In this example, I will show you some different solutions to count the occurrence of an element in a list:

1. Loop the list and count
2. Use list.where( ) method
3. Use extension

void main() {
  List<String> letters = [
    'a',
    'b',
    'c',
    'd',
    'a',
    'a',
    'a',
    'b',
    'a',
    'a',
    'b',
    'b'
  ];

  print('1. Count occurrences using loop:');
  print('Letter \'a\' appears ${countOccurrencesUsingLoop(letters, 'a')} times');
  print('Letter \'b\' appears ${countOccurrencesUsingLoop(letters, 'b')} times');
  print('Letter \'f\' appears ${countOccurrencesUsingLoop(letters, 'f')} times');

  print('\n***********\n');

  print('2. Count occurrences using where method:');
  print('Letter \'a\' appears ${countOccurrencesUsingWhereMethod(letters, 'a')} times');
  print('Letter \'b\' appears ${countOccurrencesUsingWhereMethod(letters, 'b')} times');
  print('Letter \'f\' appears ${countOccurrencesUsingWhereMethod(letters, 'f')} times');

  print('\n***********\n');

  print('2. Count occurrences using extensions:');
  print('Letter \'a\' appears ${countOccurrencesUsingExtension(letters, 'a')} times');
  print('Letter \'b\' appears ${countOccurrencesUsingExtension(letters, 'b')} times');
  print('Letter \'f\' appears ${countOccurrencesUsingExtension(letters, 'f')} times');
}

int countOccurrencesUsingLoop(List<String> list, String element) {
  if (list == null || list.isEmpty) {
    return 0;
  }

  int count = 0;
  for (int i = 0; i < list.length; i++) {
    if (list[i] == element) {
      count++;
    }
  }

  return count;
}

int countOccurrencesUsingWhereMethod(List<String> list, String element) {
  if (list == null || list.isEmpty) {
    return 0;
  }

  var foundElements = list.where((e) => e == element);
  return foundElements.length;
}

int countOccurrencesUsingExtension(List<String> list, String element) {
  return list.findOccurrences(element);
  // OR
  // return ListExt(list).findOccurrences(element);
}

extension ListExt on List {
  int findOccurrences(String element) {
    var foundLetters = this.where((letter) => letter == element);
    return foundLetters.length;
  }
}

Output:

1. Count occurrences using loop:
Letter 'a' appears 6 times
Letter 'b' appears 4 times
Letter 'f' appears 0 times

***********

2. Count occurrences using where method:
Letter 'a' appears 6 times
Letter 'b' appears 4 times
Letter 'f' appears 0 times

***********

3. Count occurrences using extension:
Letter 'a' appears 6 times
Letter 'b' appears 4 times
Letter 'f' appears 0 times

Phuc Tran

Creator of Coflutter.
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