Peak Finding in C# and Python

February 12, 2018

Do you have a data, from which peak values need to be extracted? Whether it's finding mountain peaks, peak performance days/points in your daily performance(workouts, running, amount of stuff done), building an app to do that, or just playing with algorithms, this article may be of use to you.

What is a Peak

Peak is the point/value that is higher than its surroundings. Therefore we can assume that in [1, 6, 1], the 6 is the peak.

Our definition is going to be exactly this, however we are not looking for the one ultimate peak, but all the peaks we can find. Anything that is higher than its neighbors will be a peak for us.

That way we will often find multiple values, as long as they are peaks. In [4,9,2,7,6,3], 9 and 7 are the peaks.

The Code

You can start from this simple example in C# and one Python app that I would like to share. In C# we will have something like this:

using System;
using System.Collections.Generic;
using System.Diagnostics;

namespace AlgosC
{
    class LinearPeaks
    {
        static void Main(string[] args)
        {
            List<int> numbersList = new List<int>();
            Console.Write("Length: ");
            int length = Int32.Parse(Console.ReadLine());

            Random rnd = new Random();
            for (int i = 0; i < length; i++)
            {
                numbersList.Add(rnd.Next(10));
            }
            Console.WriteLine("The list contains " + numbersList.Count + " numbers.");

            Stopwatch sw = new Stopwatch();
            List<int> findPeaks(List<int> numbers)
            {
                sw.Start();
                List<int> peaks = new List<int>();
                for (int i = 0; i < numbers.Count; i++)
                {
                    if (i == 0)
                    {
                        if (numbers[i] >= numbers[i + 1])
                        {
                            peaks.Add(numbers[i]);
                        }
                    }

                    if (i > 0 && i < (numbersList.Count - 1))
                    {
                        if (numbers[i] >= numbers[i + 1] && numbers[i] >= numbers[i - 1])
                        {
                            peaks.Add(numbers[i]);
                        }
                    }

                    if (i >= (numbersList.Count - 1))
                    {
                        if (numbers[i] >= numbers[i - 1])
                        {
                            peaks.Add(numbers[i]);
                        }
                    }
                }
                sw.Stop();
                return peaks;
            }

            Console.WriteLine("\n There are " + findPeaks(numbersList).Count + " peaks.");
            Console.WriteLine("\n It took " + sw.Elapsed + " to find the peaks in " + numbersList.Count + " numbers.");
            Console.ReadKey();
        }
    }
}

Once you input the X amount in length, and press enter, a list of X random numbers 1-10 will be generated and searched for peaks. After the process is done, you'll also see how long it took in seconds. Of course you can adjust the code to your needs and play around with it however you see fit.

Now same thing in Python:

import random
import sys
import time

numbers_list = []
length = int(input("Enter the number: "))

for x in range(0, length):
    numbers_list.append(random.randrange(1, 10))

print("The list contains ", len(numbers_list), " numbers.")

def find_peaks(numbers):
    start_time = time.time()
    peaks = [];
    for x in range(0, len(numbers)):
        if x == 0:
            if numbers[x] >= numbers[x + 1]:
                peaks.append(numbers[x])
        elif ((x > 0) and (x < (len(numbers_list) - 1))):
            if ((numbers[x] >= numbers[x - 1]) and (numbers[x] >= numbers[x + 1])):
                peaks.append(numbers[x])
        elif x >= len(numbers_list):
            if numbers[x] >= numbers[x - 1]:
                peaks.append(numbers[x])
    elapsed_time = time.time() - start_time
    return peaks, elapsed_time

peaks, elapsed_time = find_peaks(numbers_list)
print("There are ", len(peaks), " peaks.")
print("It took ", elapsed_time, " s to find peaks for ", len(numbers_list), " numbers.")

Python is doing the exact same thing, but it takes less code and more time, as expected.

If you like speed, choose C#, or even better C++/C. If you want to get up and running fast, choose Python. And of course, if you're a Trainspotting fan, choose life!