Databases
StackOverflow2013
Postsdbo
POOptimizing python for loops
 

Note that there are some explanatory texts on larger screens.

plurals
  1. POOptimizing python for loops
    primarykey
    data
    text
    <p>Here are two programs that naively calculate the number of prime numbers &lt;= n.<br> One is in Python and the other is in Java.<br></p> <pre><code>public class prime{ public static void main(String args[]){ int n = Integer.parseInt(args[0]); int nps = 0; boolean isp; for(int i = 1; i &lt;= n; i++){ isp = true; for(int k = 2; k &lt; i; k++){ if( (i*1.0 / k) == (i/k) ) isp = false; } if(isp){nps++;} } System.out.println(nps); } } `#!/usr/bin/python` import sys n = int(sys.argv[1]) nps = 0 for i in range(1,n+1): isp = True for k in range(2,i): if( (i*1.0 / k) == (i/k) ): isp = False if isp == True: nps = nps + 1 print nps </code></pre> <p>Running them on n=10000 I get the following timings.<br> shell:~$ time python prime.py 10000 &amp;&amp; time java prime 10000<br> 1230<br></p> <p>real 0m49.833s<br> user 0m49.815s<br> sys 0m0.012s<br> 1230<br></p> <p>real 0m1.491s<br> user 0m1.468s<br> sys 0m0.016s<br></p> <p>Am I using for loops in python in an incorrect manner here or is python actually just this much slower?<br> </p> <p>I'm not looking for an answer that is specifically crafted for calculating primes but rather I am wondering if python code is typically utilized in a smarter fashion.</p> <p>The Java code was compiled with javac 1.6.0_20<br> Run with java version "1.6.0_18" <br> OpenJDK Runtime Environment (IcedTea6 1.8.1) (6b18-1.8.1-0ubuntu1~9.10.1) OpenJDK Client VM (build 16.0-b13, mixed mode, sharing)</p> <p>Python is:<br> Python 2.6.4 (r264:75706, Dec 7 2009, 18:45:15)</p>
    singulars
    1. PO
      singulars
      1. PTAnswer
    1. This table or related slice is empty.
    1. PTQuestion
    1. This table or related slice is empty.
    1. USfthinker
    plurals
    1. PL
      singulars
      1. LTLinked
    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. PO
      singulars
      1. PTAnswer
    2. PO
      singulars
      1. PTAnswer
    3. PO
      singulars
      1. PTAnswer
    1. VO
      singulars
      1. POOptimizing python for loops
      1. This table or related slice is empty.
      1. VTUpMod
    2. VO
      singulars
      1. POOptimizing python for loops
      1. This table or related slice is empty.
      1. VTUpMod
    3. VO
      singulars
      1. POOptimizing python for loops
      1. This table or related slice is empty.
      1. VTUpMod
    1. COOne obvious thing you might want to do is change `range` to `xrange` - should make a bit of a difference for the memory usage, possibly also speed.
      singulars
      1. POOptimizing python for loops
      1. USKos
    2. COIn general, it is enough to divide `i` by the numbers up to square root of `i`. And probably even better would be the [Sieve of Eratosthenes](http://en.wikipedia.org/wiki/Sieve_of_Eratosthenes) but that is not the scope of the question ;)
      singulars
      1. POOptimizing python for loops
      1. USFelix Kling
    3. COInstead of "for i in range(a, b) doStuff();" try something like "i = a ; while (i <= b) { doStuff(); i = i + 1 }"
      singulars
      1. POOptimizing python for loops
      1. USLaurențiu Dascălu
 

Querying!

 
Guidance
A row detail

Detail views are divided into sections. All the information in the data section comes from columns in the selected row. The other sections display data from other, related rows.

Related data can be related in a to-one or a to-many fashion. Captions of data related in a to-many fashion link to a list view showing a filtered view of the table.

Try moving around until you find a non-empty to-many entry and click on the label to get to one. You can move back to the root by clicking on the database name in the header.

SQuiL has stopped working due to an internal error.

If you are curious you may find further information in the browser console, which is accessible through the devtools (F12).

Reload