Question

I guess that there should be someway to write the below piece of code without using “for” loops and by just using STL algorithms and iterators. If I am not wrong can anyone guide me on how to do this?

std::vector<double> A(N);
std::vector<double> B(N);
std::vector<double> C(N);
std::vector<double> D(N);

for(int i = 0; i < N; ++i)
  A[i] = myFunction1(i);

for(int i = 0; i < N; ++i)
  B[i] = myFunction2(A[i], i);

for(int i = 0; i < N; ++i)
  C[i] = myFunction3(A[i], B[i]);

for(int i = 0; i < N; ++i)
  D[i] = myFunction4(A[i], B[i], i);

Answer 1

typedef boost::counting_iterator<int> counter;

std::transform(counter(0), counter(N), A.begin(), myFunction1);
std::transform(A.begin(), A.end(), counter(0), B.begin(), myFunction2);
std::transform(A.begin(), A.end(), B.begin(), C.begin(), myFunction3);

Now write your own version of std::transform that takes a ternary function:

template <typename In1, typename In2, typename In3, typename Out, typename FUNC>
Out transform3(In1 first1, In1 last1, In2 first2, In3 first3, Out out, FUNC f) {
    while (first1 != last1) {
        *out++ = f(*first1++, *first2++, *first3++);
    }
    return out;
}

transform3(A.begin(), A.end(), B.begin(), counter(0), D.begin(), myFunction4);

I guess there might be something you can do with variadic templates in C++0x to get a transform_N, but if so I don’t know what it is, I’ve never used them. Not sure if you can forward a variable number of arguments with modifications (in this case wrapping * ++ around each one, as it were).