Today, we are venturing into nested collections in Dart. Think of nested collections like boxes within a larger box. Our goal is to understand how to create, retrieve, modify, and manipulate these nested boxes or collections, using illustrative, real-life examples.
Nested Lists in Dart are comparable to a comprehensive grocery list that holds individual, day-based lists. The creation of a nested list in Dart mirrors the creation of a regular list, except that each element in the list is also a list. Consider the following example:
Dart1var nestedList = [ 2 ['apple', 'banana', 'grapes'], // First list representing Groceries for Monday 3 ['milk', 'bread'], // Second list representing Groceries for Tuesday 4]; 5print(nestedList); // The output will be the combined list for groceries 6// Output: [[apple, banana, grapes], [milk, bread]]
To access and modify items in nested lists in Dart, we apply index numbers twice to both the inner and outer list:
Dart1print(nestedList[0][1]); 2// Output: 'banana'
You can also add new elements to a nested list, as demonstrated here:
Dart1nestedList[0].add('orange'); 2print(nestedList); 3// Output: [[apple, banana, grapes, orange], [milk, bread]]
Nested Sets, which are Sets contained within a Set, do not have direct support in Dart. However, we can create a Set of Sets to accommodate unique groups with unique elements. Here's how it can be done:
Dart1Set<Set<String>> nestedSet = { 2 {'USA', 'UK'}, // First set representing English-speaking countries 3 {'France', 'Germany'}, // Second set representing European countries 4}; 5print(nestedSet); 6// Output: {{USA, UK}, {France, Germany}}
We can utilize standard Set operations with the nested Set:
Dart1nestedSet.add({'Australia', 'New Zealand'}); // Adds a third set representing island countries 2print(nestedSet); 3// Output: {{USA, UK}, {France, Germany}, {Australia, New Zealand}}
It's important to note that accessing elements within a nested Set can be cumbersome. Unlike Lists or Maps, Sets do not maintain their elements in a specified order and do not support indexing. This means that unlike retrieving an element from a List or a Map with a key, accessing specific elements from a nested Set isn't straightforward. You might need to employ iteration or conversion to a more accessible collection type, such as a List, to access specific elements within a nested Set. This challenge makes manipulation and retrieval of data from nested Sets less direct than with nested Lists or Maps.
Nested Maps are Maps inside a Map, akin to a dictionary containing many dictionaries. To create a nested Map, we need to construct Maps within a Map:
Dart1var nestedMap = { 2 'John': {'Math': 85, 'English': 90}, // John's scores 3 'Jane': {'Math': 93, 'English': 87}, // Jane's scores 4}; 5print(nestedMap); 6// Output: {John: {Math: 85, English: 90}, Jane: {Math: 93, English: 87}}
To interact with and manipulate values in nested Maps, Dart offers a safe and concise way to access deeply nested data without the risk of encountering a null pointer exception. This is achieved through the use of the conditional member access operator ?. Here's how it works in the context of a nested Map:
Dart1print(nestedMap['John']?['Math']); // Safely outputs: 85, John's Math score 2nestedMap['John']?['Math'] = 95; // Safely updates John's Math score to 95 3print(nestedMap['John']?['Math']); // Safely outputs: 95, showcasing John's improved Math score 4 5// Attempting to access an invalid key 'History' 6print(nestedMap['John']?['History']); // Safely returns null, since 'History' key does not exist
In this example, nestedMap['John']?['Math'] utilizes ? to access the 'Math' key within John's Map only if nestedMap['John'] is not null, demonstrating how to safely access and modify nested Map values. Additionally, an attempt to access a non-existent 'History' key for John demonstrates how Dart's null-aware operations protect against runtime errors by returning null when accessing invalid keys. This approach ensures safety and robustness when dealing with uncertain data structures in Dart.
In Dart, the power to nest different types of collections — Lists, Sets, and Maps — allows for the representation of complex data structures. This capability is showcased through the example of a school model, where we employ a mix of these collection types to organize details about classes, including student names, surnames, and the topics covered in each class.
Consider the following Dart code snippet:
Dart1var schoolWithDetails = [ 2 // This is a List of Maps, each Map representing a class 3 { 4 'class': 'Math', // A String representing the class name 5 'students': <Map<String, String>>[ 6 // A List of Maps, with each Map hosting a student's name and surname 7 {'name': 'John', 'surname': 'Doe'}, 8 {'name': 'Jane', 'surname': 'Smith'}, 9 ], 10 // A Set of Strings, ensuring each topic covered is unique 11 'topicsCovered': {'Algebra', 'Geometry', 'Calculus'}, 12 }, 13 // Other Maps for more classes 14 // ... 15];
In this model:
'class', 'students', 'topicsCovered'), providing a way to associate string keys with their respective values or collections.'name' and 'surname' keys.The following code shows how to access a student's name and a topic within a combined snippet:
Dart1// Accessing the first student's name from the first class 2var studentName = schoolWithDetails[0]['students'][0]['name']; 3print(studentName); // Output: 'John' 4 5// Accessing the first topic from the first class, after converting the Set to a List for index access 6var topic = schoolWithDetails[0]['topicsCovered'].toList()[0]; 7print(topic); // Outputs could vary, example: 'Algebra'
This code first accesses the name 'John' from the first class's student list and then retrieves a topic, noting for this example as 'Algebra', from the topicsCovered set of the first class. Remember, since sets are unordered, the output of the first topic might vary.
Congratulations on mastering nested collections! We have explored how to create and manage nested Lists, Sets, and Maps in Dart. Up next, apply these concepts in hands-on practice to reinforce your learning and to tackle complex problem scenarios involving nested collections in Dart. Keep going; you're progressing well!