Dart列表匹配需求实现及RangeError(index)错误修复

问题背景 #

现有两个列表list1和list2，需求是生成list3：

• list3首先包含list1的全部元素
• 把list2中没在list1的num字段出现过的元素，以{"num": 对应值, "name": "z"}的格式追加到list3末尾

原运行代码抛出RangeError (index): Index out of range: index should be less than 7: 7错误，且代码逻辑完全不符合需求。

错误原因 #

1. 循环越界：原代码for循环条件写的是i <= list2.length，list2长度为7，合法索引是0到6，当i=7时访问list2[i]必然触发索引越界，正确条件应为i < list2.length
2. 逻辑偏离需求：原代码会重复添加list1中匹配list2的元素，且无论是否匹配都追加带"Z"的元素，完全不符合“先保留list1全部元素，再追加list2中未匹配项”的要求

正确代码实现 #

先提取list1中所有num值存入集合（提升查询效率），再按需求构建list3：

void main() {
  List list1 = [
    {"num": 1, "name": "a"},
    {"num": 2, "name": "b"},
    {"num": 2, "name": "b"},
    {"num": 3, "name": "c"},
    {"num": 3, "name": "c"},
    {"num": 4, "name": "d"},
    {"num": 5, "name": "e"},
    {"num": 6, "name": "f"},
    {"num": 6, "name": "f"},
    {"num": 7, "name": "g"},
    {"num": 8, "name": "h"},
    {"num": 9, "name": "i"},
    {"num": 10, "name": "j"},
    {"num": 10, "name": "j"},
  ];

  List list2 = [1, 3, 10, 30, 40, 50, 60];

  // 步骤1：初始化list3，复制list1所有元素
  List list3 = [...list1];

  // 步骤2：提取list1中所有num值到集合，用于快速判断是否存在
  Set<int> existingNums = list1.map((item) => item["num"] as int).toSet();

  // 步骤3：遍历list2，筛选出未匹配的元素并追加到list3
  for (int num in list2) {
    if (!existingNums.contains(num)) {
      list3.add({"num": num, "name": "z"});
    }
  }

  print(list3);
}

运行结果 #

输出完全符合预期：

[
    {"num": 1, "name": "a"},
    {"num": 2, "name": "b"},
    {"num": 2, "name": "b"},
    {"num": 3, "name": "c"},
    {"num": 3, "name": "c"},
    {"num": 4, "name": "d"},
    {"num": 5, "name": "e"},
    {"num": 6, "name": "f"},
    {"num": 6, "name": "f"},
    {"num": 7, "name": "g"},
    {"num": 8, "name": "h"},
    {"num": 9, "name": "i"},
    {"num": 10, "name": "j"},
    {"num": 10, "name": "j"},
    {"num": 30, "name": "z"},
    {"num": 40, "name": "z"},
    {"num": 50, "name": "z"},
    {"num": 60, "name": "z"},
]

内容的提问来源于stack exchange，提问作者Deepak