Slicing a vector in C++

Solution 1:

This can easily be done using std::vector's copy constructor:

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

As @AlessandroFlati suggested I should clarify that this will include v1.end()

Solution 2:

I know it's late but have a look at valarray and its slices. If you are using a vector of some sort of NumericType, then it's worth giving it a try.

Solution 3:

In C++20 it is pretty easy:

#include <span>
#include <vector>
#include <iostream>

template<int left = 0, int right = 0, typename T>
constexpr auto slice(T&& container)
{
    if constexpr (right > 0)
    {
        return std::span(begin(std::forward<T>(container))+left, begin(std::forward<T>(container))+right);
    }
    else
    {
        return std::span(begin(std::forward<T>(container))+left, end(std::forward<T>(container))+right);
    }
}



int main()
{
    std::vector v{1,2,3,4,5,6,7,8,9};

    std::cout << "-------------------" << std::endl;
    auto v0 = slice<1,0>(v);
    for (auto i : v0)
    {
        std::cout << i << std::endl;
    }

    std::cout << "-------------------" << std::endl;
    auto v1 = slice<0,-1>(v);
    for (auto i : v1)
    {
        std::cout << i << std::endl;
    }

    std::cout << "-------------------" << std::endl;
    auto v2 = slice<1,3>(v);
    for (auto i : v2)
    {
        std::cout << i << std::endl;
    }

    std::cout << "-------------------" << std::endl;
    auto v3 = slice<1,-1>(v);
    for (auto i : v3)
    {
        std::cout << i << std::endl;
    }

    std::cout << "-------------------" << std::endl;
    auto v4 = slice<3,3>(v);
    for (auto i : v4)
    {
        std::cout << i << std::endl;
    }

}

Result:

Program returned: 0
-------------------
2
3
4
5
6
7
8
9
-------------------
1
2
3
4
5
6
7
8
-------------------
2
3
-------------------
2
3
4
5
6
7
8
-------------------

You can also add boundary checks and other cases like negative left indices etc... but this is only an example.

Run in compiler explorer: https://godbolt.org/z/qeaxvjdbj

Solution 4:

It depends on whether you want a view or a copy.

Python's slicing for lists copies references to the elements, so it cannot be simply regarded as a view or a copy. For example,

list1 = [1, 2, 3]
list2 = list1[1:]  
list2[1] = 5
print(list1) # does not change, still [1, 2, 3]

list1 = [1, 2, [3]]
list2 = list1[1:]  
list2[1][0] = 5
print(list1) # changes, becomes [1, 2, [5]]

See this post for details.

DimChtz's anwer models the copy situation. If you just want a view, in C++20, you can use ranges (besides std::views::drop, std::views::take and std::views::counted are also useful):

auto v2 = v1 | std::views::drop(1); // #include <ranges>
for (auto &e: v2) std::cout << e << '\n';  

or std::span:

std::span v2{v1.begin() + 1, v1.end()}; // #include <span>
for (auto &e: v2) std::cout << e << '\n';