Here is my implementation of the k-means algorithm in python. I would love to get any feedback on how it could be improved or any logical errors that you may see. I've left off a lot of the boilerplate code like the command line argument parsing, error handling for data read in from CSV file, etc. and just added the meat of the algorithm.

import os
import numpy as np

# kmeans clustering algorithm
# data = set of data points
# k = number of clusters
# c = initial list of centroids (if provided)
def kmeans(data, k, c):
    centroids = []

    centroids = randomize_centroids(data, centroids, k)  

    old_centroids = [[] for i in range(k)] 

    iterations = 0
    while not (has_converged(centroids, old_centroids, iterations)):
        iterations += 1

        clusters = [[] for i in range(k)]

        # assign data points to clusters
        clusters = euclidean_dist(data, centroids, clusters)

        # recalculate centroids
        index = 0
        for cluster in clusters:
            old_centroids[index] = centroids[index]
            centroids[index] = np.mean(cluster, axis=0).tolist()
            index += 1

    print("The total number of data instances is: " + str(len(data)))
    print("The total number of iterations necessary is: " + str(iterations))
    print("The means of each cluster are: " + str(centroids))
    print("The clusters are as follows:")
    for cluster in clusters:
        print("Cluster with a size of " + str(len(cluster)) + " starts here:")
        print("Cluster ends here.")


# Calculates euclidean distance between
# a data point and all the available cluster
# centroids.      
def euclidean_dist(data, centroids, clusters):
    for instance in data:  
        # Find which centroid is the closest
        # to the given data point.
        mu_index = min([(i[0], np.linalg.norm(instance-centroids[i[0]])) \
                            for i in enumerate(centroids)], key=lambda t:t[1])[0]
        except KeyError:
            clusters[mu_index] = [instance]

    # If any cluster is empty then assign one point
    # from data set randomly so as to not have empty
    # clusters and 0 means.        
    for cluster in clusters:
        if not cluster:
            cluster.append(data[np.random.randint(0, len(data), size=1)].flatten().tolist())

    return clusters

# randomize initial centroids
def randomize_centroids(data, centroids, k):
    for cluster in range(0, k):
        centroids.append(data[np.random.randint(0, len(data), size=1)].flatten().tolist())
    return centroids

# check if clusters have converged    
def has_converged(centroids, old_centroids, iterations):
    if iterations > MAX_ITERATIONS:
        return True
    return old_centroids == centroids
  • \$\begingroup\$ See this question and its answer. \$\endgroup\$ Commented Feb 12, 2015 at 16:21
  • \$\begingroup\$ It would be nice if you could accept the answer. Moreover, where is c used in your code? \$\endgroup\$
    – gsamaras
    Commented Jan 31, 2016 at 13:43
  • \$\begingroup\$ what is the data type for data ? Numpy array or list of lists/tuple of tuples? \$\endgroup\$
    – webmaker
    Commented Apr 28, 2016 at 17:40

2 Answers 2


Avoid comments that are identical to the code:

# check if clusters have converged  <-- Remove this
def has_converged(centroids, old_centroids, iterations):


Constants should be put at the top of the file to be easier to tweak.

Separate calculations from IO, your function kmeans should return the values and then another function (maybe pretty_format_k_means) should create a human readable message.

# k = number of clusters
# c = initial list of centroids (if provided)

Multi-character variable names are allowed, rename your variables (and function arguments) to more meaningful names, and then you can delete the comments.

  • 1
    \$\begingroup\$ I will agree with Caridorc, but renaming k in k-means will not be such a good idea, I think, +1 though! \$\endgroup\$
    – gsamaras
    Commented Jan 29, 2016 at 23:05

To expand Caridorc's answer, I would change that:

# c = initial list of centroids (if provided)
def kmeans(data, k, c):


# c = initial list of centroids (if provided)
def kmeans(data, k, c=None):

since c may not be provided. So I used the None keyword.

However, note that in your code, you do not use c!


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