Passing column of 2d vector to function as a reference

[kamos.jura]

Hi!
Can you tell me, how to pass a reference to the column in 2d vector to function with following declaration: void SetData( const std::vector<float> &xs,const std::vector<float> &ys); This function is from wxMathPlot.

My vector's declaration looks like this:

vector<vector <double> > sekce(8);
vector<vector<time_t> > cas_t_all(8);

I would like to do something like this(doesn't work): plot->SetData( &(float)cas_t_all[1], &sekce[5]); = pass second column of vector cas_t_all and fifth column of vector sekce to funkcion SetData(...). Issue here is also type casting - this I can avoid by my working code - see lower.

Current situation is that I have to copy whole column from vector cas_t_all to new vector xs and column from vector sekce to new vector ys:

std::vector<float> xs,ys;
for ( int i = 0; i < sekce[0].size(); i++ )
        xs.push_back(cas_t_all[0][i]);
ys.assign(sekce[0].begin(),sekce[0].end());
plot->SetData( xs, ys);

Thanks in advance.

J.

[Tom Gunn]

Normally you just pass the object and the compiler does the rest. No special syntax is needed:

#include <iostream>
#include <vector>

void PrintAll(std::vector<int> const& v)
{
    std::vector<int>::const_iterator x = v.begin();

    while (x != v.end()) std::cout << *x++ << '\t';

    std::cout << '\n';
}

int main()
{
    std::vector<std::vector<int> > v;

    for (int x = 0; x < 3; ++x)
    {
        v.push_back(std::vector<int>());

        for (int y = 0; y < 5; ++y)  v.back().push_back(y);
    }

    for (int x = 0; x < 3; ++x) PrintAll(v[x]);
}

The problem you have is incompatible types. vector<double> is not compatible with vector<float>, and vector<double> is not compatible with vector<time_t>. There is no way around copying the values into a new vector with whatever explicit or implicit conversions are needed. Your working code is the answer if you cannot change the types of the vectors to match.

[mrnutty]

just pass 2d vector and use only its column?

[Tom Gunn]

just pass 2d vector and use only its column?

The function being called is from wxMathPlot, and wxMathPlot is a third party library. It is a good assumption that he either does not have the code to change, or rightfully thinks that changing the code is a really bad idea.

[mrnutty]

The function being called is from wxMathPlot, and wxMathPlot is a third party library. It is a good assumption that he either does not have the code to change, or rightfully thinks that changing the code is a really bad idea.

Should have read the rest of his comment/questions, if it even says
it there. Didn't know there was a 3rd party. In which I wasn't invited?

[kamos.jura]

Tom Gunn - unfortunatelly seems you are right. I tried to search on Google, but only helpful thing I found about type casting of vectors is this:
http://bytes.com/topic/c/answers/133936-convert-vector-float-vector-double
and it doesn't work for my case.

Anyway thanks for your help.

J.