10 examples of 'python program to find prime number' in Python

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20def prime(num):
21 # num is actually a string because input() returns strings. We'll convert it to int
22 num = int(num)
23
24 if num < 0:
25 print("Negative integers can not be prime")
26 quit()
27 if num is 1:
28 print("1 is neither prime nor composite")
29 # See how I lazily terminated program otherwise it'd forward "None"(default behaviour of python when function
30 # returns nothing) rather than True or False. Which could mess up the program.
31 # If we hit this if statement above statement is printed then program exits.
32 quit() # Now you don't need to get sys.exit() to exit python has quit to handle the same thing
33 if num in [2, 3]:
34 # if given argument is 2 or 3, it is prime. We used list without defining a variable which is perfectly valid
35 return True
36 if num % 2 == 0: # excluding all even numbers except two.
37 return False
38 else:
39 # Here we are starting counter variable from 3 in range. Second argument excludes numbers above one third
40 # of the given argument. Third argument in range sets steps to take to 2. This makes loop to iterate odds
41 for x in range(3, int(num/3), 2):
42 # Checking if argument is divisible by counter. % is modulus operator which returns remainder of division
43 if num % x == 0:
44 return False
45 # It's okay to have more than one return statement when program hits return statement it exits the function.
46 return True
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12def primes():
13 yield 2
14 yield 3
15
16 for i in itertools.count(start=5, step=2):
17 if is_prime(i):
18 yield i
7def isPrime(num):
8 # Returns True if num is a prime number, otherwise False.
9
10 # Note: Generally, isPrime() is slower than primeSieve().
11
12 # all numbers less than 2 are not prime
13 if num < 2:
14 return False
15
16 # see if num is divisible by any number up to the square root of num
17 for i in range(2, int(math.sqrt(num)) + 1):
18 if num % i == 0:
19 return False
20 return True
23def findprimes(start, end):
24 for i in range(start, end):
25 if i not in Checked:
26 Checked.append(i)
27 if is_prime(i):
28 Primes.append(i)
6def is_prime(n):
7 '''
8 checks if a number is prime
9 '''
10 if n < 2:
11 return False
12 if n == 2:
13 return True
14 for x in range(2, int(n**0.5)+1, 2):
15 if n % x == 0:
16 return False
17 return True
3def is_prime(n):
4 i = 3
5 while i * i <= n:
6 if n % i == 0: return False
7 i += 2
8 return True
1def is_prime(n):
2 if n < 2:
3 return False
4
5 i = 2
6
7 while i * i <= n:
8 if n % i == 0:
9 return False
10
11 i += 1
12
13 return True
6def countPrimes(self, n):
7 notPrimes = []
8 for k in range(n + 1):
9 notPrimes.append(False)
10 count = 0
11 for i in range(2, n):
12 if notPrimes[i]:
13 continue
14 count += 1
15 for j in range(i * i, n, i):
16 notPrimes[j] = True
17 return count
21def is_prime(n):
22 for i in xrange(2, int(sqrt(n)) + 1):
23 if n % i == 0:
24 return False
25
26 return True
3def is_prime(n):
4 if n <= 1:
5 return False
6 elif n == 2:
7 return True
8 elif n % 2 == 0:
9 return False
10 for i in xrange(3, int(sqrt(n))+1, 2):
11 if n % i == 0:
12 return False
13 return True

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