title: Finding prime numbers up till a number
body: <p>I am trying to list down all the prime numbers up till a specific number e.g. 1000. The code gets slower as the number increase. I am pretty sure it is because of the for loop where (number -1) is checked by all the prime_factors. Need some advise how I can decrease the processing time of the code for larger numbers. Thanks</p>
  <pre><code>import time
  
  t0 = time.time()
  prime_list = [2]
  number = 0
  is_not_prime = 0
  count = 0
  while number &lt; 1000:
      print(number)
      for i in range (2,number):
          count = 0
          if (number%i) == 0:
              is_not_prime = 1
          if is_not_prime == 1:
              for j in range (0,len(prime_list)):
                  if(number-1)%prime_list[j] != 0:
                      count += 1
              if count == len(prime_list):
                  prime_list.append(number-1)
                  is_not_prime = 0
                  count = 0
                  break
          
      number += 1
  print(prime_list)
  t1 = time.time()
  
  total = t1-t0
  print(total)
  </code></pre>