Function Parameter in Dart

Parameter In Dart

The parameter is the process of passing values to the function. The values passed to the function must match the number of parameters defined. A function can have any number of parameters.

// here a and b are parameters
void add(int a, int b) { 
} 

Positional Parameter In Dart

In positional parameters, you must supply the arguments in the same order as you defined on parameters when you wrote the function. If you call the function with the parameter in the wrong order, you will get the wrong result.

Example 1: Use Of Positional Parameter

In the example below, the function printInfo takes two parameters. You must pass the person’s name and gender in the same order. If you pass values in the wrong order, you will get the wrong result.

void printInfo(String name, String gender) {
  print("Hello $name your gender is $gender.");
}

void main() {
  // passing values in wrong order
  printInfo("Male", "John");

  // passing values in correct order
  printInfo("John", "Male");

}

Hello Male your gender is John.
Hello John your gender is Male.

Example 2: Providing Default Value On Positional Parameter

In the example below, function printInfo takes two positional parameters and one optional parameter. The title parameter is optional here. If the user doesn’t pass the title, it will automatically set the title value to sir/ma’am.

void printInfo(String name, String gender, [String title = "sir/ma'am"]) {
  print("Hello $title $name your gender is $gender.");
}

void main() {
  printInfo("John", "Male");
  printInfo("John", "Male", "Mr.");
  printInfo("Kavya", "Female", "Ms.");
}

Hello sir/ma'am John your gender is Male.
Hello Mr. John your gender is Male.
Hello Ms. Kavya your gender is Female.

Example 3: Providing Default Value On Positional Parameter

In the example below, function add takes two positional parameters and one optional parameter. The num3 parameter is optional here with default value 0.

void add(int num1, int num2, [int num3=0]){
   int sum;
  sum = num1 + num2 + num3;
   
   print("The sum is $sum");
}

void main(){
  add(10, 20);
  add(10, 20, 30);
}

The sum is 30
The sum is 60

Named Parameter In Dart

Dart allows you to use named parameters to clarify the parameter’s meaning in function calls. Curly braces {} are used to specify named parameters.

Example 1: Use Of Named Parameter

In the example below, function printInfo takes two named parameters. You can pass value in any order. You will learn about ? in null safety section.

void printInfo({String? name, String? gender}) {
  print("Hello $name your gender is $gender.");
}

void main() {
  // you can pass values in any order in named parameters.
  printInfo(gender: "Male", name: "John");
  printInfo(name: "Sita", gender: "Female");
  printInfo(name: "Reecha", gender: "Female");
  printInfo(name: "Reecha", gender: "Female");
  printInfo(name: "Harry", gender: "Male");
  printInfo(gender: "Male", name: "Santa");
}

Hello John your gender is Male.
Hello Sita your gender is Female.  
Hello Reecha your gender is Female.
Hello Reecha your gender is Female.
Hello Harry your gender is Male.   
Hello Santa your gender is Male.

Example 2: Use Of Required In Named Parameter

In the example below, function printInfo takes two named parameters. You can see a required keyword, which means you must pass the person’s name and gender. If you don’t pass it, it won’t work.

void printInfo({required String name, required String gender}) {
  print("Hello $name your gender is $gender.");
}

void main() {
  // you can pass values in any order in named parameters.
  printInfo(gender: "Male", name: "John");
  printInfo(gender: "Female", name: "Suju");
}

Hello John your gender is Male.
Hello Suju your gender is Female.

Optional Parameter In Dart

Dart allows you to use optional parameters to make the parameter optional in function calls. Square braces [] are used to specify optional parameters.

Example: Use Of Optional Parameter

In the example below, function printInfo takes two positional parameters and one optional parameter. First, you must pass the person’s name and gender. The title parameter is optional here. Writing [String? title] makes title optional.

void printInfo(String name, String gender, [String? title]) {
  print("Hello $title $name your gender is $gender.");
}

void main() {
  printInfo("John", "Male");
  printInfo("John", "Male", "Mr.");
  printInfo("Kavya", "Female", "Ms.");
}

Hello  John your gender is Male.
Hello Mr. John your gender is Male.
Hello Ms. Kavya your gender is Female.

Video

Watch our video on functions parameter in Dart.