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ML | Implementation of KNN classifier using Sklearn
  • Last Updated : 28 Nov, 2019

Prerequisite: K-Nearest Neighbours Algorithm

K-Nearest Neighbors is one of the most basic yet essential classification algorithms in Machine Learning. It belongs to the supervised learning domain and finds intense application in pattern recognition, data mining and intrusion detection. It is widely disposable in real-life scenarios since it is non-parametric, meaning, it does not make any underlying assumptions about the distribution of data (as opposed to other algorithms such as GMM, which assume a Gaussian distribution of the given data).

This article will demonstrate how to implement the K-Nearest neighbors classifier algorithm using Sklearn library of Python.

Step 1: Importing the required Libraries

import numpy as np
import pandas as pd
from sklearn.model_selection import train_test_split
from sklearn.neighbors import KNeighborsClassifier
import matplotlib.pyplot as plt 
import seaborn as sns

Step 2: Reading the Dataset

cd C:\Users\Dev\Desktop\Kaggle\Breast_Cancer
# Changing the read file location to the location of the file
df = pd.read_csv('data.csv')
y = df['diagnosis']
X = df.drop('diagnosis', axis = 1)
X = X.drop('Unnamed: 32', axis = 1)
X = X.drop('id', axis = 1)
# Separating the dependent and independent variable
X_train, X_test, y_train, y_test = train_test_split(
             X, y, test_size = 0.3, random_state = 0)
# Splitting the data into training and testing data

Step 3: Training the model

K = []
training = []
test = []
scores = {}
for k in range(2, 21):
    clf = KNeighborsClassifier(n_neighbors = k), y_train)
    training_score = clf.score(X_train, y_train)
    test_score = clf.score(X_test, y_test)
    scores[k] = [training_score, test_score]

Step 4: Evaluating the model

for keys, values in scores.items():
    print(keys, ':', values)

We now try to find the optimum value for ‘k’ ie the number of nearest neighbors.

Step 5: Plotting the training and test scores graph

ax = sns.stripplot(K, training);
ax.set(xlabel ='values of k', ylabel ='Training Score')
# function to show plot

ax = sns.stripplot(K, test);
ax.set(xlabel ='values of k', ylabel ='Test Score')

plt.scatter(K, training, color ='k')
plt.scatter(K, test, color ='g')
# For overlapping scatter plots

From the above scatter plot, we can come to the conclusion that the optimum value of k will be around 5.


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