse(variable);
  print('String to int: $numero');
  
  // int to String
  String valor = numero.toString();
  print('int to String: $valor');
  
  // int to double
  int numeroUno = 123;
  double dobleUno = numeroUno.toDouble();
  print('int to double: $dobleUno');
  
  // double to int
  double dobleDos = 1234.56789;
  int entero = dobleDos.toInt();
  print('double to int: $entero');
  
  // String to double
  String textoDecimal = '99.99';
  double precio = double.parse(textoDecimal);
  print('String to double: $precio');
  
  // double to String
  String precioTexto = precio.toString();
  print('double to String: $precioTexto');
}

Practical Use Cases

User Input Conversion

void main() {
  // User inputs are always strings
  String userInput = '25';  // Simulating user input
  
  // Convert to int for calculations
  int edad = int.parse(userInput);
  
  if (edad >= 18) {
    print('You are an adult');
  } else {
    print('You are a minor');
  }
}

Calculations with Mixed Types

void main() {
  int quantity = 5;
  double pricePerUnit = 12.50;
  
  // Convert int to double for precise calculation
  double total = quantity.toDouble() * pricePerUnit;
  
  print('Total: \$${total}');  // Total: $62.5
}

Formatting Numbers for Display

void main() {
  int score = 95;
  double average = 87.5;
  
  // Convert to strings for display
  String message = 'Score: ${score.toString()}, Average: ${average.toString()}';
  print(message);
}

Conversion Methods Summary

From	To	Method	Example
String	int	int.parse()	int.parse('42') → 42
String	int (safe)	int.tryParse()	int.tryParse('42') → 42
String	double	double.parse()	double.parse('3.14') → 3.14
int	String	.toString()	42.toString() → '42'
int	double	.toDouble()	42.toDouble() → 42.0
double	String	.toString()	3.14.toString() → '3.14'
double	int	.toInt()	3.14.toInt() → 3
double	int (round)	.round()	3.14.round() → 3
double	int (floor)	.floor()	3.9.floor() → 3
double	int (ceil)	.ceil()	3.1.ceil() → 4

Error Handling

Always validate string input before parsing to avoid runtime errors:

void main() {
  String input = 'not a number';
  
  // ❌ BAD: Will crash if input is invalid
  // int number = int.parse(input);  // FormatException!
  
  // ✅ GOOD: Safe parsing
  int? number = int.tryParse(input);
  if (number != null) {
    print('Valid number: $number');
  } else {
    print('Invalid input');
  }
}

Key Takeaways

• Use int.parse() or int.tryParse() to convert String to int
• Use double.parse() or double.tryParse() to convert String to double
• Use .toString() to convert any number to String
• Use .toDouble() to convert int to double
• Use .toInt() to convert double to int (truncates decimals)
• Use .round(), .floor(), or .ceil() for rounding doubles
• Always handle potential parsing errors with tryParse() methods

Next Steps

Now that you understand type conversions, you’re ready to explore operators and how to perform calculations and comparisons in Dart.