#Template Interview Question

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sour rapids
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What standard of C++ would this be targeting?

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(also I assume that's meant to be template <typename... vectors> given the usage example?)

green path
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Up to 20, and yes sorry

sour rapids
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The obvious approach (valid since ~C++11) would be a recursive helper template, along the lines of ```cpp
template <int... I> struct Vector;

namespace detail {
template <typename... Vs> struct multiply_vectors;

// base cases
template <typename V> struct multiply_vectors<V> {
using type = V;
};
template <int... I1, int... I2> struct multiply_vectors<Vector<I1...>, Vector<I2...>> {
using type = Vector<(I1 * I2)...>;
};

// recursive case
template <typename V1, typename V2, typename... Rest> struct multiply_vectors<V1, V2, Rest...> {
using type = typename multiply_vectors<typename multiply_vectors<V1, V2>::type, Rest...>::type;
};
}

template <typename... Vs>
struct zip {
using type = typename detail::multiply_vectors<Vs...>::type;
};

// ...

static_assert(std::is_same<zip<Vector<1, 2, 3>, Vector<4, 5, 6>, Vector<7, 8, 9>>::type, Vector<28, 80, 162>>::value);

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a more fancy version since C++17 (that's potentially better compile times) would be to use an operator* overload + fold expressions and decltype

green path
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using type = Vector<(I1 * I2)...>;

How did I not know you could do this 🤦‍♂️

sour rapids
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template <int... I> struct Vector;

namespace detail {
  template <typename V> struct multiply_helper { using underlying = V; };
  template <int... I1, int... I2>
  auto operator*(multiply_helper<Vector<I1...>>, multiply_helper<Vector<I2...>>)
    -> multiply_helper<Vector<(I1 * I2)...>>;
};

template <typename... Vs>
struct zip {
  using type = typename decltype((... * detail::multiply_helper<Vs>{}))::underlying;
};

// ...

static_assert(std::is_same<zip<Vector<1, 2, 3>, Vector<4, 5, 6>, Vector<7, 8, 9>>::type, Vector<28, 80, 162>>::value);
green path
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These both work. Thank you wreien

sour rapids
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again, could simplify if you're allowed to change the definition of Vector itself perhaps

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but you get the idea 🙂

green path
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How do I give you points for solving it? Is that still a thing? I haven't been on here for a while haha

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!solved

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sour rapids
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I think that was a meme thing during april fools 😛

green path
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Lmao

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Thank you again!