How to interleave two vectors of different sizes? - c++

Question

I have two vectors
vector<int> first_v = {1, 4, 9, 16, 8, 56};
vector<int> second_v = {20, 30};
And the goal is to combine those vectors in specific order like that (basically program first prints one value of the first_v vector and then one value of second_v vector):
Expected Output:
1 20 4 30 9 16 8 56
I'm very close to solution but the problem is that if one vector is shorter than another, then program will print '0'.
Problem output:
1 20 4 30 9 0 16 0
Here is the code I tried to solve this problem
merge_vect(first_v, second_v);
vector<int> merge_vect(const vector<int> & a, const vector<int> & b){
vector<int> vec(a.size() + b.size());
for (int i = 0; i < vec.size(); i++){
cout << a[i] << " ";
cout << b[i] << " ";
}
return vec;
}
What can I do in order to solve this problem?

Answer 1

You could keep one iterator to each vector and add from the vector(s) that have not reached their end() iterator.
Example:
#include <iostream>
#include <vector>
std::vector<int> merge_vect(const std::vector<int>& avec,
const std::vector<int>& bvec)
{
std::vector<int> result;
result.reserve(avec.size() + bvec.size());
for(auto ait = avec.begin(), bit = bvec.begin();
ait != avec.end() || bit != bvec.end();)
{
if(ait != avec.end()) result.push_back(*ait++);
if(bit != bvec.end()) result.push_back(*bit++);
}
return result;
}
int main() {
std::vector<int> first_v = {1, 4, 9, 16, 8, 56};
std::vector<int> second_v = {20, 30};
auto result = merge_vect(first_v, second_v);
for(auto val : result) std::cout << val << ' ';
}
Output:
1 20 4 30 9 16 8 56
A possible optimization could copy from both vectors for as long as both have elements and then copy the rest from the larger vector in one go:
std::vector<int> merge_vect(const std::vector<int>& avec,
const std::vector<int>& bvec)
{
std::vector<int> result;
result.reserve(avec.size() + bvec.size());
auto ait = avec.begin(), bit = bvec.begin();
// copy while both have more elements:
for(; ait != avec.end() && bit != bvec.end(); ++ait, ++bit) {
result.push_back(*ait);
result.push_back(*bit);
}
// copy the rest
if(ait != avec.end()) result.insert(result.end(), ait, avec.end());
else if(bit != bvec.end()) result.insert(result.end(), bit, bvec.end());
return result;
}