No output in my code

Question

i tried making a primality test, i tried to input 237028843(this is a prime) but there is no output

here's the code :
p = int(input())

if p < 2 :
print(p, 'is not a prime')
print(' ')
print('No prime number is less than 2')

r = p//2
if r % 2 == 0 :
r+= 1

if p > 1 :
for n in range(3, r+1, 2) :
if p % 2 == 0 and p != 2 :
print(p, 'is not a prime')
print(' ')
print(p, 'is 2*'+str(p//2))
break
elif p == 2 :
print(p, 'is a prime')
break
if p % n == 0 :
print(p, 'is not a prime')
print(' ')
print(p, 'is '+str(n)+'*'+str(p//n))
break
else :
if p > 1 :
print(p, 'is a prime')

can someone tell me why is there no output in my code

Answer 1

Valent Inkiriwang

Running your code on large numbers seems to result in an "Execution Timed Out!" error. This can be shown by adding a `print(n)` at the start of the loop. Not entirely sure how or why this happens, but it seems to be because the loop executes the same few lines of code too many times without an output:
https://www.sololearn.com/discuss/2076661/?ref=app

This can be fixed with some simple optimizing:
1) The part of the loop that tests for p==2 and p%2==0 can be taken out of the loop and into the top level of the code, since these conditions don't depend on n.
2) Instead of testing for divisibility of n up to r=p//2, you can test up to r=math.sqrt(p) to further reduce the number of iterations.

https://en.m.wikipedia.org/wiki/Primality_test

Hope this helps :)

Answer 2

# Hi ,Valent Inkiriwang, you can take a look at this one:

def isPrime(n):
"""isPrime(n): int -> None"""
if n < 2 or (n > 2 and (n % 2) == 0):
print(n, "is not a prime!")
elif n in [2, 3]:
print(n, "is a prime!")
else:
for i in range(3, n, 2):
if n % i == 0:
print(n, "is not a prime!")
break
elif i > n ** (1/2):
print(n, "is a prime!")
break

n = 237028843
isPrime(n)

Answer 3

your code works well. however, you need to optimize it to do things better because 237028843 is a large number and your code processes a lot of steps

my suggest code:
from math import sqrt
def isPrime(p):
if p<2 or p%2==0: return False
if p==2 or p==3: return True
for i in range(3, int(sqrt(p)+1), 2):
if p%i==0: return False
return True