Question

我有List categories及其products，我想执行这些操作：

地图
组
查找
查询汇总功能
按多个字段排序

如何在Dart中完成这项工作？

void main() {
  var books = new Category(0, "Books");
  var magazines = new Category(1, "Magazines");
  var categories = [books, magazines];
  var products = [
    new Product(0, "Dr. Dobb's", magazines),
    new Product(1, "PC Magazine", magazines),
    new Product(2, "Macworld", magazines),
    new Product(3, "Introduction To Expert Systems", books),
    new Product(4, "Compilers: Principles, Techniques, and Tools", books),
  ];      

  // How to map product list by id?

  // How to group product list by category?

  // How to create lookup for product list by category?

  // How to query aggregate functions?

  // How to sort product list by category name and product name?      
}

class Category {
  int id;
  String name;

  Category(this.id, this.name);

  operator ==(other) {
    if(other is Category) {
      return id == other.id;
    }

    return false;
  }

  String toString() => name;
}

class Product {
  int id;
  String name;
  Category category;

  Product(this.id, this.name, this.category);

  operator ==(other) {
    if(other is Product) {
      return id == other.id;
    }

    return false;
  }

  String toString() => name;
}

这些列表与数据表中的记录类似。

Answer 1

这是我的解决方案，它基于使用queries包。

import 'package:queries/collections.dart';

void main() {
  var books = new Category(0, "Books");
  var magazines = new Category(1, "Magazines");
  var categories = [books, magazines];
  var products = [
    new Product(0, "Dr. Dobb's", magazines),
    new Product(1, "PC Magazine", magazines),
    new Product(2, "Macworld", magazines),
    new Product(3, "Introduction To Expert Systems", books),
    new Product(4, "Compilers: Principles, Techniques, and Tools", books),
  ];

  var prodTable = new Collection<Product>(products);

  print("========================");
  print("Map the product list by id");
  print("========================");
  var result1 = prodTable.toDictionary<int>((product) => product.id);

  for (var pair in result1.asIterable()) {
    var key = pair.key;
    var value = pair.value;
    print("$key: $value");
  }

  print("========================");
  print("Group the product list by category");
  print("========================");
  var result2 = prodTable.groupBy<Category>((product) => product.category);

  for (var group in result2.asIterable()) {
    var key = group.key;
    print(key);
    for (var element in group.asIterable()) {
      print("  $element");
    }
  }

  print("========================");
  print("Create the lookup for product list by category");
  print("========================");
  var result3 = prodTable.toLookup<Category>((product) => product.category);

  for (var category in categories) {
    var group = result3[category];
    var key = group.key;
    print(key);
    for (var element in group) {
      print("  $element");
    }
  }

  print("========================");
  print("Query the aggregate functions");
  print("========================");
  var result4 = prodTable
      .groupBy<Category>((product) => product.category)
      .select<Map>((group) => {
            "category": group.key,
            "products": group.count(),
            "max name length":
                group.max$1<int>((product) => product.name.length)
          });

  for (var row in result4.asIterable()) {
    print(row);
  }

  print("========================");
  print("Sort the product list by category name and product name");
  print("========================");
  var result5 = prodTable
      .orderBy<String>((product) => product.category.name)
      .thenBy<String>((product) => product.name);

  for (var product in result5.asIterable()) {
    print("${product.category}: $product");
  }
}

class Category {
  int id;
  String name;

  Category(this.id, this.name);

  operator ==(other) {
    if (other is Category) {
      return id == other.id;
    }

    return false;
  }

  String toString() => name;
}

class Product {
  int id;
  String name;
  Category category;

  Product(this.id, this.name, this.category);

  operator ==(other) {
    if (other is Product) {
      return id == other.id;
    }

    return false;
  }

  String toString() => name;
}

========================
Map the product list by id
========================
0: Dr. Dobb's
1: PC Magazine
2: Macworld
3: Introduction To Expert Systems
4: Compilers: Principles, Techniques, and Tools
========================
Group the product list by category
========================
Magazines
  Dr. Dobb's
  PC Magazine
  Macworld
Books
  Introduction To Expert Systems
  Compilers: Principles, Techniques, and Tools
========================
Create the lookup for product list by category
========================
Books
  Introduction To Expert Systems
  Compilers: Principles, Techniques, and Tools
Magazines
  Dr. Dobb's
  PC Magazine
  Macworld
========================
Query the aggregate functions
========================
{category: Magazines, products: 3, max name length: 11}
{category: Books, products: 2, max name length: 44}
========================
Sort the product list by category name and product name
========================
Books: Compilers: Principles, Techniques, and Tools
Books: Introduction To Expert Systems
Magazines: Dr. Dobb's
Magazines: Macworld
Magazines: PC Magazine

Answer 2

可以使用.map方法完成映射，如下所示： products.map((product) => product.id) 这将返回产品ID的列表。

将使用fold方法进行分组。例如，如果您要按类别对产品进行分组：

product.fold<Map<String, List<Product>>>({}, (productMap, currentProduct) {
  if (productMap[currentProduct.category.name] == null) {
    productMap[currentProduct.category.name] = [];
  }
  productMap[currentProduct.category.name].add(currentProduct);
  return productMap;
});

使用.firstWhere完成查找。例如： products.firstWhere((product) => product.category.name === 'books');

如何为“列表”执行这些操作：map，group，lookup，aggregate，sort？

2 个答案: