CODEWITHSUNDEEP
c++vectorslice
Wizard
Wizard

Reputation: 1663

Slicing a vector in C++

Is there an equivalent of list slicing [1:] from Python in C++ with vectors? I simply want to get all but the first element from a vector.

Python's list slicing operator:

list1 = [1, 2, 3]
list2 = list1[1:]  

print(list2) # [2, 3]

C++ Desired result:

std::vector<int> v1 = {1, 2, 3};
std::vector<int> v2;
v2 = v1[1:];

std::cout << v2 << std::endl;  //{2, 3}

Upvotes: 95

Views: 112905

Answers (7)

DimChtz
DimChtz

Reputation: 4333

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

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

However please note that this will make a copy of the existing elements. For a large array this may be slow, and there is additional overhead due to the additional allocation required.

If you do not need a copy, and simply want to read but not modify the elements, use a std::span instead.

Upvotes: 151

Mohammad Anas Khan
Mohammad Anas Khan

Reputation: 9

To find a sub vector from index a to b, then, simply do this:

vector<int> sub_vec(v.begin() + a, v.begin() + b + 1);

This will create a Sub Vector from the a index to the b index and we also add 1 in the end of the range while slicing as because end index will be excluded (value b-1 will be taken as the last index). So, we add 1 to include our last index also.

Upvotes: 0

xskxzr
xskxzr

Reputation: 13040

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';

Upvotes: 16

mnesarco
mnesarco

Reputation: 2778

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

Upvotes: 19

uuu777
uuu777

Reputation: 901

It seems that the cheapest way is to use pointer to the starting element and the number of elements in the slice. It would not be a real vector but it will be good enough for many uses.

Upvotes: -1

Ananth
Ananth

Reputation: 462

You can follow the above answer. It's always better to know multiple ways.

int main
{
    std::vector<int> v1 = {1, 2, 3};
    std::vector<int> v2{v1};
    v2.erase( v2.begin() );
    return 0;
}

Upvotes: 1

Adrian
Adrian

Reputation: 903

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.

Upvotes: 8

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