K-Nearest Neighbors (KNN)

Machine learning algorithms can be implemented from scratch (for the purpose of understanding how it works) or it can be used by implementing the module which is already present. 

In this post, we will understand what KNN or k-nearest neighbours is, and how it can be implemented in Python. 

What is kNN? 

It is a supervised algorithm that is widely used in data mining, pattern recognition and many other techniques. It is used in classification as well as regression problems. It is also known as ‘lazy learning’, ‘instance-based learning’ and ‘non-parametric learning. The reason behind calling kNN with these names will be cleared when you read this post. 

For example: Consider we have 2 classes-class ‘A’ and class ‘B’. Class ‘A’ consists of chocolates and class‘B’ consists of chips and other spicy items. Suppose one more entity needs to be put into one of these classes (‘A’ or ‘B’), this will be done based on the characteristics of the item. Suppose the item is spicy (assume someone has already tasted it), looks crispy and is orange/red in color, it can be classified as belonging to the spicy class or class ‘B’. On the other hand, if the item is sweet (again, assume someone has tasted it), looks soft, it can be classified as an item belonging to class ‘B’. 

Note: This is a hypothetical example, since not all eatables which are red belong to the spicy class andnot all eatables which are soft belong to the sweet class. 

kNN algorithm doesn’t really learn anything from the data it has been given, but tries to find a match for the newly given data point based on how similar it is to one of the classes in the training dataset. Hence the name ‘lazy learning’ is also used to refer to kNN. This means predictions are made using the training dataset itself. Hence the name ‘instance-based learning’. 

When a new instance of data is provided, it tries to find ‘k’ (a fixed integer) values from the training dataset which are very similar to the new instance of data. It doesn’t assume anything about the data while classifying new instances into one of the classes. Hence the name ‘non-parametric algorithm’. 

Below is an image showing what kNN is used for: 

To understand which of these ‘k’ values need to be calculated, the shortest distance between the new instance and the data point from the training set is considered. In general, for real-valued data, and data that is similar in type (classifying as sweet or spicy, finding heights, weights), Euclidean distance is used. 

But based on the nature of our data, various other methods can be used, such as Hamming distance, Jaccard distance, Cosine distance or Manhattan distance. 

How to find which value of ‘k’ is right for my dataset? 

The answer to this is, ‘it depends on the dataset’. Usually, the trial and error method is used to see what value of ‘k’ gives the highest accuracy in that specific case. Values ranging from 2 to 15 are experimented with. 

We will see the implementation of kNN from scratch. We will be using the iris dataset to implement this algorithm. Download it from here.

It will be downloaded as a zip file which needs to be unzipped and the path of this CSV file has to be supplied to the below code. 

Implementation of kNN from scratch using Euclidean Distance 

Here’s the implementation: 

import csv 
import random 
import operator 
import math 
iris_data_path = "path to iris_data.csv" 
def loadDataset(filename, split, trainingSet=[] , testSet=[]): 
with open(filename, 'r') as csvfile: 
lines = csv.reader(csvfile) 
dataset = list(lines) 
for x in range(len(dataset)-1): 
for y in range(4): 
dataset[x][y] = float(dataset[x][y]) 
if random.random() < split: 
trainingSet.append(dataset[x]) 
else: 
testSet.append(dataset[x]) 
def euclideanDistance(instance1, instance2, length): 
distance = 0 
for x in range(length): 
distance += pow((instance1[x] - instance2[x]), 2) 
return math.sqrt(distance) 
def getNeighbors(trainingSet, testInstance, k): 
distances = [] 
length = len(testInstance)-1 
for x in range(len(trainingSet)): 
dist = euclideanDistance(testInstance, trainingSet[x], length) 
distances.append((trainingSet[x], dist)) 
distances.sort(key=operator.itemgetter(1)) 
neighbors = [] 
for x in range(k): 
neighbors.append(distances[x][0]) 
return neighbors 
def getResponse(neighbors): 
classVotes = {} 
for x in range(len(neighbors)): 
response = neighbors[x][-1] 
if response in classVotes: 
classVotes[response] += 1 
else: 
classVotes[response] = 1 
sortedVotes = sorted(classVotes.items(), key=operator.itemgetter(1), reverse=True) 
return sortedVotes[0][0] 
def getAccuracy(testSet, predictions): 
correct = 0 
for x in range(len(testSet)): 
if testSet[x][-1] == predictions[x]: 
correct += 1 
return (correct/float(len(testSet))) * 100.0 
def main(): 
# prepare data 
trainingSet=[] 
testSet=[] 
split = 0.67 
loadDataset(iris_data_path, split, trainingSet, testSet) 
print('Train set: ' + repr(len(trainingSet))) 
print('Test set: ' + repr(len(testSet))) 
generate predictions predictions=[] 
k = 3 
for x in range(len(testSet)): 
neighbors = getNeighbors(trainingSet, testSet[x], k) 
result = getResponse(neighbors) 
predictions.append(result) 
print('> predicted=' + repr(result) + ', actual=' + repr(testSet[x][-1])) 
accuracy = getAccuracy(testSet, predictions) 
print('Accuracy: ' + repr(accuracy) + '%') 
main() 

Advantages of kNN Algorithm 

1. Easy to understand and implement. 
2. No need to build a model or tune parameters or make assumptions. 
3. Versatile algorithm- can be used for classification and regression. 

Disadvantages of kNN Algorithm 

• It becomes slow when the number of independent variables increases.

Conclusion 

In today's post, we understood how kNN algorithm works, how the value of 'k' is chosen based on different factors, its implementation from scratch, its advantages and its disadvantages.