Creating a new C++ subvector?

Question

Say I have a vector with values [1,2,3,4,5,6,7,8,9,10]. I want to create a new vector that refers to, for example, [5,6,7,8]. I imagine this is just a matter of creating a vector with pointers or do I have to push_back all the intermediary values I need?

The given answers so far are all good options. While you *could* create a vector of pointers to your subrange, you probably don't want to. If the original vector ever grows to the point where it has to re-allocate its internal storage, your pointers would all become invalid. Especially when dealing with such small ranges, working with a copy is preferable. — Michael Kristofik, Mar 14 '12 at 16:18
Define "refers to". What should happen if the original vector changes? — Karl Knechtel, Mar 14 '12 at 16:43
Possible duplicate of [Best way to extract a subvector from a vector?](http://stackoverflow.com/questions/421573/best-way-to-extract-a-subvector-from-a-vector) — TobiMcNamobi, Apr 12 '17 at 08:38

score 52 · Accepted Answer · answered Mar 14 '12 at 16:02

52

One of std::vector's constructor accepts a range:

std::vector<int> v;

// Populate v.
for (int i = 1; i <= 10; i++) v.push_back(i);   

// Construct v1 from subrange in v.
std::vector<int> v1(v.begin() + 4, v.end() - 2);

answered Mar 14 '12 at 16:02

hmjd

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score 4 · Answer 2 · edited Mar 14 '12 at 16:33

4

You don't have to use push_back if you don't want to, you can use std::copy:

std::vector<int> subvector;
copy ( v1.begin() + 4, v1.begin() + 8, std::back_inserter(subvector) );

edited Mar 14 '12 at 16:33

Michael Kristofik

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answered Mar 14 '12 at 16:04

Luchian Grigore

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Shouldn't you initialize subvector to the correct size before copying into it? – obmarg Mar 14 '12 at 16:07
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@obmarg, or you could use some form of `insert_iterator`. – Michael Kristofik Mar 14 '12 at 16:09
@obmarg yes or use a back insertion iterator - `std::back_inserter` will make one for you – Flexo Mar 14 '12 at 16:09
@Kristo (and @awoodland) Yes. Though from my experience they tend to be slow, do they not just use push_back behind the scenes? – obmarg Mar 14 '12 at 16:11
Inserting one element at a time (as `insert_iterator`s do) will usually be slower than inserting a range all at once. But for vectors of such small size as this example, I doubt it matters. – Michael Kristofik Mar 14 '12 at 16:15
you can always use `reserve()` with an insertion iterator to avoid reallocation too. – Flexo Mar 14 '12 at 16:20
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Since it's been 30 minutes and @Luchian hasn't noticed our comments yet, I went ahead and edited the code. – Michael Kristofik Mar 14 '12 at 16:33

score 4 · Answer 3 · answered Mar 14 '12 at 16:08

This is fairly easy to do with std::valarray instead of a vector:

#include <valarray>
#include <iostream>
#include <iterator>
#include <algorithm>

int main() {
  const std::valarray<int> arr={0,1,2,3,4,5,6,7,8,9,10};

  const std::valarray<int>& slice = arr[std::slice(5, // start pos
                                                   4, // size
                                                   1  // stride
                                                  )];

}

Which takes a "slice" of the valarray, more generically than a vector.

For a vector you can do it with the constructor that takes two iterators though:

const std::vector<int> arr={0,1,2,3,4,5,6,7,8,9,10};
std::vector<int> slice(arr.begin()+5, arr.begin()+9);

score 3 · Answer 4 · answered Mar 14 '12 at 16:07

I would do the following:

#include <vector>
#include <iostream>

using namespace std;

void printvec(vector<int>& v){
        for(int i = 0;i < v.size();i++){
                cout << v[i] << " ";
        }
        cout << endl;
}

int main(){
        vector<int> v;

        for(int i = 1;i <= 10;i++) v.push_back(i);
        printvec(v);

        vector<int> v2(v.begin()+4, v.end()-2);
        printvec(v2);
        return 0;
}

~

Creating a new C++ subvector?

4 Answers4

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