Cross Concatenate Elements of 2 Vectors to Produce a Third

Question

I have 2 vectors and want to distribute one across the other to form a third vector like:

V1 = (a,b,c)
V2 = (d,e,f)

Result:

V3          


        

Answer 1

In C++:

std::vector v3 { v1[0] * v2[0], v1[0] * v2[1], v1[0] * v2[2], v1[1] * v2[0], v1[1] * v2[1], v1[1] * v2[2], v1[2] * v2[0], v1[2] * v2[1], v1[2] * v2[2] };

...though, I usually would format it in multiple lines.

As MCVE:

#include 
#include 
using namespace std;

template 
ostream& operator << (ostream &out, const vector &vec)
{
  const char *sep = "{ ";
  for (const ELEM &elem : vec) { out << sep << elem; sep = ", "; }
  return out << " }";
}

int main()
{
  vector v1{ 1.0, 2.0, 3.0 }, v2{ 4.0, 5.0, 6.0 };
  // in one line:
  vector v3 { v1[0] * v2[0], v1[0] * v2[1], v1[0] * v2[2], v1[1] * v2[0], v1[1] * v2[1], v1[1] * v2[2], v1[2] * v2[0], v1[2] * v2[1], v1[2] * v2[2] };
  // output:
  cout << v3 << endl;
  // done
  return 0;
}

Output:

{ 4, 5, 6, 8, 10, 12, 12, 15, 18 }

(Tested on ideone.)

Thanks, Rory Daulton, for the inspiration. In C++, I can do this also:

#include 
#include 
#include 

using namespace std;

string operator*(const string &str1, const string &str2)
{
  return str1 + str2;
}

template 
ostream& operator << (ostream &out, const vector &vec)
{
  const char *sep = "{ ";
  for (const ELEM &elem : vec) { out << sep << elem; sep = ", "; }
  return out << " }";
}

int main()
{
  vector v1{ "a", "b", "c" }, v2{ "d", "e", "f" };
  // in one line:
  vector v3 { v1[0] * v2[0], v1[0] * v2[1], v1[0] * v2[2], v1[1] * v2[0], v1[1] * v2[1], v1[1] * v2[2], v1[2] * v2[0], v1[2] * v2[1], v1[2] * v2[2] };
  // output:
  cout << v3 << endl;
  // done
  return 0;
}

Output:

{ ad, ae, af, bd, be, bf, cd, ce, cf }

After a little improvement, it will even work for mixed types:

#include 
#include 
#include 

using namespace std;

string operator*(const string &arg1, int arg2)
{
  string ret; for (int i = 0; i < arg2; ++i) ret += arg1;
  return ret;
}

template 
ostream& operator << (ostream &out, const vector &vec)
{
  const char *sep = "{ ";
  for (const ELEM &elem : vec) { out << sep << elem; sep = ", "; }
  return out << " }";
}

int main()
{
  vector v1{ "a", "b", "c" }; vector v2{ 1, 2, 3 };
  // in one line:
  vector v3 { v1[0] * v2[0], v1[0] * v2[1], v1[0] * v2[2], v1[1] * v2[0], v1[1] * v2[1], v1[1] * v2[2], v1[2] * v2[0], v1[2] * v2[1], v1[2] * v2[2] };
  // output:
  cout << v3 << endl;
  // done
  return 0;
}

Output:

{ a, aa, aaa, b, bb, bbb, c, cc, ccc }