Re: Repetitive Appending of Strings

FROM : Adam P Jenkins
DATE : Tue Feb 12 23:51:15 2008

I tried it.  It takes ~ 0.004 seconds for a million 
vector<char>::push_back calls, if I build with -O3, so only very 
slightly slower than the direct assignment into a char array.   
That's not surprising since vector<char>::push_back is not a virtual 
method and can be inlined, so the machine code should be the same as a 
simple array element assignment + size increment.  In fact, if instead 
of using push_back I simply resize the vector to have a million 
elements and then use simple element assignments, then the vector 
performance is the same as using a raw array, in fact a little faster 
for some reason.

Here's the output, and I've also attached the modified program.

String append with initial capacity 0 took 0.409247 seconds
String append with initial capacity 1000000 took 0.407078 seconds
NSMutableData appendBytes took 0.033926 seconds
Assigning into char array took 0.003776 seconds
vector.push_back took 0.004577 seconds
vector[i]=ch took 0.002886 seconds



On Feb 12, 2008, at 5:01 PM, John Stiles wrote:

> If you've got a little geekery left in you, I'd be highly curious to 
> know the times for doing a million push_back's into a vector<char>. 
> I wonder how it compares to NSMutableData.


#import <Foundation/Foundation.h>
#include <vector>

int main (int argc, const char * argv[]) {
    NSAutoreleasePool * pool = [NSAutoreleasePool new];

   NSDate *tic, *toc;
   NSMutableString *theString;
   int niters = 1000000;
   int i;
   
   // start the string off at 0 capacity and let appendFormat expand it
   tic = [NSDate date];
   theString = [NSMutableString stringWithCapacity:0];
   for (i = 0; i < niters; i++)
       [theString appendFormat:@"%d", (i % 10)];
   toc = [NSDate date];
   printf("String append with initial capacity 0 took %f seconds\n",
         [toc timeIntervalSinceDate:tic]);
   
   [pool release];
   pool = [NSAutoreleasePool new];
   
   // start the string off with enough capacity to hold the whole 
string, so no
   // reallocations are necessary
   tic = [NSDate date];
   theString = [NSMutableString stringWithCapacity:niters];
   for (i = 0; i < niters; i++)
       [theString appendFormat:@"%d", (i % 10)];
   toc = [NSDate date];
   printf("String append with initial capacity %d took %f seconds\n",
         niters, [toc timeIntervalSinceDate:tic]);
   
   [pool release];
   pool = [NSAutoreleasePool new];

   
   // try using a buffer, using appendBytes:length
   tic = [NSDate date];
   NSMutableData *buffer = [NSMutableData dataWithCapacity:niters];
   for (i = 0; i < niters; i++) {
       char ch = (i % 10) + '0';
       [buffer appendBytes:&ch length:1];
   }
   toc = [NSDate date];
   printf("NSMutableData appendBytes took %f seconds\n",
         [toc timeIntervalSinceDate:tic]);
   
   [pool release];
   pool = [NSAutoreleasePool new];
   
   // finally, try assigning directly to a memory buffer
   tic = [NSDate date];
   buffer = [NSMutableData dataWithCapacity:niters];
   char *chars = (char*)[buffer mutableBytes];
   for (i = 0; i < niters; i++) {
       char ch = (i % 10) + '0';
       chars[i] = ch;
   }
   toc = [NSDate date];
   printf("Assigning into char array took %f seconds\n",
         [toc timeIntervalSinceDate:tic]);
   
   std::vector<char> vecchars;
   vecchars.reserve(niters);
   tic = [NSDate date];
   for (i = 0; i < niters; i++)
       vecchars.push_back((i%10) + '0');
   toc = [NSDate date];
   printf("vector.push_back took %f seconds\n",
         [toc timeIntervalSinceDate:tic]);
   
   vecchars.clear();
   vecchars.resize(niters);
   tic = [NSDate date];
   for (i = 0; i < niters; i++)
       vecchars[i] = (i%10) + '0';
   toc = [NSDate date];
   printf("vector[i]=ch took %f seconds\n",
         [toc timeIntervalSinceDate:tic]);
   
   [pool release];
    return 0;
}