Python Prime number checker

Question

I have been trying to write a program that will take an inputed number  and check and see if it is a prime number   The code that I have made so far works perfectly if the number is in fact a prime number   If the number is not a prime number it acts strange   I was wondering if anyone could tell me what the issue is with the code    a 2 num 13 while num  gt  a     if num a  0  amp  a  num      print  not prime       a a 1   else      print  prime       a  num  1   the result given when 24 is inputed is  not prime not prime not prime prime  How would i fix the error with the reporting prime on every odd and not prime for every even

User · Answer

The two main problems with your code are:

After designating a number not prime, you continue to check the rest of the divisors even though you already know it is not prime, which can lead to it printing "prime" after printing "not prime". Hint: use the `break' statement.
You designate a number prime before you have checked all the divisors you need to check, because you are printing "prime" inside the loop. So you get "prime" multiple times, once for each divisor that doesn't go evenly into the number being tested. Hint: use an else clause with the loop to print "prime" only if the loop exits without breaking.

A couple pretty significant inefficiencies:

You should keep track of the numbers you have already found that are prime and only divide by those. Why divide by 4 when you have already divided by 2? If a number is divisible by 4 it is also divisible by 2, so you would have already caught it and there is no need to divide by 4.
You need only to test up to the square root of the number being tested because any factor larger than that would need to be multiplied with a number smaller than that, and that would already have been tested by the time you get to the larger one.

User · Answer

def isprime n          check if integer n is a prime           make sure n is a positive integer     n   abs int n          0 and 1 are not primes     if n  lt  2          return False        2 is the only even prime number     if n    2           return True            all other even numbers are not primes     if not n  amp  1           return False        range starts with 3 and only needs to go up        the square root of n for all odd numbers     for x in range 3  int n  0 5    1  2           if n   x    0              return False      return True   Taken from       http   www daniweb com software-development python code 216880 check-if-a-number-is-a-prime-number-python

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This example is use reduce    but slow it   def makepnl pnl  n       for p in pnl          if n   p    0              return pnl     pnl append n      return pnl  def isprime n       return True if n    reduce makepnl  range 3  n   1  2    2   -1  else False  for i in range 20       print i  isprime i    It use Sieve Of Atkin  faster than above   def atkin limit       if limit  gt  2          yield 2     if limit  gt  3          yield 3      import math     is prime    False     limit   1       for x in range 1 int math sqrt limit   1           for y in range 1 int math sqrt limit   1               n   4 x  2   y  2              if n lt  limit and  n 12  1 or n 12  5                     print  1st if                                                                                                                                      is prime n    not is prime n              n   3 x  2 y  2             if n lt   limit and n 12  7                    print  Second if                                                                                                                                   is prime n    not is prime n              n   3 x  2 - y  2             if x gt y and n lt  limit and n 12  11                    print  third if                                                                                                                                    is prime n    not is prime n       for n in range 5 int math sqrt limit             if is prime n               for k in range n  2 limit 1 n  2                   is prime k    False      for n in range 5 limit           if is prime n   yield n  def isprime n       r   list atkin n 1       if not r  return False     return True if n    r -1  else False  for i in range 20       print i  isprime i

User · Answer

After you determine that a number is composite  not prime   your work is done  You can exit the loop with break   while num  gt  a     if num a  0  amp  a  num      print  not prime       break            not going to update a  going to quit instead   else      print  prime       a  num  1   Also  you might try and become more familiar with some constructs in Python  Your loop can be shortened to a one-liner that still reads well in my opinion   any num   a    0 for a in range 2  num

User · Answer

def is prime n       return all n j for j in xrange 2  int n  0 5  1   and n gt 1

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This would do the job   number int raw input  Enter a number to see if its prime     if number  lt   1      print  number is not prime  else      a 2     check   True     while a    number          if number a    0              print  Number is not prime              check   False             break         a  1     if check    True          print  Number is prime

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max int input  Find primes upto what numbers     primeList    for x in range 2 max 1       isPrime True     for y in range 2 int x  0 5  1            if x y  0              isPrime False             break      if isPrime          primeList append x  print primeList

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is digit prime  we will see  Coder  Chikak   def is prime x       flag   False     if x  lt  2          return False     else          for count in range 2  x               if x   count    0                  flag   True                 break         if flag    True              return False         return True

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This is a slight variation in that it keeps track of the factors   def prime a       list        x 2     b True      while x lt a          if a x  0              b False             list append x          x  1      if b  False          print  Not Prime          print list     else          print  Prime

User · Answer

You need to stop iterating once you know a number isn t prime   Add a break once you find prime to exit the while loop   Making only minimal changes to your code to make it work   a 2 num 13 while num  gt  a     if num a  0  amp  a  num      print  not prime       break   i    1 else    loop not exited via break   print  prime     Your algorithm is equivalent to   for a in range a  num       if a   num    0          print  not prime           break else    loop not exited via break     print  prime     If you throw it into a function you can dispense with break and for-else   def is prime n       for i in range 3  n           if n   i    0              return False     return True   Even if you are going to brute-force for prime like this you only need to iterate up to the square root of n   Also  you can skip testing the even numbers after two   With these suggestions   import math def is prime n       if n   2    0 and n  gt  2           return False     for i in range 3  int math sqrt n     1  2           if n   i    0              return False     return True   Note that this code does not properly handle 0  1  and negative numbers   We make this simpler by using all with a generator expression to replace the for-loop   import math def is prime n       if n   2    0 and n  gt  2           return False     return all n   i for i in range 3  int math sqrt n     1  2

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a input  Enter number     def isprime          total 0     factors  1 a   The only factors of a number     pfactors range 1 a 1   considering all possible factors       if a  1 or a  0   One and Zero are not prime numbers         print   d is NOT prime  a       elif a  2    Two is the only even prime number         print   d is  prime  a       elif a 2  0  Any even number is not prime except two         print   d is NOT prime  a          else  a number is prime if its multiples are 1 and itself            The sum of the number that return zero moduli should be equal to the  only  factors         for number in pfactors               if  a number   0                   total  number         if total  sum factors               print   d is NOT prime  a          else               print   d is  prime  a isprime

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Prime number check   def is prime x       if x  lt  2          return False     else          if x    2              return True         else              for i in range 2  x                   if x   i    0                      return False             return True x   int raw input  enter a prime number    print is prime x

User · Answer

Your problem is that the loop continues to run even thought you ve  made up your mind  already  You should add the line break after a a 1

User · Answer

Begginer here  so please let me know if I am way of  but I d do it like this   def prime n       count   0     for i in range 1   n 1              if n   i    0                count    1     if count  gt  2          print  Not a prime      else          print  A prime

[python] Python Prime number checker

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