如果连接的组合是素数，则接受用户输入并打印

Question

I am trying to figure how to take a user input of numbers that are comma separated (eg: 1,7) and print out if their concatenated combinations are prime numbers (eg. 7, 17, 71).

So far, the steps i take is:

#convert user input into a list
values = input("Input some comma separated numbers : ")

num = values.split(",")

But i do not know what to do after to come up with all their combination in a form of a list like: num_combination = [1, 7, 17, 71].

If i can convert them like the above, i think i can write a function like:

answer = []
for i in num_combination:
    if i > 1 and i%i ==0:
        answer.append(i)
    else:
        pass

Thank you very much for your help!

Answer 1

So, I wasn't planning to answer this question, because I felt that the comments I had left would put you on the right track, but the question you have posted seems to have some interesting sub-problems, that would be interesting (and hopefully useful) to tackle.

1. How do I convert a line of text into an array of numbers?

You currently have this code:

#convert user input into a list
values = input("Input some comma separated numbers : ")

num = values.split(",")

Now, this code is part of what needs to be done. There's actually one step missing in the code mentioned above, because right now, num is an array containing strings

>>> num
["1", "7"]

To convert it into an array of numbers, you will also need to run the int function on all elements in the array. There are multiple ways of doing this:

new_num = list(map(int, num))
# OR
new_num = []
for v in num: new_num.append(int(v))
# OR
new_num = [int(v) for v in num]

2. How do I get all possible permutations and combinations of elements in an array?

Now, luckily for us, Python provides us with a utility that does at least part of our work for us. itertools.permutations . This function takes two parameters: the array, and the number of elements to provide as permutations:

>>> itertools.permutations(['A','B','C'], 2)
[('A', 'B'), ('A', 'C'), ('B', 'A'), ('B', 'C'), ('C', 'A'), ('C', 'B')]

But for our case, we need all length permutations, so we'll have to do something a little more complex:

num_permutations = []
for i in range(1,len(num)):
    num_permutations += list(itertools.permutations(num, i))

There are other ways to get this list of permutations, without having a native function, but they're a little more detailed than necessary for this solution. I'll cover them if I have time later.

3. How do I find Prime numbers?

The definition of a prime number is a number that has prime factors of just 1 and itself. That means, to check if k is a prime number, one needs to ensure that all the numbers in ( 2 , 3 , ..., k-2 , k-1 ) do not divide k .

The easiest way to do that is

def is_prime(k):
    prime = True
    for i in range(2,k):
        if k%i == 0:
            prime = False
            break

    return prime

There are a lot of optimisations that can be done on this, but again, I won't go into them right now.

4. Putting it all together

There is one small edge-case that I have left out above. How do I convert a tuple of digits into an actual number? That's part of what the code below does:

def is_prime(k):
    prime = True
    for i in range(2, k):
        if k%i == 0:
            prime = False
            break

    return prime


values = input("Input some comma separated numbers : ")
num = map(int, values.split(","))

permutations = []
for i in range(1, len(num)):
    for t in itertools.permutations(num, i):
        str_num = int("".join(map(str, t)))
        permutations.append(str_num)

answer = []
for i in permutations:
    if is_prime(i):
        answer.append(i)

print(answer)