将 stl 算法应用于多维向量（向量<vector<t> &gt;) </vector<t>

Question

How, generally, can an stl algorithm be applied to a multidimensional vector (ie a vector<vector<T> > )?

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For example, if I wanted to fill some vector, myVector , with values according to some function, myFunc() , I might use something like:

std::generate(myVector.begin() myVector.end(), myFunc())

Suppose now that myVec is a vector<vector<T> > . How might I use std::generate to populate every element of every vector in myVec according to myFunc ? Need I use a loop (barring all other considerations)?

Would I simply write something like:

std::generate(myVec.begin(), myVec.end(), std::generate(...))

Surprisingly, I cannot find anything on this here or elsewhere.

Answer 1

The generator passed to std::generate() needs to return a type that is assignable to the element type of the container. So, in your example, the element type of myVector is another vector , so myFunc() would need to return a whole vector , eg:

template<typename T>
vector<T> myFunc()
{
    vector<T> v;
    // populate v as needed...
    return v;
}

vector<vector<T> > myVector(some size);
std::generate(myVector.begin() myVector.end(), myFunc<T>);

Otherwise, you will have to do something more like this instead:

template<typename T>
void myFunc(vector<T> &v)
{
    // populate v as needed...
}

vector<vector<T>> myVector(some size);
for(auto &v : myVector) {
    myFunc(v);
}

Answer 2

With range library (as range-v3 ), you might flatten your vectors to work with vector of less dimension:

std::vector<std::vector<int>> v(4, std::vector<int>(3));

auto flattened = v | ranges::view::join;

std::generate(begin(flattened), end(flattened), [n = 0]() mutable{ return n++; });

Demo

Else, regular loop seems the simpler:

auto gen = [n = 0]() mutable{ return n++; }
for (auto& inner : v) {
    std::generate(begin(inner), end(inner), gen);
}

You cannot really nest generate calls as it doesn't take current element to know size of each inner vector.