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exercises > sum-of-first-n

Exercise #17: Sum of First N Numbers
by Zoran Horvat @zoranh75
December 14, 2013

Problem Statement

Given a positive number N, write a function which returns sum of numbers 1 + 2 + ... + N.

Example: If N is 5, then return value should be 15 (1 + 2 + 3 + 4 + 5 = 15).

Problem Analysis

This task can be solved quickly by iterating through the sequence:

function Sum(n)
    sum = 0
    for i = 1 to n
        sum = sum + i
    return sum

This function runs in time O(N) and space O(1). Now we can ask if there is a better solution in terms of running time. In order to make the function run in constant time, we need to entail a definite form of the sum. This is easy to do and here is one possible way:

The function which calculates the sum of the sequence in O(1) time and O(1) space is then:
function Sum(n)
    return n * (n + 1) / 2


Below is the console application in C# which lets the user enter N and then prints the sum of the sequence of N elements.

using System;

namespace SumOfSequence

    public class Program

        static int Sum(int n)
            return n * (n + 1)  2;

        static void Main(string[] args)

            while (true)

                Console.Write("Enter sequence length (zero to exit): ");
                int n = int.Parse(Console.ReadLine());

                if (n <= 0)

                Console.WriteLine("Sum of the sequence is {0}\n", Sum(n));





Here is the possible output of the application listed above:

Enter sequence length (zero to exit): 4
Sum of the sequence is 10

Enter sequence length (zero to exit): 5
Sum of the sequence is 15

Enter sequence length (zero to exit): 43127
Sum of the sequence is 929990628

Enter sequence length (zero to exit): 0

See also:

Published: Dec 14, 2013


Zoran is software architect dedicated to clean design and CTO in a growing software company. Since 2014 Zoran is an author at Pluralsight where he is preparing a series of courses on object-oriented and functional design, design patterns, writing unit and integration tests and applying methods to improve code design and long-term maintainability.

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