How to chain C++ `transform` and `inner_product` calls? - c++

Question

I would like to do something like:
vector<int> v;
v.push_back(0);
v.push_back(1);
v
.transform([](auto i) { return i + 2; })
.transform([](auto i) { return i * 3; })
.inner_product(0);
In other words, just implicitly use begin() and end() for the first and last iterators and chain the results.
Is there anything (eg some library) that would allow this?

Answer 1

Just write your own class to augment std::vector.
#include <algorithm>
#include <iostream>
#include <numeric>
#include <vector>
#include <utility>
template < typename T >
class augmented_vector
{
std::vector < T > m_vec;
public:
augmented_vector() : m_vec() {}
explicit augmented_vector(std::vector<T> const& in) : m_vec(in) {}
void push_back ( T&& value )
{
m_vec.push_back(std::forward<T>(value));
}
template < typename F >
augmented_vector < T > transform(F const& f)
{
std::vector < T > new_vec(m_vec.size());
std::transform( m_vec.begin(), m_vec.end(), new_vec.begin(), f);
return augmented_vector < T > ( new_vec );
}
T inner_product(T value)
{
return std::inner_product( m_vec.begin(), m_vec.end(), m_vec.begin(), value);
}
};
int main()
{
augmented_vector<int> v;
v.push_back(0);
v.push_back(1);
auto val = v
.transform([](auto i) { return i + 2; })
.transform([](auto i) { return i * 3; })
.inner_product(0);
std::cout << val << '\n';
}
Or use the D programming language. It has universal function call syntax.
import std.algorithm : fold, map;
import std.stdio : writeln;
void main()
{
auto v = [0, 1];
auto x = v
.map!(a => a+2)
.map!(a => a*3)
.fold!((a,b) => a + b^^2)(0);
writeln(x);
}

Answer 2

Wrap the functions, providing the boilerplate in the wrapper functions:
template<typename Container, typename Transform>
void transform_container(Container & container, Transform transform) {
std::transform(std::begin(container), std::end(container),
std::begin(container), /* requires output iterator */
transform);
}
template<typename T, typename Container>
auto inner_product_self(Container&& container, T initial) {
return std::inner_product(std::begin(container), std::end(container),
std::begin(container),
initial);
}
Your code then becomes:
int main() {
std::vector<int> v(2);
std::itoa(std::begin(v), std::end(v), 0);
transform_container(v, [](auto i) { return i + 2; });
transform_container(v, [](auto i) { return i * 3; });
auto result = inner_product_self(container, 0);
std::cout << "result: " << result;
}
(Live on ideone)
You're not chained to object oriented programming!