Switching data from array to vector - c++

Question

I'm trying to convert my array to a vector, yet I'm having trouble printing it.
It says in int main() in my for loop that v is undefined. When I define
vector v; inside int main() the program compiles and runs and yet prints nothing. What am I doing wrong?
#include <iostream>
#include <vector>
using namespace std;
vector<int> a2v(int x[], int n)
{
vector<int> v(n);
for(int i = 0; i < n; i++)
{
v.push_back(x[i]);
}
return(v);
}
int main()
{
vector<int> a2v(int x[], int n);
int array[] = {11,12,13,14,15,16,17,18};
a2v(array, 8);
for(int i = 0; i < v.size(); i++)
{
cout << v[i] <<" ";
}
cout << endl;
return(0);
}

Answer 1

This is your program corrected:
#include <iostream>
#include <vector>
using namespace std;
vector<int> a2v(int x[], int n)
{
vector<int> v(0);
v.reserve(n); //optional
for(int i = 0; i < n; i++)
{
v.push_back(x[i]);
}
return(v);
}
int main()
{
int array[] = {11,12,13,14,15,16,17,18};
auto v = a2v(array, 8);
for(size_t i = 0; i < v.size(); i++)
{
cout << v[i] <<" ";
}
cout << endl;
return(0);
}
There were 2 errors:
In the function a2v, you instantiated a vector of 0 with length n, and then you pushed back other elements;
You were not defining v inside the main, as the return value of a2v

Answer 2

The vector you want to read is the return of the a2v function.
But there is a lot more simpler than that to go from C-array to vector array , I put here the example in found the vector reference web page :
http://www.cplusplus.com/reference/vector/vector/vector/
int myints[] = {16,2,77,29};
std::vector<int> fifth (myints, myints + sizeof(myints) / sizeof(int) );

Answer 3

When you define a variable, such as your vector v, you always do it in a certain scope. The body of a function such as a2v, or main, is an example of a possible scope. So if you only do it in the function a2v, that's the only scope where it will be visible.

Answer 4

It says in int main() in my for loop that v is undefined.
Well it will be because there is no v in main()
a2v(array, 8);
The above function call returns a vector so you need to collect the returned vector like
vector<int> v=a2v(array,8)
Also,
vector<int> v(n);//creates a vector of size n
for(int i = 0; i < n; i++)
{
v.push_back(x[i]);//adds n more elements to the vector
}
The returned vector has 2n elements and not n
Finally you could directly create a vector from an array as
vector<int> v(arr,arr+n);//where n is the number of elements in arr