K – means clustering is an unsupervised algorithm that is used in customer segmentation applications. In this algorithm, we try to form clusters within our datasets that are closely related to each other in a high-dimensional space.
In this article, we will see how to use the k means algorithm to identify the clusters of the digits.
Load the Datasets
from sklearn.datasets import load_digits
digits_data = load_digits().data
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Output:
array([[ 0., 0., 5., ..., 0., 0., 0.],
[ 0., 0., 0., ..., 10., 0., 0.],
[ 0., 0., 0., ..., 16., 9., 0.],
...,
[ 0., 0., 1., ..., 6., 0., 0.],
[ 0., 0., 2., ..., 12., 0., 0.],
[ 0., 0., 10., ..., 12., 1., 0.]])Each handwritten digit in the data is an array of color values of pixels of its image. For better understanding, let’s print how the data of the first digit looks like and then display its’s respective image
import matplotlib.pyplot as plt
print("First handwritten digit data: " + digits_data[0])
sample_digit = digits_data[0].reshape(8, 8)
plt.imshow(sample_digit) plt.title("Digit image")
plt.show() |
Output:
First handwritten digit data: [ 0. 0. 5. 13. 9. 1. 0. 0. 0. 0. 13. 15. 10. 15. 5. 0. 0. 3.
15. 2. 0. 11. 8. 0. 0. 4. 12. 0. 0. 8. 8. 0. 0. 5. 8. 0.
0. 9. 8. 0. 0. 4. 11. 0. 1. 12. 7. 0. 0. 2. 14. 5. 10. 12.
0. 0. 0. 0. 6. 13. 10. 0. 0. 0.]
Sample image from the dataset
In the next step, we scale the data. Scaling is an optional yet very helpful technique for the faster processing of the model. In our model, we scale the pixel values which are typically between 0 – 255 to -1 – 1, easing the computation and avoiding super large numbers. Another point to consider is that a train test split is not required for this model as it is unsupervised learning with no labels to test. Then, we define the k value, which is 10 as we have 0-9 digits in our data. Also setting up the target variable.
from sklearn.preprocessing import scale
scaled_data = scale(digits_data)
print(scaled_data)
Y = load_digits().target
print(Y)
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Output:
[[ 0. -0.33501649 -0.04308102 … -1.14664746 -0.5056698
-0.19600752]
[ 0. -0.33501649 -1.09493684 … 0.54856067 -0.5056698
-0.19600752]
[ 0. -0.33501649 -1.09493684 … 1.56568555 1.6951369
-0.19600752]
…
[ 0. -0.33501649 -0.88456568 … -0.12952258 -0.5056698
-0.19600752]
[ 0. -0.33501649 -0.67419451 … 0.8876023 -0.5056698
-0.19600752]
[ 0. -0.33501649 1.00877481 … 0.8876023 -0.26113572
-0.19600752]]
[0 1 2 … 8 9 8]
Defining k-means clustering:
Now we define the K-means cluster using the KMeans function from the sklearn module.
Method 1: Using a Random initial cluster.
- Setting the initial cluster points as random data points by using the ‘init‘ argument.
- The argument ‘n_init‘ is the number of iterations the k-means clustering should run with different initial clusters chosen at random, in the end, the clustering with the least total variance is considered’
- The random state is kept to 0 (any number can be given) to fix the same random initial clusters every time the code is run.
from sklearn.cluster import KMeans
k = 10
kmeans_cluster = KMeans(init = "random",
n_clusters = k,
n_init = 10,
random_state = 0)
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Method 2: Using k-means++
It is similar to method-1 however, it is not completely random, and chooses the initial clusters far away from each other. Therefore, it should require fewer iterations in finding the clusters when compared to the random initialization.
kmeans_cluster = KMeans(init="k-means++", n_clusters=k, n_init=10, random_state=0)
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Model Evaluation
We will use scores like silhouette score, time taken to reach optimum position, v_measure and some other important metrics.
def bench_k_means(estimator, name, data):
initial_time = time()
estimator.fit(data)
print("Initial-cluster: " + name)
print("Time taken: {0:0.3f}".format(time() - initial_time))
print("Homogeneity: {0:0.3f}".format(
metrics.homogeneity_score(Y, estimator.labels_)))
print("Completeness: {0:0.3f}".format(
metrics.completeness_score(Y, estimator.labels_)))
print("V_measure: {0:0.3f}".format(
metrics.v_measure_score(Y, estimator.labels_)))
print("Adjusted random: {0:0.3f}".format(
metrics.adjusted_rand_score(Y, estimator.labels_)))
print("Adjusted mutual info: {0:0.3f}".format(
metrics.adjusted_mutual_info_score(Y, estimator.labels_)))
print("Silhouette: {0:0.3f}".format(metrics.silhouette_score(
data, estimator.labels_, metric='euclidean', sample_size=300)))
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We will now use the above helper function to evaluate the performance of our k means algorithm.
kmeans_cluster = KMeans(init="random", n_clusters=k, n_init=10, random_state=0)
bench_k_means(estimator=kmeans_cluster, name="random", data=digits_data)
kmeans_cluster = KMeans(init="k-means++", n_clusters=k,
n_init=10, random_state=0)
bench_k_means(estimator=kmeans_cluster, name="random", data=digits_data)
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Output:
Initial-cluster: random Time taken: 0.302 Homogeneity: 0.739 Completeness: 0.748 V_measure: 0.744 Adjusted random: 0.666 Adjusted mutual info: 0.741 Silhouette: 0.191 Initial-cluster: random Time taken: 0.386 Homogeneity: 0.742 Completeness: 0.751 V_measure: 0.747 Adjusted random: 0.669 Adjusted mutual info: 0.744 Silhouette: 0.175
Visualizing the K-means clustering for handwritten data:
- Plotting the k-means cluster using the scatter function provided by the matplotlib module.
- Reducing the large dataset by using Principal Component Analysis (PCA) and fitting it to the previously defined k-means++ model.
- Plotting the clusters with different colors, a centroid was marked for each cluster.
from sklearn.decomposition import PCA
import numpy as np
# Reducing the dataset pca = PCA(2)
reduced_data = pca.fit_transform(digits_data)
kmeans_cluster.fit(reduced_data) # Calculating the centroids centroids = kmeans_cluster.cluster_centers_
label = kmeans_cluster.fit_predict(reduced_data)
unique_labels = np.unique(label)
# plotting the clusters: plt.figure(figsize=(8, 8))
for i in unique_labels:
plt.scatter(reduced_data[label == i, 0],
reduced_data[label == i, 1],
label=i)
plt.scatter(centroids[:, 0], centroids[:, 1],
marker='x', s=169, linewidths=3,
color='k', zorder=10)
plt.legend() plt.show() |
Output:
Clusters of the data points
Conclusion
From the above graph, we can observe the clusters of the different digits are approximately separable from one another.