Dart – Advance Concepts of Iterable Collections

In this article, we will look into some of the important concepts related to iterables in Dart. 

1. Indexing with [ ] operator is invalid:

In dart, there is no operator [ ] in Iterable. To better understand let’s take a look at the below example.

Example:

Dart

void main() {

  // invalid program
  Iterable<int> iterable = [1, 2, 3, 4, 5 ]; // line 1
int value = iterable[1]; // line 2
}

Output: 

In this example, we have a list of 5 elements. What if  we want to fetch the number from the particular index?  In line 2 we tried to fetch numbers from the index by writing iterable[1] but the [ ] operator isn’t defined in the class. As you can see the above error we got.

So the question arises how do we fetch the number from a particular index in dart? To do that we use elementAt() in dart. 

Syntax:

elementAt(index_number)

Here,

• index_number: It is the index whose value is to be fetched.

Take a look at the below example.

Example: 

Here we want to fetch the value at index 3. So let’s see how we can do that with the help of elementAt().

Dart

void main() {
 Iterable<int> iterable = [1, 2, 3, 4, 5];
 print('Element at 3rd index is ${iterable.elementAt(3)}');
}

Output:

Explanation: So here we passed index no 3 because we wanted the value stored at that index. So we got the result 4. This is how we can get the value stored at a particular index using elementAt().

2. Mapping Iterable

We can map an existing iterable and derive modified values. To do so follow the below steps:

• Take a collection
• Transform all its items according to a condition.
• Return a new collection

Example: 

Dart

void main() {
  const list = [5,4,3,2,1];
  final value = list.map((number) => number*2);
  print('new values are: ${value}');
}

Output:

What makes map operators very powerful in dart?

It’s the ability to transform the process into a very short piece of code. Let’s see an example.

Same above code if we write using for loop then we can see below as it takes many lines of code. 

Dart

void main() {
   const list = [5,4,3,2,1];

  final element = [];
  for(var value in list){
    element.add(value * 2);
  }
  print('new values are: ${element}');
}

 Output :

new values are: [10, 8, 6, 4, 2]

We see that although the output is the same, the lines of code are more. That’s why a map is powerful because using it we can write code in fewer lines and get the desired result.

Important note:

According to the official docs, the map returns lazy Iterable.

What does lazy Iterable mean? 

Let’s take a look at the below code:

Dart

void main() {

 const list = [5,4,3,2,1];

 final value = list.map((number) => number*2);

 print('new values are: ${value}');
}

Here, the anonymous function isn’t evaluated until the result is used. 

This means:

• It means map will not evaluate the anonymous function if the result is not getting printed or it’s isn’t in any use anywhere.
• It’s a performance optimization technique ensuring that all the resulting items are only calculated when needed.