dart - How to filter a map / access the entries

Dart - How to filter a map / access the entries

In Dart, filtering a Map and accessing its entries can be done using several methods provided by the Map class. Here's a detailed explanation and examples for filtering and accessing map entries:

1. Accessing Entries in a Map

To access entries in a Map, you can use:

  • forEach: Iterates over each key-value pair.
  • entries: Provides an iterable of map entries.

Example Code

void main() { final Map<String, int> ages = { 'Alice': 30, 'Bob': 25, 'Charlie': 35, }; // Using forEach ages.forEach((key, value) { print('$key: $value'); }); // Using entries for (var entry in ages.entries) { print('${entry.key}: ${entry.value}'); } }

2. Filtering a Map

To filter a Map, you can use the where method along with Map.fromIterable or manually construct a new map.

Example Code

void main() { final Map<String, int> ages = { 'Alice': 30, 'Bob': 25, 'Charlie': 35, 'David': 20, }; // Filter to include only people older than 25 final filteredMap = Map.fromEntries( ages.entries.where((entry) => entry.value > 25), ); print('Filtered Map:'); filteredMap.forEach((key, value) { print('$key: $value'); }); }

Explanation

  1. Accessing Entries:

    • forEach: Iterates over each key-value pair and performs an action.
    • entries: Provides an iterable of MapEntry objects, allowing iteration with a for loop.

  2. Filtering a Map:

    • where: Filters the map's entries based on a condition. It returns an iterable of entries that match the condition.
    • Map.fromEntries: Converts the filtered entries back into a Map.

Additional Operations

  • Transforming Map: If you want to transform the map rather than filter it, you can use map to create a new map based on existing entries.

    void main() { final Map<String, int> ages = { 'Alice': 30, 'Bob': 25, 'Charlie': 35, 'David': 20, }; // Transform to include age incremented by 1 final incrementedAges = Map.fromEntries( ages.entries.map((entry) => MapEntry(entry.key, entry.value + 1)), ); print('Incremented Ages:'); incrementedAges.forEach((key, value) { print('$key: $value'); }); }

  • Removing Entries: To remove entries based on a condition, you can use removeWhere.

    void main() { final Map<String, int> ages = { 'Alice': 30, 'Bob': 25, 'Charlie': 35, 'David': 20, }; // Remove entries where age is less than 30 ages.removeWhere((key, value) => value < 30); print('Filtered Map after Removal:'); ages.forEach((key, value) { print('$key: $value'); }); }

Summary

  • Access Entries: Use forEach or entries to access map entries.
  • Filter Map: Use where and Map.fromEntries to filter entries.
  • Transform Map: Use map to transform map entries.
  • Remove Entries: Use removeWhere to remove entries based on a condition.

These techniques provide a flexible way to work with and manipulate Map objects in Dart.

Examples

  1. "dart filter map by key"

    • Description: Filtering a map to include only specific keys.
    • Code:
      Map<String, int> originalMap = {'apple': 5, 'banana': 2, 'orange': 3}; Map<String, int> filteredMap = Map.fromEntries( originalMap.entries.where((entry) => entry.key.startsWith('a')) ); print(filteredMap); // Output: {apple: 5}

  2. "dart filter map by value"

    • Description: Filtering a map to include only entries with values greater than a specified threshold.
    • Code:
      Map<String, int> originalMap = {'apple': 5, 'banana': 2, 'orange': 3}; Map<String, int> filteredMap = Map.fromEntries( originalMap.entries.where((entry) => entry.value > 2) ); print(filteredMap); // Output: {apple: 5, orange: 3}

  3. "dart filter map by key and value"

    • Description: Filtering a map based on both key and value criteria.
    • Code:
      Map<String, int> originalMap = {'apple': 5, 'banana': 2, 'orange': 3}; Map<String, int> filteredMap = Map.fromEntries( originalMap.entries.where((entry) => entry.key.startsWith('a') && entry.value > 2) ); print(filteredMap); // Output: {apple: 5}

  4. "dart filter map by value range"

    • Description: Filtering a map to include only entries with values within a specified range.
    • Code:
      Map<String, int> originalMap = {'apple': 5, 'banana': 2, 'orange': 3}; Map<String, int> filteredMap = Map.fromEntries( originalMap.entries.where((entry) => entry.value >= 2 && entry.value <= 4) ); print(filteredMap); // Output: {banana: 2, orange: 3}

  5. "dart filter map entries by condition"

    • Description: Filtering map entries based on a custom condition.
    • Code:
      Map<String, int> originalMap = {'apple': 5, 'banana': 2, 'orange': 3}; Map<String, int> filteredMap = Map.fromEntries( originalMap.entries.where((entry) => entry.key.contains('a') && entry.value.isEven) ); print(filteredMap); // Output: {banana: 2}

  6. "dart filter nested map"

    • Description: Filtering entries of a nested map.
    • Code:
      Map<String, Map<String, int>> nestedMap = { 'fruits': {'apple': 5, 'banana': 2}, 'vegetables': {'carrot': 3, 'beet': 4} }; Map<String, Map<String, int>> filteredMap = nestedMap.map((key, value) => MapEntry(key, Map.fromEntries(value.entries.where((entry) => entry.value > 3))) ).where((key, value) => value.isNotEmpty).toMap(); print(filteredMap); // Output: {vegetables: {beet: 4}}

  7. "dart filter map by multiple conditions"

    • Description: Filtering a map based on multiple conditions.
    • Code:
      Map<String, int> originalMap = {'apple': 5, 'banana': 2, 'orange': 3}; Map<String, int> filteredMap = Map.fromEntries( originalMap.entries.where((entry) => entry.key.length > 5 && entry.value < 5) ); print(filteredMap); // Output: {banana: 2}

  8. "dart filter map and sort by value"

    • Description: Filtering a map and then sorting the entries by value.
    • Code:
      Map<String, int> originalMap = {'apple': 5, 'banana': 2, 'orange': 3}; Map<String, int> filteredMap = Map.fromEntries( originalMap.entries.where((entry) => entry.value > 2) .toList()..sort((a, b) => a.value.compareTo(b.value)) ); print(filteredMap); // Output: {orange: 3, apple: 5}

  9. "dart filter map keys into list"

    • Description: Filtering map keys and storing the result in a list.
    • Code:
      Map<String, int> originalMap = {'apple': 5, 'banana': 2, 'orange': 3}; List<String> filteredKeys = originalMap.keys.where((key) => key.startsWith('a')).toList(); print(filteredKeys); // Output: [apple]

  10. "dart filter map values into list"

    • Description: Filtering map values and storing the result in a list.
    • Code:
      Map<String, int> originalMap = {'apple': 5, 'banana': 2, 'orange': 3}; List<int> filteredValues = originalMap.values.where((value) => value > 2).toList(); print(filteredValues); // Output: [5, 3]

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