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.

Solution 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]]
}

Solution 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);
  }

Solution 3

 void matchItem()
  {
    var firstList = ["Travel", "Shopping", "Transport"];
    var secondList = ["Travel", "Shopping", "Shopping", "Travel", "Transport", "Transport", "Travel", "Travel"];
    var foundItemPositions = [];
    for(int i=0;i<firstList.length;i++)
      {
        if(secondList.contains(firstList[i]))
          {
            for(int j=0;j<secondList.length;j++)
              {
                if(firstList[i] == secondList[j])
                  {
                    foundItemPositions.add(j);
                  }
              }
          }
      }
    print(foundItemPositions);
  }

Solution 4

we can achieve this with a single loop.

Try this

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

View more solutions

Comments

[kl23]

The "Travel", here is an example, it should be matching dynamically such as, 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[].

[Ravi Limbani]

you can get any from firstList but you need to store alls result in indexList. You need to use if condition like if(firstList[i] == secondList[i]){

[Josteve]

Check this out: stackoverflow.com/a/70893478/11452511

[Josteve]

From effective dart, it is recommended to not use forEach in this fashion. Use the normal for loop instead; Check this: dart.dev/guides/language/effective-dart/…

[Farman Ali Khan]

Thanks @Josteve , it's updated now.