CODEWITHSUNDEEP
c++stlvectoriostream
kolistivra
kolistivra

Reputation: 4429

Getting input directly into a vector in C++

Consider the following code piece:

...
int N,var;
vector<int> nums;
cin >> N;
while (N--)
{
   cin >> var;
   nums.push_back(var);
}
...

Is it possible to do this without using an auxillary variable, in this case var?

Upvotes: 5

Views: 3901

Answers (5)

Trinetra
Trinetra

Reputation: 11

Hinted from Diego Sevilla answer. Use a range constructor

std::vector<int> nums( std::istream_iterator<int>(std::cin),
                       std::istream_iterator<int>() );

Upvotes: 1

Diego Sevilla
Diego Sevilla

Reputation: 29021

Assuming you have already read the initial N, there is a nice trick using istream_iterator:

std::vector<int> nums;
nums.reserve(N);
std::copy(std::istream_iterator<int>(std::cin), 
          std::istream_iterator<int>(),
          std::back_inserter(nums));

The back_inserter object turns itself into an iterator that adds elements to the vector at the end. Iterator streams can be parameterized by the type of the elements read, and, if no parameter given, signals the end of input.

Upvotes: 11

wilhelmtell
wilhelmtell

Reputation: 58675

If you don't have already copy_n() in your toolbelt then you should. Very useful.

template<class In, class Size, class Out>
Out copy_n(In first, In last, Size n, Out result)
{
    while( n-- > 0 && first != last )
        *result++ = *first++;
    return result;
}

With this utility it's convenient and elegant to copy n elements into a vector:

#include<iterator>
#include<vector>
#include<iostream>

// ...
int n = 0;
std::cin >> n;
std::vector<int> v(n);
copy_n(std::istream_iterator<int>(std::cin), std::istream_iterator<int>(),
       n,
       v.begin());

Upvotes: 4

Steve Townsend
Steve Townsend

Reputation: 54168

No need to allocate the vector and then resize it.

Iterators are preferable to index usage.

size_t N;
std::cin >> N;

std::vector<int> values(N);
for (vector<int>::iterator iter = values.begin(); iter != values.end(); ++iter)
{
  std::cin >> *iter;
}

Upvotes: 0

Oliver Charlesworth
Oliver Charlesworth

Reputation: 272557

vector<int> nums(N);
for (int i = 0; i < N; i++)
{
    cin >> nums[i];
}

In the general case, this is actually more efficient. Calling std::vector::push_back() repeatedly without an initial reserve will lead to lots of reallocations.

Upvotes: 3

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