Python Checking if prime number

Question

so...this is my code down below. I altered it all ways I can think of, but regardless of what I do it will ether say all the numbers are prime or all the numbers are not prime. I was hoping someone can point out the obvious error. Currently this code says all numbers are not prime. Thanks.

import math

x = int(input('Enter a number: '))

def isPrime(x):
  if x==2:
      print ("The number you entered is not Prime.")
      return

  i = 2
  x = int(math.sqrt(x)) 
  while i < x+1:
      if x%i==0:            
          print ("The number you entered is not Prime.")
          return
      i = i+1
  print ("This number is Prime")
  return

isPrime(x)

You need to fix the indentation, your code won't run as it stands. — zehnpaard, Jan 19 '15 at 06:13
Please fix the indentation. Without it, we can only guess about the structure of your code. — NPE, Jan 19 '15 at 06:13
There are about a million questions on SO regarding prime numbers and Python. I suggest you read through some of them. — MattDMo, Jan 19 '15 at 06:13

NPE · Accepted Answer · 2015-01-19T08:23:39.017

3

The main bug is here:

x = int(math.sqrt(x))

You are altering x, so the subsequent divisibility checks incorrectly use this altered value of x.

You should store the square root in a different variable:

sqrt_x = int(math.sqrt(x)) 
while i < sqrt_x + 1:
   ...

Also, the number 2 is prime since it's only divisible by itself and one.

edited Jan 19 '15 at 08:23

answered Jan 19 '15 at 06:14

NPE

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score 1 · Answer 2 · answered Jan 19 '15 at 06:20

the square root line should be removed, and a minor edit will make it work.

and yes, please fix your indent

import math

def isPrime(x): 

    i = 2
    #x = int(math.sqrt(x)) 
    while i < x:#x+1:
        z = x%i
        print "i: ", i, " x: ", x, "x%i: ", z
        if z==0:
            print ("The number you entered is not Prime.")
            return
        i = i+1
    print ("This number is Prime")
    return

x = int(input('Enter a number: '))
isPrime(x)

Python Checking if prime number

2 Answers2