How to compare vector with array in efficient way in cpp?

Question

I would like to write a code in a better (faster) way. I have got a container vector and an simple array. I would like to compare content of the vector with content of the array. Suppose that I have got classes like this:

struct A
{
    float aa;
    struct(float p_aa) : aa(p_aa) {}
};

struct B : public A
{
    A bb;
    struct(float p_aa) : A(p_aa) {}
};

And I have also a container and an simple array:

std::vector<B> l_v = {B(1), B(3)};
B l_b[2] = {B(1), B(3)};

The function, which compare the container with the array is:

bool isTheSame(const std::vector<B> &l_v, B *l_b)
{
    unsigned int count = 0;
    for(auto it = l_v.begin(); it!= l_v.end(); ++it)
    {
        if(l_b[count].aa != it->aa)
        {
            return false;
        }
        ++count;
    }
    return true;
}

I would like to write it in a better way using lambda or foreach. Do you have any ideas? Thanks.

std::equal

Borgleader
– Borgleader

2015-05-14 15:26:00 +00:00
Commented May 14, 2015 at 15:26 — Borgleader
– Borgleader, Commented May 14, 2015 at 15:26

Barry · Accepted Answer · 2015-05-14 15:44:32Z

8

Use std::equal:

bool isTheSame(const std::vector<B> &l_v, B *l_b)
{
    return std::equal(l_v.begin(), l_v.end(), l_b,
                      [](const B& lhs, const B& rhs){
                          return lhs.aa == rhs.aa;
                      });
}

Note that both this and your code presupposes that the vector and the array have the same size. A better implementation would be to additionally pass in the length of l_b so that you can ensure that you don't read uninitialized memory from l_b:

bool isTheSame(const std::vector<B> &l_v, B *l_b, size_t len)
{ 
    return len == l_v.size() && 
        std::equal(l_v.begin(), l_v.end(), l_b, same_pred);
}

Or as an array:

template <size_t N>
bool isTheSame(const std::vector<B> &l_v, const B (&l_b)[N])
{ 
    return N == l_v.size() && 
        std::equal(l_v.begin(), l_v.end(), l_b, same_pred);
}

edited May 14, 2015 at 15:44

answered May 14, 2015 at 15:24

Barry

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1 Comment

juanchopanza Over a year ago

The only reason you need to pass the length is because you decided to pass a pointer instead of a reference to an array. There's no need for that.

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