How to Sort a Map by Keys in Dart

How to Sort a Map by Keys in Dart ?

Answer

To sort a map by keys in Dart, you can use the `SplayTreeMap` from the `dart:collection` library, which automatically sorts the keys. Alternatively, you can convert the map to a list of key-value pairs, sort the list, and then convert it back to a map.

✐ Examples

1 Sorting a Map by Keys Using SplayTreeMap

We can sort a map by keys in Dart using the `SplayTreeMap`, which automatically sorts the keys.

For example,

We start by importing the `dart:collection` library, which provides the `SplayTreeMap` class.
We declare and initialize a map named originalMap with some key-value pairs. In this example, the map has integer keys and string values.
We create a `SplayTreeMap` named sortedMap and pass the `originalMap` to it, which automatically sorts the keys.
We print the sorted map to the console using the print function to verify the sorting.

Dart Program

import 'dart:collection';

void main() {
    // Declare and initialize a map
    Map<int, String> originalMap = {
        3: 'three',
        1: 'one',
        2: 'two'
    };

    // Create a SplayTreeMap to sort the keys
    SplayTreeMap<int, String> sortedMap = SplayTreeMap.from(originalMap);

    // Print the sorted map
    print('Sorted Map by Keys:');
    sortedMap.forEach((key, value) => print('$key: $value'));
}

Output

Sorted Map by Keys:
1: one
2: two
3: three

2 Sorting a Map by Keys Using List Conversion

We can sort a map by keys in Dart by converting the map to a list of key-value pairs, sorting the list, and then converting it back to a map.

For example,

We declare and initialize a map named originalMap with some key-value pairs. In this example, the map has integer keys and string values.
We convert the map to a list of key-value pairs and sort the list based on the keys.
We create a new map from the sorted list of key-value pairs.
We print the sorted map to the console using the print function to verify the sorting.

Dart Program

void main() {
    // Declare and initialize a map
    Map<int, String> originalMap = {
        3: 'three',
        1: 'one',
        2: 'two'
    };

    // Convert the map to a list of key-value pairs and sort the list by keys
    var sortedEntries = originalMap.entries.toList()
      ..sort((a, b) => a.key.compareTo(b.key));

    // Create a new map from the sorted list of key-value pairs
    Map<int, String> sortedMap = {
      for (var entry in sortedEntries) entry.key: entry.value
    };

    // Print the sorted map
    print('Sorted Map by Keys:');
    sortedMap.forEach((key, value) => print('$key: $value'));
}

Output

Sorted Map by Keys:
1: one
2: two
3: three

3 Sorting a Map by Keys in Descending Order Using SplayTreeMap

We can sort a map by keys in descending order in Dart using the `SplayTreeMap` with a custom comparator.

For example,

We start by importing the `dart:collection` library, which provides the `SplayTreeMap` class.
We declare and initialize a map named originalMap with some key-value pairs. In this example, the map has integer keys and string values.
We create a `SplayTreeMap` named sortedMap with a custom comparator to sort the keys in descending order, and pass the `originalMap` to it.
We print the sorted map to the console using the print function to verify the sorting.

Dart Program

import 'dart:collection';

void main() {
    // Declare and initialize a map
    Map<int, String> originalMap = {
        3: 'three',
        1: 'one',
        2: 'two'
    };

    // Create a SplayTreeMap with a custom comparator for descending order
    SplayTreeMap<int, String> sortedMap = SplayTreeMap<int, String>((a, b) => b.compareTo(a))
      ..addAll(originalMap);

    // Print the sorted map
    print('Sorted Map by Keys (Descending):');
    sortedMap.forEach((key, value) => print('$key: $value'));
}

Output

Sorted Map by Keys (Descending):
3: three
2: two
1: one

Summary

In this tutorial, we learned How to Sort a Map by Keys in Dart language with well detailed examples.

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