Dart for...in Loop

In this chapter, we will learn about the dart for…in loop with the help of examples.

The for loop is used to iterate with a particular condition. The dart for..in loop is useful when we need loop over the Dart objects such as list, map, and set.

---

for…in Loop

The dart for…in loop iterates over the element of an object one at a time.

The syntax for for…in loop is:

for(variable in Object) {
    // statements
}

The variable represents the variable name that accesses each element of the list one at a time.

Object contains the collection of ordered groups of elements.

Access Each element from a List

void main() { 
   var numbers = [1, 2, 3, 4, 5]; 
   
   for (var num in numbers) { 
      print(num); 
   } 
}

Output

1
2
3
4
5

Here, the numbers list consist of a group of numbers.

The num variable gets access to each element one at a time.

• During the first iteration, the num variable gets 1 and prints it.

• During the second iteration, the num variable gets 2 and prints it.

This continues for all the elements available. And the loop terminates when there are no more elements to access.

---

Performing Operations on Elements

Once each element is accessed using the dart for…in statement, we can perform various operations using those elements. For example,

 void main() { 
   var numbers = [1, 2, 3, 4, 5]; 
   int sum = 0;
   for (var num in numbers) {
     sum += num; // sum = sum + num;
   }
  print('Final sum is: $sum');
}

Output

Final sum is: 15

In this program, num variable access each element and adds to the previous sum.

First, the value of sum is 0.

• On the first iteration, the sum variable adds 1 to it. The sum is now 1.

• On the second iteration, the sum variable adds 2 to it. The sum has now value 3.

• During the third iteration, the sum variable adds 3 to it. And the sum has now value 6.

• During the fourth iteration, the sum variable adds 4 to it. The sum has now value 10.

• During the fifth iteration, the sum variable adds 5 to it. The sum has now value 15. And since all the elements have been accessed, the loop terminates. Hence the final sum is 15.

---

To see the flow of the program, we can print the num variable and the sum variable inside each iteration. For example,

void main() { 
   var numbers = [1, 2, 3, 4, 5]; 
   int sum = 0;
   for (var num in numbers) {
     sum += num;
     print('num value: $num');
     print('sum value: $sum');
   }
  print('Final sum is: $sum');
}

Output

num value: 1
sum value: 1
num value: 2
sum value: 3
num value: 3
sum value: 6
num value: 4
sum value: 10
num value: 5
sum value: 15
Final sum is: 15