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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.

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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

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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.

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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
}

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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)
}

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Working with Large Numbers

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

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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.

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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

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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)
}

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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
}

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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');
}

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Practical Use Cases

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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');
  }
}

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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
}

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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);
}

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Conversion Methods Summary

FromToMethodExample
Stringintint.parse()int.parse('42') → 42
Stringint (safe)int.tryParse()int.tryParse('42') → 42
Stringdoubledouble.parse()double.parse('3.14') → 3.14
intString.toString()42.toString() → '42'
intdouble.toDouble()42.toDouble() → 42.0
doubleString.toString()3.14.toString() → '3.14'
doubleint.toInt()3.14.toInt() → 3
doubleint (round).round()3.14.round() → 3
doubleint (floor).floor()3.9.floor() → 3
doubleint (ceil).ceil()3.1.ceil() → 4

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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');
  }
}

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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

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Next Steps

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

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