python中的素数列表不正确

Question

I had assumed my Python prime number list was good. But it's printing out numbers the moment it sees that it is not divisible by 2. I've tried to iterate loops through the list that contains other prime numbers such as 3, 5, 7 ... but it does not seem to work still.

primes = []
input = raw_input("> ")

input2 = raw_input("> ")

for num in range(int(input), int(input2)):
    for j in primes:
        if not primes:
            break
        else:
            if j % num ==0:
                break
    for i in range(2, num):
        if num % i == 0:    
            break
        else:
            primes.append(num)
            break
print primes

This prints out numbers divisible by 3 but not divisible by 2.

for num in range(int(input), int(input2)):
    if not primes:
        break
    else:
        for j in primes:
            if j % num ==0:
                break
    for i in range(2, num):
        if num % i == 0:    
            break
        else:
            primes.append(num)
            break
print primes

This code is printing out an empty list. It made sense to me logically but the code is not executing the way I expect it to. What is wrong here???

Answer 1

Your first for loop isnt doing anything useful:

for num in range(int(input), int(input2)):
    for j in primes:            #there is nothing in list 'primes'
        if not primes:          #this won't execute
            break               #this won't execute
        else:                   #this will execute
            if j % num ==0:     #this will execute
                break           #break if statement, repeat for loop
    for i in range(2, num):
        if num % i == 0:    
            break
        else:
            primes.append(num)
            break
print primes  

With that said, this would be the most convenient way to get your desired output:

primes=[]
input = raw_input("> ")
input2 = raw_input("> ")

for num in range(int(input), int(input2)):
    if all(num%i!=0 for i in range(2,num)):
       primes.append(num)

print primes

Answer 2

your second "else" seems to be in the wrong place. try this modified version of your program :

primes = []
input = raw_input("> ")
input2 = raw_input("> ")

for num in range(int(input), int(input2)+1):
    # prime numbers are greater than 1
    if num > 1 :
        for i in range(2, num):
            if num % i == 0:
                break
        else:
                primes.append(num)
print primes