How to use algorithms to fill vector of vectors

Question

I have

typedef std::vector<int> IVec;
typedef std::vector<IVec> IMat;

and I would like to know how I can fill an IMat by using std algorithms, ie how to do the following with less code (all the IVecs have the same size) ?

void fill(IMat& mat){
    for (int i=0;i<mat.size();i++){
        for (int j=0;j<mat[i].size();j++){
            mat[i][j] = i*j;
        }
    }
}

PS: already a way to fill the matrix with a constant would help me. And preferably with pre-C++11 algorithms.

Answer 1

The best solution is the one that you have already implemented. It takes advantage of using i/j as both offsets and as inputs to compute the algorithm.

Standard algorithms will have to use iterators for the elements and maintain counters. This data mirroring as a sure sign of a problem. But it can be done, even on one line if you wanna be fancy:

for_each(mat.begin(), mat.end(), [&](auto& i) { static auto row = 0; auto column = 0; generate(i.begin(), i.end(), [&]() { return row * column++; }); ++row; });

But as stated just cause it could be done doesn't mean that it should be done. The best way to approach this is the for-loop. Even doing it on one line is possible if that's your thing:

for(auto i = 0U;i < mat.size();i++) for(auto j = 0U;j < mat[i].size();j++) mat[i][j] = i*j;

---

Incidentally my standard algorithm works fine on Clang 3.7.0, gcc 5.1, and on Visual Studio 2015. However previously I used transform rather than generate. And there seem to be some implementation bugs in gcc 5.1 and Visual Studio 2015 with the captures of lambda scope static variables.