Fibonacci series in C#

Write down a program to generate Fibonacci no series -This is yet another commonly asked problem in programing interviews.

Although there are multiple ways of solving this problem. Here I would show you the most simple method to do it. Also this method is efficient from a performance perspective.

        public static List<int> GetFibonacciSeries(int number)
        {
            List<int> lstFibonacciNos = new List<int>();
            int previousNo = 0, latestNo = 1;
            switch (number)
            {
                case 0:
                    break;
                case 1:
                    lstFibonacciNos.Add(previousNo);
                    break;
                case 2:
                    lstFibonacciNos.Add(previousNo);
                    lstFibonacciNos.Add(latestNo);
                    break;
                default:
                    lstFibonacciNos = FetchFibonacciNo(number);
                    break;
            }
            return lstFibonacciNos;
        }

The description of Fibonacci series can be found here.

In the function GetFibonacciSeries() a number greater than zero is given as an input. For input numbers 1,2 it is an edge/boundary value scenario, that’s why we return hardcoded values. From number starting from 2 we do the actual calculations in FetchFibonacciNo() sub method shown below:

       private static List<int> FetchFibonacciNo(int number)
        {
            List<int> lstFibonacciNos = new List<int>();
            int previousNo = 0, latestNo = 1, newFibonacciNo = 0;
            lstFibonacciNos.Add(previousNo);
            lstFibonacciNos.Add(latestNo);
            for (int i = 2; i < number; i++)
            {
                //adding previous two nos fetches the new Fibonacci series
                newFibonacciNo = previousNo + latestNo;
                lstFibonacciNos.Add(newFibonacciNo);
                previousNo = latestNo;
                latestNo = newFibonacciNo;
            }
            return lstFibonacciNos;
        }

As you can see in the code above we initialize the first 2 nos in the series :

int previousNo = 0, latestNo = 1,

and we run a loop for getting the numbers in the series till nth term. The new number in the series fetched by adding last 2 previous numbers in the series as shown below:

newFibonacciNo = previousNo + latestNo;

The Unit test function(using Nunit) for the code is:

        [Test, TestCaseSource(nameof(GetFibonacciSeriesTestCases))]
        public void Test_GetFibonacciSeries(TestFibonacci testData)
        {
            //Arrange             
            //Act
            var result = Num.GetFibonacciSeries(testData.No);

            //Assert
            Assert.AreEqual(testData.ExpectedResult, result,
                "Fibonacci series has wrong result for the number :" + testData.No);

        }

Some of the test scenarios are written in function (GetFibonacciSeriesTestCases) as shown below:

        private static List<TestFibonacci> GetFibonacciSeriesTestCases()
        {
            List<TestFibonacci> lstFibonacciNos = new List<TestFibonacci>();
            int[] fiboSeries = { 0 };

            lstFibonacciNos.Add(GetFibonacciTestData(0, null));
            lstFibonacciNos.Add(GetFibonacciTestData(1, fiboSeries));
            fiboSeries = new int[] { 0, 1 };
            lstFibonacciNos.Add(GetFibonacciTestData(2, fiboSeries));
            fiboSeries = new int[] { 0, 1, 1 };
            lstFibonacciNos.Add(GetFibonacciTestData(3, fiboSeries));
            fiboSeries = new int[] { 0, 1, 1, 2 };
            lstFibonacciNos.Add(GetFibonacciTestData(4, fiboSeries));
            fiboSeries = new int[] { 0, 1, 1, 2, 3 };
            lstFibonacciNos.Add(GetFibonacciTestData(5, fiboSeries));
            fiboSeries = new int[] { 0, 1, 1, 2, 3, 5 };
            lstFibonacciNos.Add(GetFibonacciTestData(6, fiboSeries));
            fiboSeries = new int[] { 0, 1, 1, 2, 3, 5, 8 };
            lstFibonacciNos.Add(GetFibonacciTestData(7, fiboSeries));
            return lstFibonacciNos;
        }

Perhaps readers might be thinking as to why I haven’t used recursive function to implement the solution. The thing is recursive function from a performance point of view is very expensive. That’s why it’s better to avoid recursion.

The source code can be found here :

Note: The project is in .NET 5 .Both the functions can found in Num.cs. Also Have a look into the unit test project(RandomUtilityTest) so that you can get an idea as to how to write better unit tests.