How to find the indices of all elements matching from one list to another in Dart/Flutter

Question

Suppose, I have a list: firstList = ["Travel", "Shopping", "Transport"];

another list: secondList = ["Travel", "Shopping", "Shopping", "Travel", "Transport", "Transport", "Travel", "Travel"]

I need to find the index of all the elements from the secondList that contains: "Travel"

Here in secondList "Travel" is present in index 0, 3, 6, and 7

Now, I have a 3rd List containing the index of the element, here "Travel".

indexList = [0, 3, 6, 7] // Since index 0, 3, 6, and 7 only contains the element "Travel".

Below is my program:

for (int i = 0; i < firstList.length; i++) {
    for (int j = 0; j < secondList.length; j++) {
           
      indexList.add(secondList.indexOf(firstList[i]));

         }
    } 

This is not working as I am getting output like this:

[0, 0, 0, 0, 0, 0, 0, 0, 0]

Seems like it stuck on the first index itself.

The "Travel", here is an example, it should be matching dynamically such, firstList[i] or any other element not just "Travel" as hardcoded. Such as if I have selected firstList[i] then find the index of the same element in the secondList[].

Please help me to identify the cause. I am new to programming.

Answer 1

Try this:

main() {
  var firstList = ["Travel", "Shopping", "Transport"];
  var secondList = [
    "Travel",
    "Shopping",
    "Shopping",
    "Travel",
    "Transport",
    "Transport",
    "Travel",
    "Travel"
  ];
  var thirdList = [];
  for (var i = 0; i < secondList.length; i++) {
    if (secondList[i] == "Travel") {
      thirdList.add(i);
    }
  }
  print(thirdList);
}

EDIT: For it to work for every item in firstList

Try this:

main() {
  var firstList = ["Travel", "Shopping", "Transport"];
  var secondList = [
    "Travel",
    "Shopping",
    "Shopping",
    "Travel",
    "Transport",
    "Transport",
    "Travel",
    "Travel"
  ];
  var thirdList = [];
  for (var i = 0; i < firstList.length; i++) {
    var sublist = [];
    for (var j = 0; j < secondList.length; j++) {
      if (secondList[j] == firstList[i]) {
        sublist.add(j);
      }
    }
    thirdList.add(sublist);
  }
  print(thirdList); // [[0, 3, 6, 7], [1, 2], [4, 5]]
}

Answer 2

  List firstList = ["Travel", "Shopping", "Transport"];
  List secondList = ["Travel", "Shopping", "Shopping", "Travel", "Transport", "Transport", "Travel", "Travel"];
  List indexList = [];

  getElement() {
    for (int i = 0; i <= secondList.length; i++) {
      if (secondList[i] == "Travel") {
        indexList.add(i);
      }
    }

    print(indexList);
  }