std::transform for a vector of vectors

问题

I have numerical data in a vector< vector < double> > and need to add scalar values to them as follows:

vector <vector<double> > data ( M, vector<double>(N) );
vector <double>scalars(N);

data[0][0]     += scalars[0];
data[0][1]     += scalars[1];
...
data[0][N-1]   += scalars[N-1];

data[1][0]     += scalars[0];
data[1][1]     += scalars[1];
...
data[1][N-1]   += scalars[N-1];

...

data[M-1][N-1] += scalars[N-1];

Of course this is possible with two for loops. I was wondering if it can be done as simply with transform, bind and plus? I'm trying to use functors where possible (although still use old C-style code out of habit).

The inside loop would need to do this for the vector 0 in data:

transform ( data[0].begin(), data[0].end(), 
            scalars[0].begin(), 
            data[0].begin(), 
            plus<double>() )

Is it possible replace data[0] in this line with another counter (related to a transform on data[0]..data[N-1])? This is probably a standard problem but I could not find a good reference.

回答1:

What about the following, simply wrapping your transform in a for_each?

std::for_each( data.begin(), data.end(), 
               [&scalars](std::vector<double>& v) {
                  transform ( v.begin(), v.end(),
                              scalars.begin(),
                              v.begin(), plus<double>() );
                } 
             );

回答2:

If you don't have lambdas available to you, you could implement a functor to transform each vector<double>:

struct transformer
{
    transformer(vector<double>& s)
    :s_(s)
    {
    }

    vector<double>& operator ()(vector<double>& v)
    {
        transform(v.begin(), v.end(), s_.begin(), v.begin(), plus<double>());
        return v;
    }

    vector<double> s_;
};

And use this in your call to transform for your vector of vector<double>:

transform(data.begin(), data.end(), data.begin(), transformer(scalars));

来源：https://stackoverflow.com/questions/15014686/stdtransform-for-a-vector-of-vectors

标签

c++

vector

nested

range

transform