Add same value multiple times to std::vector (repe

问题:

I want to add a value multiple times to an std::vector. E.g. add the interger value 1 five times to the vector:

std::vector<int> vec;
vec.add(1, 5);

vec should be of the form {1,1,1,1,1} afterwards. Is there a clean c++ way to do so?

回答1:

It really depends what you want to do.

Make a vector of length 5, filled with ones:

std::vector<int> vec(5, 1);

Grow a vector by 5 and fill it with ones:

std::vector<int> vec;
// ...
vec.insert(vec.end(), 5, 1);

Or resize it (if you know the initial size):

std::vector<int> vec(0);
vec.resize(5, 1);

You can also fill with elements using one of the many versions of fill, for example:

fill_n(back_inserter(vec), 5, 1);

and so on.... Read the library documentation, some of these functions return useful information, too.

回答2:

You can just use the std::vector constructor for this:

std::vector<int> vec (5,1);

The signature for this is:

vector (size_type n, const value_type& val)

The standard algorithm header has a number of functions which can be used in cases like this. std::fill_n would work for your case.:

std::fill_n (std::back_inserter(vec), 5, 1);

回答3:

Just use std::vector::insert.

#include <vector>
#include <iostream>

int main()
{
    std::vector<int> a;
    a.insert(a.end(), 5, 1);
    for(auto const& e : a)
        std::cout << e << std::endl;
    return 0;
}

回答4:

You can use the assign method:

vec.assign(5, 1);

This will delete any existing elements in the vector before adding the new ones.