Re: C++ is Slow?
* nw:
Hi all,
I'm constantly confronted with the following two techniques, which I
believe often produce less readable code, but I am told are faster
therefore better. Can anyone help me out with counter examples and
arguments?
1. The STL is slow.
More specifically vector. The argument goes like this:
"Multidimensional arrays should be allocated as large contiguous
blocks. This is so that when you are accessing the array and reach the
end of a row, the next row will already be in the cache. You also
don't need to spend time navigating pointers when accessing the array.
So a 2 dimensional array of size 100x100 should be created like this:
const int xdim=100;
const int ydim=100;
int *myarray = malloc(xdim*ydim*sizeof(int));
and accessed like this:
myarray[xdim*ypos+xpos] = avalue;
Is this argument reasonable? (Sounds reasonable to me, though the
small tests I've performed don't usually show any significant
difference).
To me this syntax looks horrible, am I wrong?
The syntax is horrible, and the code is brittle.
Is vector the wrong
container to use?
No, not necessarily. Doing the above with vector gives you about the
same performance but less unsafe (no manual memory management) and more
convenient. Using a library matrix class even better.
(My usual solution would be a vector<vector<int> >).
Would using a valarray help?
I don't think valarray would help, I think on the contrary. Also
consider that that's essentially a not-quite-complete not-quite-kosher
part of the standard library. Which will never be completed.
2. iostream is slow.
I've encountered this is work recently. I'd not considered it before,
I like the syntax and don't do so much IO generally... I'm just now
starting to process terabytes of data, so it'll become an issue. Is
iostream slow?
Depends very much on the implementation. As a general rule, naive code
using C library i/o will be faster than equally naive code using
iostreams, or at least it was that way some years ago. However, you can
probably speed up things considerably for iostreams by using less naive
code, essentially buying speed by paying in complexity and code size.
specifically I encountered the following example
googling around. The stdio version runs in around 1second, the
iostream version takes 8seconds. Is this just down to a poor iostream
implementation? (gcc 4 on OS X). Or are there reasons why iostream is
fundamentally slower for certain operations? Are there things I should
be keeping in mind to speed up io?
// stdio version
#include <cstdio>
using namespace std;
const int NULA = 0;
int main (void) {
for( int i = 0; i < 100000000; ++i )
printf( "a" );
return NULA;
}
//cout version
#include <iostream>
using namespace std;
const int NULA = 0;
int main (void) {
std::ios_base::sync_with_stdio(false);
for( int i = 0; i < 100000000; ++i )
cout << "a" ;
return NULA;
}
Guideline: reserve all uppercase names for macros.
The C++ macro denoting success return value for main is EXIT_SUCCESS.
Cheers, & hth.,
- Alf
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