Author: Graham Laight
Date: 07:00:51 01/03/03
Go up one level in this thread
On January 03, 2003 at 07:52:46, Graham Laight wrote:
>On January 01, 2003 at 11:50:58, Lieven Clarisse wrote:
>
>>I was wondering if there is a good book about how to write efficient C code. I
>>am not talking about algorithms, but the way *how* to write things, eg
>>
>>which is faster :
>>do {} while() or while () {}
>>
>>-------
>>I know for instance that:
>>
>>ptr=&R[i];
>>if((*ptr==3)||(*ptr==7)) {;}
>>
>>is faster then:
>>
>>if((R[i]==3)||(R[i]==7)) {;}
>>
>>Is there a good book that reviews all those kinds of tricks?
>>
>>regards,
>>
>>Lieven
>
>Forgetting the pointer and saving to a variable DOES save time - and now I can
>prove it!
>
>Below is a web page I have written, with the button actions written in
>JavaScript.
>
>The "easy to maintain" button does a million loops using the
>
>if((R[1]==3)||(R[1]==7))
>
>construct, wheras the "Potentially Quicker" button does a million loops using
>the
>
> x = R[1];
> if((x==3)||(x==7))
>
>construct. On my computer, with IE 5.5, it takes 2.013 seconds the easy way, but
>only 1.883 seconds the quick way.
On further testing, I found that the times to execute the code can vary
significantly (presumably depending on what else my computer is doing). I have
therefore tested again several times, and just taken the fastest time for each
button.
New Results
===========
Easy to maintain function: 1.992 seconds
Potentially quicker function: 1.873 seconds
This is a difference of 119 milliseconds, or about 6%
Also note that because this is true in JavaScript doesn't necessarily make it
true in C - I'll leave that to someone with a C compiler to test.
-g
>If you want to prove this for yourself:
>
>1. copy the web page below
>
>2. paste it into Notepad
>
>3. save it to your c:\ drive as Graham_Demo.htm
>
>4. point your web browser at it (I wrote and tested under IE 5.5 - should also
>work under Netscape or Mozilla)
>
>-g
n.b. if you copy the web page below now, you should go to the parent message to
copy it - otherwise you'll have to remove the > from the start of each line.
><!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
>
><html>
><head>
><title>Graham's Optimisation Demo</title>
></head>
><body>
><SCRIPT LANGUAGE="JavaScript">
>function easyToMaintain()
>{
> var R = new Array();
> R[0] = 1;
> R[1] = 2;
> R[2] = 3;
>
> var p = 0;
> var startTime = new Date();
>
> for (k = 0; k < 1000000; k++)
> {
> if((R[1]==3)||(R[1]==7))
> p = p + 1;
> }
>
> var endTime = new Date();
> var totalTime = endTime.getTime() - startTime.getTime();
> alert("The easy to maintain version took " + totalTime + " milliseconds to
>run");
>}
>
>function possiblyQuicker()
>{
> var R = new Array();
> R[0] = 1;
> R[1] = 2;
> R[2] = 3;
>
> var p = 0;
> var x = 0;
> var startTime = new Date();
>
> for (k = 0; k < 1000000; k++)
> {
> x = R[1];
> if((x==3)||(x==7))
> p = p + 1;
> }
>
> var endTime = new Date();
> var totalTime = endTime.getTime() - startTime.getTime();
> alert("The potentially quicker version took " + totalTime + " milliseconds to
>run");
>}
>
></SCRIPT>
><input type="button" value="Test Easy-To-Maintain Version Of Code"
>onclick="easyToMaintain()">
><input type="button" value="Test Possibly-Quicker Version Of Code"
>onclick="possiblyQuicker()">
></body>
></html>
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