The Fastest Python Method To Compute All Primes < N

Overview

Computing primes from a given series of numbers might not be a big issue. However, computing primes in the most effective manner keeping the time complexity and other factors in mind might be a challenging issue. After all, one of the major criteria of an effective code is its efficiency. Therefore, in this article, we will compare and contrast the complexities of different codes to compute all primes less than N, where N denotes an arbitrary number of values in the given series as entered by the user.

Example: Given below is a simple example of what is to follow next in this article:

import time
start_time = time.time()
n = int(input("Enter the last range of the series: "))
for i in range(1,n+1):
  if i>1:
    for j in range(2,i):
        if(i % j==0):
            break
    else:
        print(i)
end_time = time.time()
print("Elapsed Time: " + str(end_time-start_time))

Output:

Enter the last range of the series: 10
2
3
5
7
Elapsed Time: 3.9661035537719727

Disclaimer: The methods used in the below script are purely based on the least time taken to compute prime numbers < N. There are other methods of computing prime numbers, however, the time taken to resolve them for a larger range is quite high. Also, in case of better methods identified in the future which yield better performance, we shall be updating the article accordingly.

Without further delay let us dive into the comparisons and visualize the output.

Code Comparison

Instead of comparing the codes one by one which would make the article longer unnecessarily, here’s a list of all the probable methods to compute prime numbers in a given range with the least possible time taken for computation.

from sympy import sieve
import numpy
import itertools

izip = itertools.zip_longest
chain = itertools.chain.from_iterable
compress = itertools.compress
import time


def method1(n):
    """ Returns  a list of primes < n """
    sieve = [True] * n
    for i in range(3, int(n ** 0.5) + 1, 2):
        if sieve[i]:
            sieve[i * i::2 * i] = [False] * ((n - i * i - 1) // (2 * i) + 1)
    return [2] + [i for i in range(3, n, 2) if sieve[i]]


def method2(n):
    """ Returns  a list of primes < n """
    sieve = [True] * (n // 2)
    for i in range(3, int(n ** 0.5) + 1, 2):
        if sieve[i // 2]:
            sieve[i * i // 2::i] = [False] * ((n - i * i - 1) // (2 * i) + 1)
    return [2] + [2 * i + 1 for i in range(1, n // 2) if sieve[i]]


def method3(n):
    """ Input n>=6, Returns a array of primes, 2 <= p < n """
    sieve = numpy.ones(n // 3 + (n % 6 == 2), dtype=numpy.bool)
    for i in range(1, int(n ** 0.5) // 3 + 1):
        if sieve[i]:
            k = 3 * i + 1 | 1
            sieve[k * k // 3::2 * k] = False
            sieve[k * (k - 2 * (i & 1) + 4) // 3::2 * k] = False
    return numpy.r_[2, 3, ((3 * numpy.nonzero(sieve)[0][1:] + 1) | 1)]


def method4(n):
    """ Input n>=6, Returns a list of primes, 2 <= p < n """
    n, correction = n - n % 6 + 6, 2 - (n % 6 > 1)
    sieve = [True] * (n // 3)
    for i in range(1, int(n ** 0.5) // 3 + 1):
        if sieve[i]:
            k = 3 * i + 1 | 1
            sieve[k * k // 3::2 * k] = [False] * ((n // 6 - k * k // 6 - 1) // k + 1)
            sieve[k * (k - 2 * (i & 1) + 4) // 3::2 * k] = [False] * (
                    (n // 6 - k * (k - 2 * (i & 1) + 4) // 6 - 1) // k + 1)
    return [2, 3] + [3 * i + 1 | 1 for i in range(1, n // 3 - correction) if sieve[i]]


def method5(n):
    primes = list(sieve.primerange(1, n))
    return primes


def method6(n):
    """ Input n>=6, Returns a list of primes, 2 <= p < n """
    zero = bytearray([False])
    size = n // 3 + (n % 6 == 2)
    sieve = bytearray([True]) * size
    sieve[0] = False
    for i in range(int(n ** 0.5) // 3 + 1):
        if sieve[i]:
            k = 3 * i + 1 | 1
            start = (k * k + 4 * k - 2 * k * (i & 1)) // 3
            sieve[(k * k) // 3::2 * k] = zero * ((size - (k * k) // 3 - 1) // (2 * k) + 1)
            sieve[start::2 * k] = zero * ((size - start - 1) // (2 * k) + 1)
    ans = [2, 3]
    poss = chain(izip(*[range(i, n, 6) for i in (1, 5)]))
    ans.extend(compress(poss, sieve))
    return ans


m1_start = time.time()
method1(10 ** 6)
m1_end = time.time()
m1_et = m1_end - m1_start
print("Method 1 Elapsed time: " + str(m1_end - m1_start))

m2_start = time.time()
method2(10 ** 7)
m2_end = time.time()
m2_et = m2_end - m2_start
print("Method 2 Elapsed time: " + str(m2_end - m2_start))

m3_start = time.time()
method3(10 ** 7)
m3_end = time.time()
m3_et = m3_end - m3_start
print("Method 3 Elapsed time: " + str(m3_end - m3_start))

m4_start = time.time()
method4(10 ** 7)
m4_end = time.time()
m4_et = m4_end - m4_start
print("Method 4 Elapsed time: " + str(m4_end - m4_start))

m5_start = time.time()
method5(10 ** 7)
m5_end = time.time()
m5_et = m5_end - m5_start
print("Method 5 Elapsed time: " + str(m5_end - m5_start))

m6_start = time.time()
method6(10 ** 7)
m6_end = time.time()
m6_et = m6_end - m6_start
print("Method 6 Elapsed time: " + str(m6_end - m6_start))

Output:

Method 1 Elapsed time: 0.06881570816040039
Method 2 Elapsed time: 0.9155552387237549
Method 3 Elapsed time: 0.045876264572143555
Method 4 Elapsed time: 0.6512553691864014
Method 5 Elapsed time: 7.0082621574401855
Method 6 Elapsed time: 0.33211350440979004

From the above analysis, it is clear that Method 3 takes the minimum time to compute primes < n, while Method 5 takes the maximum time to do so. To compare and contrast the different methods and the time taken by each method we calculated the time to compute all primes within the range of 1 to 107 and then deduced our conclusion. Therefore,

Our Winner: Method 3

Disclaimer: The values of Elapsed Time Taken by each method as calculated by the time module may vary based on the system/hardware in use and the version of Python you are using.

In case you are still using Python 2.x, you might fancy the following methods given below:

from math import sqrt
import time


def method1(max_n):
    numbers = range(3, max_n + 1, 2)
    half = (max_n) // 2
    initial = 4

    for step in range(3, max_n + 1, 2):
        for i in range(initial, half, step):
            numbers[i - 1] = 0
        initial += 2 * (step + 1)

        if initial > half:
            return [2] + filter(None, numbers)


def method2(n):
    """sieveOfEratosthenes(n): return the list of the primes < n."""
    if n <= 2:
        return []
    sieve = range(3, n, 2)
    top = len(sieve)
    for si in sieve:
        if si:
            bottom = (si * si - 3) // 2
            if bottom >= top:
                break
            sieve[bottom::si] = [0] * -((bottom - top) // si)
    return [2] + [el for el in sieve if el]


def method3(n):
    s = range(3, n, 2)
    for m in xrange(3, int(n ** 0.5) + 1, 2):
        if s[(m - 3) / 2]:
            for t in xrange((m * m - 3) / 2, (n >> 1) - 1, m):
                s[t] = 0
    return [2] + [t for t in s if t > 0]


def method4(size):
    prime = [True] * size
    rng = xrange
    limit = int(sqrt(size))
    for i in rng(3, limit + 1, +2):
        if prime[i]:
            prime[i * i::+i] = [False] * len(prime[i * i::+i])

    return [2] + [i for i in rng(3, size, +2) if prime[i]]


m1_start = time.time()
method1(10 ** 6)
m1_end = time.time()
print("Method 1 Elapsed time: " + str(m1_end - m1_start))

m2_start = time.time()
method2(10 ** 6)
m2_end = time.time()
print("Method 2 Elapsed time: " + str(m2_end - m2_start))

m3_start = time.time()
method3(10 ** 6)
m3_end = time.time()
print("Method 3 Elapsed time: " + str(m3_end - m3_start))

m4_start = time.time()
method4(10 ** 6)
m4_end = time.time()
print("Method 4 Elapsed time: " + str(m4_end - m4_start))

Output:

Method 1 Elapsed time: 0.891271114349
Method 2 Elapsed time: 0.178880214691
Method 3 Elapsed time: 0.526117086411
Method 4 Elapsed time: 0.29536986351

Graphical Comparison

Considering that the above snippet is written in a file with the name plot.py, here’s a graphical analysis of the times taken by each method to compute all the prime numbers less than N. The code given below is used to plot the bar-graph for comparing the different methods used to compute primes < n.

import plot
import matplotlib.pyplot as plt
import numpy as np

method = ['Method 1', 'Method 2', 'Method 3', 'Method 4', 'Method 5', 'Method 6']
et = [plot.m1_et, plot.m2_et, plot.m3_et, plot.m4_et, plot.m5_et, plot.m6_et]
c = ["red", "green", "orange", "blue", "black", "purple"]
ypos = np.arange(len(method))
plt.xticks(ypos, method)
plt.bar(ypos, et, 0.4, color=c)
plt.title("Time To Compute Primes")
plt.xlabel("Methods")
plt.ylabel("Elapsed Time (seconds)")
plt.show()

Plot/Graph Output:

❖ Given below is another graphical comparison using a dashed line graph which compares the time taken by each method:

Output:

The code to generate the above graph is given below (The code containing the major methods has been mentioned above. We consider it to be present in a plot.py file and then we import it into our main class file to plot the graph.)

import plot
import matplotlib.pyplot as plt
import numpy as np
method = ['Method 1', 'Method 2', 'Method 3', 'Method 4', 'Method 5', 'Method 6']
et = [plot.m1_et, plot.m2_et, plot.m3_et, plot.m4_et, plot.m5_et, plot.m6_et]
ypos = np.arange(len(method))
plt.xticks(ypos, method)
plt.plot(ypos, et, color='green', linestyle='dashed', linewidth = 3,
         marker='o', markerfacecolor='blue', markersize=12)
plt.title("Time To Compute Primes")
plt.xlabel("Methods")
plt.ylabel("Elapsed Time (seconds)")
plt.show()

Conclusion

In this article, we compared several methods and found the best method in terms of the least time taken to compute all primes < n. The clear cut winner in our case was Method 3 as depicted in the output as well as in the graphs. However, this is just a comparison as of now and if we come across any changes or better methods in terms of performance shall be updated. Also, note that the comparisons are purely based on the time taken to compute each method, however, the memory utilization check might give you different results. Please feel free to check that as a practice for yourself. If you want to learn, how to plot graphs in Python, we have a tutorial for you here.

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Reference: https://stackoverflow.com/questions/2068372/fastest-way-to-list-all-primes-below-n