How to write multiple vectors to multidimensional array? - c++

Question

I have 2 vectors of ints:
vector<int> even = {2, 4, 6, 8};
vector<int> odd = {1, 3, 5, 7, 9};
I am trying to pass these vectors into a 2d array so that they end up looking like this:
array[2][5] = {2, 4, 6, 8,
1, 3, 5, 7, 9};
Writing a function to do this but I cant get it to work, this is the code so far:
void copy(vector<int> even, vector<int> odd,int arr[][COLMAX]){
int vectors = 2;
for(int row = 0; row < vectors; row++){
for(int col = 0; col < even.size(); col++){
arr[row][col] = even[col];
}
cout << endl;
}
}
Edit: Added function code

Answer 1

Here is one way:
void copy(vector<int> even, vector<int> odd,int arr[][COLMAX]){
for(int col = 0; col < even.size(); col++){
arr[0][col] = even[col];
}
for(int col = 0; col < odd.size(); col++){
arr[1][col] = odd[col];
}
}

Answer 2

Why go for arrays if you can use vectors?
If you can use a 2D vector instead of a 2D array, you'd have much more flexibility. And it'll be very simple:
vector<int> even = {2, 4, 6, 8};
vector<int> odd = {1, 3, 5, 7, 9};
vector<vector<int>> v {even,odd}; // that's sufficient !!
for (auto &r: v) { // and this is only the printing of the results
std::copy(r.cbegin(), r.cend(), ostream_iterator<int>(cout," "));
cout <<endl;
}
If you must really use raw arrays...
Now if you really want to use fixed size 2D arrays, you can update your function:
const size_t COLMAX=10;
void mycopy(vector<int> even, vector<int> odd,int arr[][COLMAX]){
vector<const vector<int> *>v ={&even,&odd};
for(int row=0; row<v.size(); row++) {
for(int col = 0; col <COLMAX; col++){
arr[row][col] = (col < v[row]->size() ? (*v[row])[col]:0);
}
}
}
It uses a pointer to the vectors to avoid repeating several time the same code for the different vectors. Moreover, you could easily make it work for 3 or 4 vecotrs (and with a little imagination you could eaily adapt it for a variable number of arguments).
You can then use it as planned:
int myarr[2][COLMAX];
mycopy (even, odd, myarr);
for (int i=0; i<2; i++) {
std::copy (myarr[i], myarr[i]+COLMAX, ostream_iterator<int>(cout," "));
cout <<endl;
}
The problem is that you do not keep track of the number of elements that are really in each line, so you'd have either al lot of truncated lines, or lines with a lot of trailing zeroes.