My For loop won't iterate through a list

Question

I have to determine if all the numbers in a list are prime numbers and then return a boolean "True" or "False" statement depending on the outcome. I made some conditional statements inside of a for loop to see if the number was prime or not.

Here's the code:

def all_primes(xs):
    is_prime = None
    for i in xs:
        if i < 2:
            is_prime = False
            return is_prime
            break
        elif (i % 2 == 0) and (i % i == 1):
            is_prime = False
            return is_prime
            break
        else:
            is_prime = True
            return is_prime

The problem is, and I saw this in the Python Visualizer, the for loop stops iterating after checking the first value in the list. I don't understand why as the syntax is the same as for loops I've used in the past.

I plugged in some example values like: all_primes([5,2,11,37]) or all_primes([5,2,4,37]) and the return value is always true since 5 is the first number on the list and the only number that is being iterated.

Any ideas as to why?

Answer 1

You have a return and a break in your if/else block, you should get rid of them. Also the return in the else should be outside, or it will just return whenever he finds a "prime".

def all_primes(xs):
  is_prime = None
  for i in xs:
      if i < 2:
          is_prime = False
          return is_prime
      elif (i % 2 == 0):
          is_prime = False
          return is_prime
      else:
          is_prime = True
  return is_prime

After this, you should know, that you are not really checking primes. Here is not the most efficient way but its clear how to:

def all_primes(xs):
    def checkPrime(n):
        if n < 2:
            return False
        for i in xrange(2, n):
            if n%i == 0:
                return False
        return True
    return all(map(checkPrime, xs))

EDIT: without the map functions, you just have to iterate with a for loop:

def all_primes(xs):
    def checkPrime(n):
        if n < 2:
            return False
        for i in xrange(2, n):
            if n%i == 0:
                return False
        return True
    for n in xs:
        if not checkPrime(n):
            return False
    return True

Answer 2

You should see the problem the other way around.

If you you find a number which is not prime you should return False, and after your loop end you should return True.