cplusplus

Let’s say our task is to convert a set or unordered_set in C++ to std::vector, what would you do?

Given the following set with integers (or other types)

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unordered_set<int> data;

unordered_set<int> data;

We want to convert it to:

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vector<int> target;

vector<int> target;

Using a Loop to copy a C++ set to std::vector

Intuitively, we can push to the vector one by one, in a loop, by reading the elements in the set.

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vector<int> target;
for (const auto &it: data) {
    target.push_back(it);
}

vector<int> target;
for (const auto &it: data) {
    target.push_back(it);
}

Alternatively, we can pre-allocate the size of the vector, and use something like this (slightly a bit faster):

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vector<int> target(data.size());
for (int i = 0; i < data.size(); ++ i) {
    target[i] = data[i];
}

vector<int> target(data.size());
for (int i = 0; i < data.size(); ++ i) {
    target[i] = data[i];
}

Using vector constructor to convert a C++ set to std::vector

In the constructor of the vector, we can pass the iterator of begin and end for the input set, like below:

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vector<int> target(data.begin(), data.end());

vector<int> target(data.begin(), data.end());

Using vector.assign to convert a C++ set to std::vector

We can use the std::vector.assign() method that takes the begin and end iterator of the source set/unordered_set – which will allow us to convert/copy a C++ set to std::vector, like below.

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vector<int> target;
target.assign(data.begin(), data.end());

vector<int> target;
target.assign(data.begin(), data.end());

Using std::copy to copy a C++ set to std::vector

Using std::copy, we can specify the begin and end iterator of the source data to copy from, and the begin iterator of the target data source, like this:

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vector<int> target(data.size());
std::copy(data.begin(), data.end(), target.begin());

vector<int> target(data.size());
std::copy(data.begin(), data.end(), target.begin());

We have to make sure the target data has enough space/storage to copy the data to, thus, the target vector has to be pre-allocated.

Alternatively, we can use the back_inserter that will insert at the end of the target vector, thus no need to allocate the target vector.

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vector<int> target;
std::copy(data.begin(), data.end(), std::back_inserter(res));

vector<int> target;
std::copy(data.begin(), data.end(), std::back_inserter(res));

–EOF (The Ultimate Computing & Technology Blog) —