Как из set сделать vector

Обновлено: 17.05.2024

This post will discuss how to convert a set to a vector in C++.

C++, copy set to vector

std::copy doesn't add elements to the container into which you are inserting: it can't; it only has an iterator into the container. Because of this, if you pass an output iterator directly to std::copy , you must make sure it points to a range that is at least large enough to hold the input range.

std::back_inserter creates an output iterator that calls push_back on a container for each element, so each element is inserted into the container. Alternatively, you could have created a sufficient number of elements in the std::vector to hold the range being copied:

Or, you could use the std::vector range constructor:

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332k 72 72 gold badges 889 889 silver badges 958 958 bronze badges or just use the cbegin and cend version: output.insert(output.cend(), input.cbegin(), input.cend()); What do you think? Thanks. Should I output.reserve(input.size()); by myself or can I hope that some compiler does it for me? Your first vector initialization is incorrect. You create an array of input,size() empty entries and then append the appends after that. I think you mean to use std::vector<double> output; output.reserve(input.size()); std::copy(. ); .

Just use the constructor for the vector that takes iterators:

Assumes you just want the content of s in v, and there's nothing in v prior to copying the data to it.

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here's another alternative using vector::assign :

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You haven't reserved enough space in your vector object to hold the contents of your set.

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18.9k 11 11 gold badges 62 62 silver badges 92 92 bronze badges I only just figured out that if I edit the answer myself, I can do an upvote. Did that, gave you a +1 for the fail-first memory allocation. Sorry!

I think the most efficient way is to preallocate and then emplace elements:

That way we will only invoke copy constructor for every element as opposed to calling default constructor first and then copy assignment operator for other solutions listed above. More clarifications below.

We need to avoid constructing a vector with the size argument which causes all elements default constructed (for nothing). Like with solution using std::copy(), for instance.

And, finally, vector::assign() method or the constructor taking the iterator range are not good options because they will invoke std::distance() (to know number of elements) on set iterators. This will cause unwanted additional iteration through the all set elements because the set is Binary Search Tree data structure and it does not implement random access iterators.

I am getting the run time error in following code. Please let me know can i copy vector elements in set?

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1. Using Range Constructor

The most elegant solution is to use the std::vector range constructor, which takes two input iterators pointing to the beginning and the end of an input sequence.

Output:

a b c d e

4 Answers 4

You are not initialising itr :

Or remove itr entirely:

In this instance, you could simply initialise kk thus (but if you want to add to kk follow the line above):

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If the goal is to create a set from the vector elements (rather than update an already existing set that might have some elements in it), then do that, by using the constructor:

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You need to initialize the iterator.

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2. Using std::copy function

Another good alternative is to use the std::copy , which inserts elements from a source container into a destination container. The destination container should be large enough to accommodate all elements.

How to Convert Set to Vector in C++?

Given the following set with integers (or other types)

We want to convert it to:

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.

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

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:

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

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:

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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