python 中给定范围内的素数，范围（）问题，

Question

This program prints prime no between 1-20, i have tried to grasp the working of(for loop,range & else associated with for) in this program

def isPrime(num):
    for i in range(2,num):
        if num%i==0:
            return False
        #else:
            #return True
    else:
        return True

#below is driver program        
for i in range(1, 20):
    if isPrime(i + 1):
        print(i + 1, end="\n")
print()

1.in this program put num//2 instead of num in range() result is diff (prints 4 as prime)why?

2. when 2 is sent to isprime() , it doesn't even enter for loop,it is not being checked by if conditions but outputs 2 as a prime by the else block associated with for. why?how? i think because of range(2,num) ,2 is passed as num.

Can someone Explain??

Answer 1

The issue is that range(2,n) is all numbers 2, 3, ..., n-1. So change your for loop to:

for i in range(2,num+1):

As is, when 2 is passed in, the range will be empty.