How to find repeated numbers in a list in Dart

In Dart, the forEach loop is used to iterate over each item in an iterable, such as a list, set, or map. It comes in handy when we want to loop through a collection or list.

Syntax

The following is the syntax for the forEach() method:

iterable.forEach((element) { ... });

Code example

The following program takes in a list of integers and returns a new list containing the repeated number.

void main() {
  // Declare list of integers
  List myList = [9,10,20,15,20,9,5,5];
  // Invoke getRepeatedNo() function
  getRepeatedNo(myList);
}
// Function that returns a list of repeated numbers in the list
getRepeatedNo(List numbers){
  List repeatedNumbers = [];
  List otherNumbers = [];
  numbers.forEach((x){
    if(otherNumbers.contains(x)){
      repeatedNumbers.add(x);
     
    }
    else{
      otherNumbers.add(x);
    }
    });
  
   print("Repeated number: $repeatedNumbers");
   print("List without repeated number: $otherNumbers");
}

Code explanation

Line 3: We define a list of integers called myList().

Line 5: We invoke the getRepeatedNo() function, which takes a list as its parameter.

Lines 9–25: We define a function called getRepeatedNo(). This function takes a list of integers and returns the list of repeated numbers in the original list.

Line 10: We define a variable called repeatedNumbers that will hold the repeated numbers.

Line 11: We define another variable called otherNumbers that will hold distinct numbers.

Lines 12–20: We use the forEach() method to iterate through the list passed into the getRepeatedNo() function. Next, we use the if...else statement to check if the value exists in the otherNumbers list. If true, the value is added to the repeatedNumbers list. Otherwise, the value is added to the otherNumbers list.

Line 22: We display the repeated numbers using the print() method.

Line 23: We display the distinct numbers using the print() method.

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