The KNN algorithm is one of the simplest yet highly effective machine learning algorithms. Its strength lies in its versatility, as it can be used for both classification and regression tasks, although it’s more commonly applied in classification. This article focuses on KNN for regression, exploring its implementation in Python, with examples and a detailed guide to understanding how KNN works in regression problems.

*My aim here is to illustrate and emphasize how KNN can be equally effective when the target variable is continuous in nature.*

**Learning Objectives**

- Understand the intuition behind KNN algorithms.
- Learn different ways to calculate distances between points.
- Practice implementing the algorithm in Python on the Big Mart Sales dataset.

If you want to understand the KNN algorithm in a course format, here is the link to our free course- **K-Nearest Neighbors (KNN) Algorithm in Python and R**

Let us start with a simple example. Consider the following table – it consists of the height, age, and weight (target) values for 10 people. As you can see, the weight value of ID11 is missing. We need to predict the weight of this person based on their height and age, utilizing the kNN algorithm.

**Note: The data in this table does not represent actual values. We’re simply employing it as an example to illustrate this concept.**

For a clearer understanding of this, below is the plot of height versus age from the above table:

In the above graph, the y-axis represents the height of a person (in feet) and the x-axis represents the age (in years). The points are numbered according to the ID values. The yellow point (ID 11) is our test point.

If I ask you to identify the weight of ID11 based on the plot, what would be your answer? You would likely say that since ID11 is **closer** to points 5 and 1, so it must have a weight similar to these IDs, probably between 72-77 kgs (weights of ID1 and ID5 from the table). That actually makes sense, but how do you think the algorithm predicts the values? We will find that out in this article.

**Here is a free video-based course to help you understand the KNN algorithm – K-Nearest Neighbors (KNN) Algorithm in Python and R**

The KNN algorithm can be used for both classification and regression. It predicts new data points based on their similarity to points in the training set. In our example, ID11 has height and age similar to ID1 and ID5, so its weight would be predicted accordingly.

For classification, the mode would be the final prediction, but in this case, the average of the weights (72 and 77) is taken as the final prediction.

Below is a stepwise explanation of the algorithm:

1. First, the distance between the new point and each training point is calculated.

2. The closest k data points are selected (based on the distance). In this example, points 1, 5, and 6 will be selected if the value of k is 3. We will further explore the method to select the right value of k later in this article.

3. The average of these data points is the final prediction for the new point. Here, we have the weight of ID11 = (77+72+60)/3 = 69.66 kg.

In the next few sections, we will discuss each of these three steps in detail.

The **first step** is to calculate the distance between the new point and each training point. There are various methods for calculating this distance, of which the most commonly known methods are – Euclidian, Manhattan (for continuous), and Hamming distance (for categorical).

**Euclidean Distance:**Euclidean distance is calculated as the square root of the sum of the squared differences between a new point (x) and an existing point (y).**Manhattan Distance**: This is the distance between real vectors using the sum of their absolute difference.

**Hamming Distance**: It is used for categorical variables. If the value (x) and the value (y) are the same, the distance D will be equal to 0. Otherwise D=1.

There is also Minkowski distance which is a generalized form of Euclidean and manhattan distances.

After measuring the distance of a new observation from the points in our training set, we proceed to select the closest points. The number of points to be considered is determined by the value of k.

The **second step** is to select the k value. This determines the number of neighbors we look at when we assign a value to any new observation.

In our example, for a value k = 3, the closest points are ID1, ID5, and ID6.

The prediction of weight for ID11 will be:

```
ID11 = (77+72+60)/3
ID11 = 69.66 kg
```

For the value of k=5, the closest point will be ID1, ID4, ID5, ID6, and ID10.

The prediction for ID11 will be:

```
ID 11 = (77+59+72+60+58)/5
ID 11 = 65.2 kg
```

We notice that based on the k value, the final result tends to change. Then how can we figure out the optimum value of k? Let us decide based on the error calculation for our train and validation set (after all, minimizing the error is our final goal!).

Have a look at the below graphs for training error and validation error for different values of k.

For a very low value of k (suppose k=1), the model is overfitting the training data, which leads to a high error rate on the validation set. On the other hand, for a high value of k, the model performs poorly on both the train and validation sets. If you observe closely, the validation error curve reaches a minimum at a value of k = 9. This value of k is the optimum value of the model (it will vary for different datasets). Researchers typically use the elbow curve, named for its resemblance to an elbow, to determine the k value.

By now, you must have a clear understanding of the algorithm. If you have any questions regarding the same, please use the comments section below, and I will be happy to answer them. We will now go ahead and implement the algorithm on a dataset. I have used the Big Mart sales dataset to show the implementation; you can download it from **this link**.

The full Python code is below, but we have a really cool coding window here where you can code your own k-Nearest Neighbor model in Python using sklearn k nearest neighbors:

```
import pandas as pd
df = pd.read_csv('train.csv')
df.head()
```

```
df.isnull().sum()
#missing values in Item_weight and Outlet_size needs to be imputed
mean = df['Item_Weight'].mean() #imputing item_weight with mean
df['Item_Weight'].fillna(mean, inplace =True)
mode = df['Outlet_Size'].mode() #imputing outlet size with mode
df['Outlet_Size'].fillna(mode[0], inplace =True)
```

```
df.drop(['Item_Identifier', 'Outlet_Identifier'], axis=1, inplace=True)
df = pd.get_dummies(df)
```

```
from sklearn.model_selection import train_test_split
train , test = train_test_split(df, test_size = 0.3)
x_train = train.drop('Item_Outlet_Sales', axis=1)
y_train = train['Item_Outlet_Sales']
x_test = test.drop('Item_Outlet_Sales', axis = 1)
y_test = test['Item_Outlet_Sales']
```

```
from sklearn.preprocessing import MinMaxScaler
scaler = MinMaxScaler(feature_range=(0, 1))
x_train_scaled = scaler.fit_transform(x_train)
x_train = pd.DataFrame(x_train_scaled)
x_test_scaled = scaler.fit_transform(x_test)
x_test = pd.DataFrame(x_test_scaled)
```

```
#import required packages
from sklearn import neighbors
from sklearn.metrics import mean_squared_error
from math import sqrt
import matplotlib.pyplot as plt
%matplotlib inline
```

```
rmse_val = [] #to store rmse values for different k
for K in range(20):
K = K+1
model = neighbors.KNeighborsRegressor(n_neighbors = K)
model.fit(x_train, y_train) #fit the model
pred=model.predict(x_test) #make prediction on test set
error = sqrt(mean_squared_error(y_test,pred)) #calculate rmse
rmse_val.append(error) #store rmse values
print('RMSE value for k= ' , K , 'is:', error)
```

**Output:**

```
RMSE value for k = 1 is: 1579.8352322344945
RMSE value for k = 2 is: 1362.7748806138618
RMSE value for k = 3 is: 1278.868577489459
RMSE value for k = 4 is: 1249.338516122638
RMSE value for k = 5 is: 1235.4514224035129
RMSE value for k = 6 is: 1233.2711649472913
RMSE value for k = 7 is: 1219.0633086651026
RMSE value for k = 8 is: 1222.244674933665
RMSE value for k = 9 is: 1219.5895059285074
RMSE value for k = 10 is: 1225.106137547365
RMSE value for k = 11 is: 1229.540283771085
RMSE value for k = 12 is: 1239.1504407152086
RMSE value for k = 13 is: 1242.3726040709887
RMSE value for k = 14 is: 1251.505810196545
RMSE value for k = 15 is: 1253.190119191363
RMSE value for k = 16 is: 1258.802262564038
RMSE value for k = 17 is: 1260.884931441893
RMSE value for k = 18 is: 1265.5133661294733
RMSE value for k = 19 is: 1269.619416217394
RMSE value for k = 20 is: 1272.10881411344
```

```
#plotting the rmse values against k values
curve = pd.DataFrame(rmse_val) #elbow curve
curve.plot()
```

As we discussed, when we take k=1, we get a very high RMSE value. The RMSE value decreases as we increase the k value. At k= 7, the RMSE is approximately 1219.06 and shoots upon further increasing the k value. We can safely say that k=7 will give us the best result in this case.

These are the predictions using our training dataset. Let us now predict the values for test data and make a submission.

```
#reading test and submission files
test = pd.read_csv('test.csv')
submission = pd.read_csv('SampleSubmission.csv')
submission['Item_Identifier'] = test['Item_Identifier']
submission['Outlet_Identifier'] = test['Outlet_Identifier']
#preprocessing test dataset
test.drop(['Item_Identifier', 'Outlet_Identifier'], axis=1, inplace=True)
test['Item_Weight'].fillna(mean, inplace =True)
test = pd.get_dummies(test)
test_scaled = scaler.fit_transform(test)
test = pd.DataFrame(test_scaled)
#predicting on the test set and creating submission file
predict = model.predict(test)
submission['Item_Outlet_Sales'] = predict
submission.to_csv('submit_file.csv',index=False)
```

On submitting this file, I get an RMSE of 1279.5159651297.

**Step 8: Implementing GridsearchCV **

For deciding the value of k, plotting the elbow curve every time is a cumbersome and tedious process. **You can simply use grid search to find the best parameter value.**

```
from sklearn.model_selection import GridSearchCV
params = {'n_neighbors':[2,3,4,5,6,7,8,9]}
knn = neighbors.KNeighborsRegressor()
model = GridSearchCV(knn, params, cv=5)
model.fit(x_train,y_train)
model.best_params_
```

**Output:**

`{'n_neighbors': 7}`

In this article, we covered the workings of the KNN algorithm and its implementation in Python. It’s one of the most basic yet effective machine-learning models. For KNN implementation in R, you can go through this tutorial: kNN Algorithm using R. You can also go for our free course – **K-Nearest Neighbors (KNN) Algorithm in Python and R**, to further your foundations of KNN.

Hope you like the article, Where we had covered the KNN model directly from the *scikit-learn* library. Also, We have Cover about the Knn regression in python, knn regression , What is knn algorithm. And If you think you know KNN well and have a solid grasp of the technique, test your skills in this MCQ quiz: **30 questions on kNN Algorithm**. Good luck!

- We have learned how to implement KNN in Python.
- We have learned to compute the optimum value of the K hyper-parameter.
- We have learned that the KNN regression model is useful in many regression problems.

A. The K nearest neighbors supervised machine learning algorithm performs classification and regression tasks effectively. In this, we calculate the distance between features of test data points against those of train data points. We calculate prediction values by taking the mode or mean.

A. Yes, we can use KNN for regression. Here, we take the k nearest values of the target variable and compute the mean of those values. Those k nearest values act like regressors of linear regression.

A. Linear regression: fast, interpretable, for linear data. KNN regression: handles non-linearity, less affected by outliers, slower, less interpretable.

Choose linear regression for speed and clear relationships, KNN for complex patterns or when outliers are a concern.

A. KNN (K-Nearest Neighbors) Classifier is a type of machine learning algorithm used for classification tasks. It is a non-parametric algorithm, which means it does not make any assumptions about the underlying distribution of the data.

In KNN Classifier, a new data point is classified based on its proximity to the K nearest neighbors in the training set. We calculate proximity using a distance metric, such as Euclidean distance or Manhattan distance. Then, we determine the predicted class of the new data point based on the majority class among its K nearest neighbors.

Hi Aishwarya, your explanation on KNN is really helpful. I have a doubt though. KNN suffers from the dimensionality curse i.e. Euclidean distance is not helpful when subjected to high dimensions as it is equidistant for different vectors. What was your viewpoint while using the KNN despite this fact ? Curious to know. Thank you.

Hi, KNN works well for dataset with less number of features and fails to perform well has the number of inputs increase. Certainly other algorithms would show a better performance in that case. With this article I have tried to introduce the algorithm and explain how it actually works (instead of simply using it as a black box).

Hi. I have been following you now for a while. Love your post. I wish you cold provide a pdf format also, because it is hard to archive and read web posts when you are offline.

Hi Osman, Really glad you liked the post. Although certain articles and cheat sheets are converted and shared as pdf, but not all articles are available in the format. Will certainly look into it and see if we can have an alternate.

Hi thanks for the explanations. Can you explain the intuition behind categorical variable calculations. For example, assume data set has height,age,gender as independent variable and weight as dependent variable. Now if an unseen data comes with male as gender, then how it works? how it predicts the weight?

Hi, Excellent question! Suppose we have gender as a feature, we would use hamming distance to find the closest point (We need to find the distance with each training point as discussed in the article). Let us take the first training point, if it has the gender male and my test point also has the gender male, then the distance D will be 0. Now we take the second training point, it it has gender female and the test point has gender male, the value of D will be 1. Simply put, we will consider the test point closer to first training point and predict the target value accordingly.