Comments on: What is the Fastest Method to Iterate Over a String? http://www.guyrutenberg.com/2007/08/30/what-is-the-fastest-method-to-iterate-over-a-string/ Keeping track of what I do Tue, 07 Feb 2012 23:33:30 +0000 hourly 1 http://wordpress.org/?v=3.3.1 By: RobC http://www.guyrutenberg.com/2007/08/30/what-is-the-fastest-method-to-iterate-over-a-string/comment-page-1/#comment-44512 RobC Sun, 17 Oct 2010 01:30:00 +0000 http://www.guyrutenberg.com/2007/08/30/what-is-the-fastest-method-to-iterate-over-a-string/#comment-44512 ^ ^ Like RichardK said, not exactly apples to apples. Also, how often do you know the size of your string at compile time? ^ ^
Like RichardK said, not exactly apples to apples. Also, how often do you know the size of your string at compile time?

]]> By: Guy http://www.guyrutenberg.com/2007/08/30/what-is-the-fastest-method-to-iterate-over-a-string/comment-page-1/#comment-23741 Guy Sat, 26 Sep 2009 19:17:40 +0000 http://www.guyrutenberg.com/2007/08/30/what-is-the-fastest-method-to-iterate-over-a-string/#comment-23741 @RichardK, Thanks for your comments and optimizations. @RichardK, Thanks for your comments and optimizations.

]]> By: RichardK http://www.guyrutenberg.com/2007/08/30/what-is-the-fastest-method-to-iterate-over-a-string/comment-page-1/#comment-23737 RichardK Sat, 26 Sep 2009 16:49:44 +0000 http://www.guyrutenberg.com/2007/08/30/what-is-the-fastest-method-to-iterate-over-a-string/#comment-23737 Your iterator code is flawed, move the .begin() and .end() code outside the for loop, otherwise the functions (which are not inline) will be run EVERY loop iteration! Also you need to change the iter 'tmp' from tmp++ to ++tmp. The former is postfix, meaning the compiler will create a temporary object to store the contents of 'tmp', then increment it. Using prefix notation will elliminate the temporary. Heres a better example: std::string::iterator tmp(hugestring.begin()); std::string::iterator const tmp_end(hugestring.end()); for(; tmp != tmp_end; ++tmp) { //->Code Here } Note how I use the constructor call for storing the begin() and end() values. If not done this way, the compiler will create a temporary for both those lines! Your iterator code is flawed, move the .begin() and .end() code outside the for loop, otherwise the functions (which are not inline) will be run EVERY loop iteration!

Also you need to change the iter ‘tmp’ from tmp++ to ++tmp. The former is postfix, meaning the compiler will create a temporary object to store the contents of ‘tmp’, then increment it. Using prefix notation will elliminate the temporary.

Heres a better example:
std::string::iterator tmp(hugestring.begin());
std::string::iterator const tmp_end(hugestring.end());
for(; tmp != tmp_end; ++tmp)
{
//->Code Here
}

Note how I use the constructor call for storing the begin() and end() values. If not done this way, the compiler will create a temporary for both those lines!

]]> By: David Sankel http://www.guyrutenberg.com/2007/08/30/what-is-the-fastest-method-to-iterate-over-a-string/comment-page-1/#comment-16506 David Sankel Fri, 06 Mar 2009 18:32:42 +0000 http://www.guyrutenberg.com/2007/08/30/what-is-the-fastest-method-to-iterate-over-a-string/#comment-16506 One method that was left out is using the .data member of std::string. This accesses the underlying data where the c_str() does not and cannot (std::string must be able to have embedded '''s). That method ought to be the fastest (hopefully tied with iterators): char * cur = s.data(); char * end = cur + s.size(); for( ; cur!=end; ++cur ) { f(*cur); } One method that was left out is using the .data member of std::string. This accesses the underlying data where the c_str() does not and cannot (std::string must be able to have embedded ”’s).

That method ought to be the fastest (hopefully tied with iterators):

char * cur = s.data();
char * end = cur + s.size();
for( ; cur!=end; ++cur )
{
f(*cur);
}

]]> By: Jancsi F. http://www.guyrutenberg.com/2007/08/30/what-is-the-fastest-method-to-iterate-over-a-string/comment-page-1/#comment-1755 Jancsi F. Fri, 29 Feb 2008 15:10:19 +0000 http://www.guyrutenberg.com/2007/08/30/what-is-the-fastest-method-to-iterate-over-a-string/#comment-1755 The results are right, without optimization. However if I turn on optimizations, with -O2 I will get the following result: jancsi@dew:~/$ ./benchmark iteration using indexes real 0m0.325s user 0m0.280s sys 0m0.040s iteration using indexes over C string real 0m0.327s user 0m0.260s sys 0m0.060s iteration using string::at() real 0m0.425s user 0m0.348s sys 0m0.068s iteration using pointers over C string real 0m0.703s user 0m0.612s sys 0m0.072s iteration using iterators real 0m0.355s user 0m0.304s sys 0m0.048s J. The results are right, without optimization.
However if I turn on optimizations, with -O2 I will get the following result:

jancsi@dew:~/$ ./benchmark
iteration using indexes
real 0m0.325s
user 0m0.280s
sys 0m0.040s

iteration using indexes over C string
real 0m0.327s
user 0m0.260s
sys 0m0.060s

iteration using string::at()
real 0m0.425s
user 0m0.348s
sys 0m0.068s

iteration using pointers over C string
real 0m0.703s
user 0m0.612s
sys 0m0.072s

iteration using iterators
real 0m0.355s
user 0m0.304s
sys 0m0.048s

J.

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