I am new to python and dont know the advanced features or stuff. Please explain the following code step by step. how it works.

Question

Its a code to get the greatest mersenne prime within an upper limit of value. Describe the sieve of eratosthenes also.
#largestmersenneprime
def SieveOfEratosthenes(n, prime):
for i in range(0, n + 1) :
prime[i] = True
p = 2
while(p * p <= n):
if (prime[p] == True) :
for i in range(p * 2, n + 1, p):
prime[i] = False
p += 1
list=[]
def mersennePrimes(n) :
prime = [0] * (n + 1)
SieveOfEratosthenes(n, prime)
k = 2
while(((1 << k) - 1) <= n ):
num = (1 << k) - 1
if (prime[num]) :
list.append(num)
k += 1
n = int(input("enter upper limit (893452 suggested) : "))
mersennePrimes(n)
print("largest Mersenne prime numbers smaller than or equal to " , n ,"is : ",list[-1])

Answer 1

https://play.google.com/store/apps/details?id=com.W3school.Anbu go to this app and download it

Answer 2

https://play.google.com/store/apps/details?id=com.lms.greatlakes and this also

Answer 3

Akash Adhikary Where is ur code