remove list from another list in dart

Question

I have two lists, lst1 and lst2. I want to perform lst1 - lst2, which returns all elements of lst1 not in lst2.

var lst1 = ["t1" , "t2" , "t3" , "t4"];
var lst2 = ["t2" , "t4" , "t5"];
//output: ["t1" , "t3"]

Answer 1

Convert to sets and take the difference, then convert back to a list (with the caveat that neither duplicates nor the ordering of elements will be preserved):

void main() {
  var lst1 = ["t1" , "t2" , "t3" , "t4"];
  var lst2 = ["t2" , "t4" , "t5"];
  var set1 = Set.from(lst1);
  var set2 = Set.from(lst2);
  print(List.from(set1.difference(set2)));
}

Output

[t1, t3]

Try it online

Answer 2

The method using sets suggested by Ray Toal is probably the fastest, but if your first list contains duplicates that you want to keep, sets will completely destroy them.

Instead, you could use a simple list filtering method.

void main() {
  var lst1 = ["t1" , "t2" , "t2", "t3" , "t3", "t4"];  // introduce duplicates
  var lst2 = ["t2" , "t4" , "t5"];

  var set1 = Set.from(lst1);
  var set2 = Set.from(lst2);
  print(List.from(set1.difference(set2)));
  // Output : [t1, t3]

  var filtered_lst = List.from(lst1.where(
    (value) => !lst2.contains(value)));
  print(filtered_lst);
  // Output: [t1, t3, t3]
}

If there is duplicates in both lists and you actually want to subtract list item per item, you could use the remove method (warning: this will actually remove items from your first list, so you might need to create a copy first).

void main() {
  var lst1 = ["t1" , "t2" , "t2", "t3" , "t3", "t4"];  // introduce duplicates
  var lst2 = ["t2" , "t4" , "t5"];

  for (var elem in lst2) {
    lst1.remove(elem);
  }
  print(lst1);
  // Output : [t1, t2, t3, t3]
  // only one occurrence of "t2" was removed.
}