Get every n number of elements in list Flutter Dart - dart

Question

I have a list of elements and I need to get a list containing the first element followed by every nth element afterwards. For example: given n = 3 and the list [banana, cherry, apple, pear, kiwi], I need to get the list [banana, pear]. I need this regardless of specific content, since the list depends on user input.
How do I do this using Dart?

Answer 1

You may access list in dart by providing an index like for example:
List<String> fruits = ["banana","cherry","apple","pear","kiwi"];
print(fruits[0]); // Will print to the console "banana";
On your case, you are trying to access index 0 and index 3 which is "banana" and "pear".
You may create a function that accepts an index like:
String getFruit(int index, List<String> fruits) => fruits[index];
print(getFruit[0]); // Will print "banana";
or if you need to actually get the specific ranges you may use:
List<String> fruits =["banana","cherry","apple","pear","kiwi"].getRange(0,4);
// Will give you "banana","cherry","apple","pear
You may check : https://api.dart.dev/be/180791/dart-core/List-class.html for more information.
Edited answer based off the comment:
List<String> getElements(List userInput, nIndex){
List elements = [];
for(int x = 0; x<userInput.length;x++){
if(x % nIndex == 0){
elements.add(userInput[x]);
}
}
return elements;
}
List fruits = ["banana","cherry","apple","pear","kiwi"];
print(getElements(fruits,2));
or you may try to look and use List.retainWhere() depending on your use case.

Answer 2

Dart has a great set of collection operators that make this type of problem pretty straightforward to solve. For example, we could do something like:
extension X<T> on List<T> {
List<T> everyNth(int n) => [for (var i = 0; i < this.length; i += n) this[i]];
}
main() {
final fruit = ["banana", "cherry", "apple", "pear", "kiwi"];
print(fruit.everyNth(3));
}
Output:
[banana, pear]

Answer 3

You can use this extension method, which will work on lists of any type:
extension GetEveryN<T> on List<T> {
List<T> elementsEveryN(int n) {
List<T> result = [];
for(int index = 0; index < length; index +=1) {
if(index % n == 0) {
result.add(this[index]);
}
}
return result;
}
}
Trying it in an example:
List<String> list = ["banana", "cherry","apple", "pear","kiwi"];
print(list.elementsEveryN(2)); // [banana, pear]