Solution Review: Sum of Lists

Task

In this challenge, you had to create a recursive function, sum, which sums together all the integers in a list.

Solution

A skeleton of the function was already provided for you. Let’s look it over.

int sum(List<int> numberList, int index) { 

}

sum takes two parameters. The first is a list of type List<int> and the second is the index of the last item. sum returns a value of type int.

General logic

We will start the summation from the last item in the list and move our way backwards. This is why we need to keep track of the index of the last item in the list.

Base ase

Since we are required to create a recursive function, the first thing we need to do is figure out the base case. The smallest possible list is an empty list. The smallest index is 0, this would mean that when index < 0, our base case should be called which will simply return 0.

if(index < 0 ){
    return 0; 
  } 

Recursive case

The second case is if the List is not empty. In this case, we separate the last element of the List from the rest of the elements.

We need to sum the last element with the recursive call to sum. This is passed index-1 and the remaining list excluding the last item as the new list will be one item smaller.

else {
    return numberList[index] + sum(numberList, index-1);
  }

If our list has 5 elements, we need to pass 4 to sum. The recursive case will be called as index < 0. sum will now look at the list starting from the second to last index, i.e., 3. This will continue until we have reached the first item of the list.

You can find the complete solution below:

You were required to write the code written from line 2 to line 6.