Flutter/Dart嵌套的For&While回圈行為怪異

我有一個用例，我使用嵌套的 For 回圈從內部回圈創建一個新串列，并使用外部回圈將其值添加到另一個串列中。這是代碼：

對于回圈版本：

void main(List<String> arguments) {
  print('Hello world!');

  List sortedMapList = [1, 2, 3];
  List pickupPolyLineList = [4, 5, 6];
  List tempList = [];
  List wayPointsList = [];

  for (int i = 0; i <= sortedMapList.length; i  ) {
    tempList = pickupPolyLineList;
    for (int j = 0; j < i; j  ) {
      tempList.add(sortedMapList[j]);
      // print("tempList at internal index j: $j is: $tempList");
    }
    print(" Before wayPointsList at internal index i: $i is: $wayPointsList \n");

    wayPointsList.add(tempList);
    print(" after wayPointsList at internal index i: $i is: $wayPointsList");
  }
}

For回圈版本輸出：

Hello world!
 Before wayPointsList at internal index i: 0 is: [] 

 after wayPointsList at internal index i: 0 is: [[4, 5, 6]]
 Before wayPointsList at internal index i: 1 is: [[4, 5, 6, 1]] 

 after wayPointsList at internal index i: 1 is: [[4, 5, 6, 1], [4, 5, 6, 1]]
 Before wayPointsList at internal index i: 2 is: [[4, 5, 6, 1, 1, 2], [4, 5, 6, 1, 1, 2]] 

 after wayPointsList at internal index i: 2 is: [[4, 5, 6, 1, 1, 2], [4, 5, 6, 1, 1, 2], [4, 5, 6, 1, 1, 2]]

While回圈版本：

void main(List<String> arguments) {
  print('Hello world!');

  List sortedMapList = [1, 2, 3];
  List pickupPolyLineList = [4, 5, 6];
  List tempList = [];
  List wayPointsList = [];

  int i = 0;
  int j = 0;

  while (i < sortedMapList.length) {
    tempList = pickupPolyLineList;
   
    while (j < i) {
      tempList.add(sortedMapList[j]);
      // print("tempList at internal index j: $j is: $tempList");
      j  ;
    }
    print("tempList at internal index is: $tempList");
    print(" Before wayPointsList at internal index i: $i is: $wayPointsList");

    wayPointsList.add(tempList);
    print(" after wayPointsList at internal index i: $i is: $wayPointsList \n");
    i  ;
  }
}

While 回圈版本輸出：

Hello world!
tempList at internal index is: [4, 5, 6]
 Before wayPointsList at internal index i: 0 is: []
 after wayPointsList at internal index i: 0 is: [[4, 5, 6]] 

tempList at internal index is: [4, 5, 6, 1]
 Before wayPointsList at internal index i: 1 is: [[4, 5, 6, 1]]
 after wayPointsList at internal index i: 1 is: [[4, 5, 6, 1], [4, 5, 6, 1]] 

tempList at internal index is: [4, 5, 6, 1, 2]
 Before wayPointsList at internal index i: 2 is: [[4, 5, 6, 1, 2], [4, 5, 6, 1, 2]]
 after wayPointsList at internal index i: 2 is: [[4, 5, 6, 1, 2], [4, 5, 6, 1, 2], [4, 5, 6, 1, 2]] 

我在 Python 中撰寫的相同代碼似乎運行良好：

adj = [1, 2, 3]
fruits = [4, 5, 6]
fruits2 = []
tempList = []

for x in range(len(fruits)):
  tempList = fruits[:]
  for i in range(x):
    tempList.append(adj[i])
  fruits2.append(tempList)
  print(fruits2)

python代碼的輸出：

[[4, 5, 6]]
[[4, 5, 6], [4, 5, 6, 1]]
[[4, 5, 6], [4, 5, 6, 1], [4, 5, 6, 1, 2]]

uj5u.com熱心網友回復：

您的第一個問題是您沒有tempList在每個主回圈中重置，第二個問題是當您按值復制串列時應該使用.addAll，試試這個：

void testy() {
    print('Hello world!');

    List sortedMapList = [1, 2, 3];
    List pickupPolyLineList = [4, 5, 6];
    List tempList = [];
    List wayPointsList = [];

    for (int i = 0; i <= sortedMapList.length; i  ) {
      tempList = [];// <--- add this
      tempList.addAll(pickupPolyLineList);// <--- change this
      for (int j = 0; j < i; j  ) {
        tempList.add(sortedMapList[j]);
        // print("tempList at internal index j: $j is: $tempList");
      }
      print(
          " Before wayPointsList at internal index i: $i is: $wayPointsList \n");

      wayPointsList.add(tempList);
      print(" after wayPointsList at internal index i: $i is: $wayPointsList");
    }
  }

輸出：

flutter: Hello world!
flutter:  Before wayPointsList at internal index i: 0 is: []
flutter:  after wayPointsList at internal index i: 0 is: [[4, 5, 6]]
flutter:  Before wayPointsList at internal index i: 1 is: [[4, 5, 6]]
flutter:  after wayPointsList at internal index i: 1 is: [[4, 5, 6], [4, 5, 6, 1]]
flutter:  Before wayPointsList at internal index i: 2 is: [[4, 5, 6], [4, 5, 6, 1]]
flutter:  after wayPointsList at internal index i: 2 is: [[4, 5, 6], [4, 5, 6, 1], [4, 5, 6, 1, 2]]
flutter:  Before wayPointsList at internal index i: 3 is: [[4, 5, 6], [4, 5, 6, 1], [4, 5, 6, 1, 2]]
flutter:  after wayPointsList at internal index i: 3 is: [[4, 5, 6], [4, 5, 6, 1], [4, 5, 6, 1, 2], [4, 5, 6, 1, 2, 3]]

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