Print prime numbers in a range of given numbers

Question

I'm trying to print all primes into a range of given numbers (low and max, given numbers included).

For example:

num1=10, num2=20

>>> 11, 13, 17, 19

My code fails in some occasions and I can't understand why:

num1 = int(input('First number is: '))
num2 = int(input('Second number is: '))
if num2 <= num1:
    num1,num2 = num2,num1
for i in range(num1, num2+1):
    for p in range(2,int(num2**0.5)+1):
        if i%p == 0:
            break
    else:
        print(i,' ',end = '')
print('\n')

Results:

1 to 7 >>> 1 3 5 7 (omits 2)

1 to 30 >>> 1 7 11 13 17 19 23 29 (omits 2,3,5)

1 to 60 >>> 1 7 11 13 17 19 23 29 (omits 2,3,5,7)

0 to 0 >>> 0 (prints 0 -> not a prime number)

0 to 7 >>> 1 3 5 7 (omits 2)

How can I correct this? Thanks a bunch!

ps. number 1 is not a prime too.

Answer 1

The mistake in your code was not having an i in the second part of the inner for loop range as opposed to num2

num1 = int(input('First number is: '))
num2 = int(input('Second number is: '))
if num2 > num1: 
    num1, num2 = num2, num1
for i in range(num1, num2+1):
    if i == 0 or i == 1: continue
    for p in range(2,int(i**0.5)+1):     # the second part should be int(i**0.5) + 1, not int(num2**0.5)+1
        if i%p == 0:    
            break       

    else:
        print(i,' ',end = '')

Also rather than having two branches for num1 < num2 and the other way around, you can do something like below. Further in terms of code design, it would be better to decompose this slightly into an is_prime method. That way, if you ever want to write a quicker primality tester, you could easily edit the helper function as opposed to messing with the main code.

def is_prime(num):
    if i == 1: return False
    for p in range(2,int(num**0.5)+1):
        if num % p == 0:
            return False
    return True

inp1 = int(input('First number is: '))
inp2 = int(input('Second number is: '))

num1 = min(inp1, inp2)
num2 = max(inp1, inp2)

for i in range(num1, num2+1):
    if is_prime(i):
        print(i,' ',end = '')

print('\n')