C++ Create a vector from attributes of objects in another vector

Question

Assuming I have a class A and a vector of instances instances

class A {
public:
    int a;
}

std::vector<A> instances;  // Contains several A objects

I now want to create a vector of int from the attribute a of objects in A.

The naive way would be

std::vector attributes;
for (auto& obj: instances) {
    attributes.push_back(obj.a);
}

Is there a way to do it in just one line? Something conceptually close to python's

attributes = [obj.a for obj in instances]

[`std::transform`](https://en.cppreference.com/w/cpp/algorithm/transform)? — Alan Birtles, May 28 '21 at 09:59
`std::vector attributes; std::transform(instances.cbegin(), instances.cend(), std::back_inserter(attributes), [](auto const &e){return e.a;});` — EOF, May 28 '21 at 10:00
With C++20, `std::ranges::transform(instances, std::back_inserter(attributes), &A::a)` using the projection argument, should work and be much more succinct than the old way. — underscore_d, May 28 '21 at 10:03

songyuanyao · Answer 1 · 2021-05-28T10:06:57.680

3

Use std::transform.

std::vector<...> attributes;
attributes.reserve(instances.size());
std::transform(instances.begin(), instances.end(), std::back_inserter(attributes),
               [](auto&& obj) { return obj.a; });

edited May 28 '21 at 10:06

answered May 28 '21 at 10:00

songyuanyao

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fwiw, thats the equivalent of OPs loop, though it would be more efficient to resize `attributes` upfront and drop the `back_inserter`. – 463035818_is_not_an_ai May 28 '21 at 10:04
2

@463035818_is_not_a_number resizing in advance would initialise all elements to `0` which could be slower, using `reserve` and keeping the `back_inserter` is likely to be more performant – Alan Birtles May 28 '21 at 10:06
@AlanBirtles oh right. The question getting closed saved me from posting a poor answer ;) – 463035818_is_not_an_ai May 28 '21 at 10:06

C++ Create a vector from attributes of objects in another vector

1 Answers1