3

How to initialize a 2 dimensional vector<int> in C++?

For instance I have 4 arrays each of length 8 ints, like the below 

int a1[] = {1,2,3,4,5,6,7,8};
int a2[] = {1,2,3,4,9,10,11,12};
int a3[] = {1,2,5,6,9,10,13,14};
int a4[] = {1,3,5,7,9,11,13,15};

and I have this 

vector< vector <int> > aa (4);

aa[i] (a1,a1+8);

But this gives error. I even tried supplying the array a1 to v1 and passed v1 to aa[i] , still it fails.

So what would be the proper way of initializing the elements of a 2 dimensional vector<int>

user470379
  • 4,879
  • 16
  • 21
bsoundra
  • 930
  • 2
  • 13
  • 27
  • Will each subarray have a length of exactly 8 ints? It might be easier to simply have a flat, one-dimensional array with `a1` occupying the first 8 spots, `a2` occupying the second 8 spots, etc. – In silico Nov 03 '10 at 21:21

3 Answers3

2
aa[i].assign(a1,a1+8);
Armen Tsirunyan
  • 130,161
  • 59
  • 324
  • 434
  • +1 - This answer is better. I was going to use `assign`, but was thrown off by the absence of `assign` on SGI's page. – aschepler Nov 03 '10 at 21:16
2
int arr[4][8] =
{
    {1, 2, 3, 4, 5,  6,  7,  8},
    {1, 2, 3, 4, 9, 10, 11, 12},
    {1, 2, 5, 6, 9, 10, 13, 14},
    {1, 3, 5, 7, 9, 11, 13, 15},
};

std::vector<std::vector<int> > vec(4, std::vector<int>(8));
for (int i = 0; i < 4; ++i)
{
    vec[i].assign(arr[i], arr[i] + 8);
}
fredoverflow
  • 256,549
  • 94
  • 388
  • 662
0

The initialization of aa also initialized all four of the contained vector<int> objects, using the default constructor for vector<int>. So you'll need to add data to those empty vectors, not initialize them.

Try for example:

std::copy(a1, a1+8, std::back_inserter(aa[i]));
aschepler
  • 70,891
  • 9
  • 107
  • 161