extracting part of a vector

Question

Which is the best way to extract part of a vector? If I have

std::vector<int> v1(9);

for(int i = 0; i < 9; i++)
    v1[i] = i + 1;

I need a code that put in

std::vector<int> v2(2);

the second and third element of v1. I have to do a cycle or there is a smartest way?

mathematician1975 · Accepted Answer · 2014-01-31T09:51:27.263

5

Try this using the iterator form of vector constructor

 std::vector<int> v2(v1.begin() + 1,v1.begin() + 3);

If v2 already exists use assign

v2.assign(v1.begin() + 1,v1.begin() + 3);

edited Jan 31 '14 at 09:51

answered Jan 31 '14 at 09:14

mathematician1975

Beat me by 5 seconds! – Tristan Brindle Jan 31 '14 at 09:15
I know this, but my vector v2 is already existing... I can't use constructor, so? – Wellen Jan 31 '14 at 09:16
@mathematician1975 it seems that your code doesn't work – Wellen Jan 31 '14 at 09:25
@Wellen works fine on my machine – mathematician1975 Jan 31 '14 at 09:34
@mathematician1975 alright, it works if I write `v2.assign(v1.begin() + 1,v1.begin() + 3);` and not `+2`. However, do you think this is the smartest way or using `std::copy` is equivalent? – Wellen Jan 31 '14 at 09:52
@Wellen - I apologise - i had a typo above. It needs to be +3. – mathematician1975 Jan 31 '14 at 09:52
@Wellen I am not certain about speed but if I were coding this myself with large subsets of vectors then I would likely actually use `memcpy` to do it. – mathematician1975 Jan 31 '14 at 10:01

score 2 · Answer 2 · answered Jan 31 '14 at 09:24

2

You can use std::copy(), Suppose if you want to insert 1st and 2nd element from v1 to the beginning of v2 then you can use following.

std::copy ( v1.begin()+1, ,v1.begin() + 3, v2.begin() );

answered Jan 31 '14 at 09:24

Shrikant

2 Answers2