Re: Vector vs. Array

From:

"Bo Persson" <bop@gmb.dk>

Newsgroups:

comp.lang.c++.moderated

Date:

Wed, 3 Nov 2010 17:18:08 CST

Message-ID:

<8jdq3vForkU1@mid.individual.net>

Peter Vermeer wrote:

Hi,

I have implemented Manhattan Distance computation one time with
arrays and and one time with vectors:

    float manDist(vector<float>& v1, vector<float>& v2)
    {
        float sum = 0;
        for (int i = 0; i < 100; i++)
        {
            sum += fabs(v1[i]-v2[i]);
        }
        return sum;
    }

    float manDist(float v1[], float v2[])
    {
        float sum = 0;
        for (int i = 0; i < 100; i++)
        {
            sum += fabs(v1[i]-v2[i]);
        }
        return sum;
    }

Curiously, the array version is much faster (factor 100). So
probably, there is something wrong with my code. I have tested also
iterators instead of [] as well as at() instead of [] for vectors.
Furthermore, I defined _SECURE_SCL 0.
Can someone enlighten me?

Are you running in debug mode? :-)

Testing them with a main function like:

     int main()
     {
        vector<float> v1(100), v2(100);

        return int(manDist(v1, v2) + manDist(&v1[0], &v2[0]));

     }

shows that the two functions produce identical assembly code. MSVC10
Express, Release mode.

Bo Persson

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