Compare smaller vector to bigger to check if it differs at the end of smaller

Question

We have two vectors of size that depends on runtime and need to check if they are equal - differ elements only after the end of smaller size vector. I used std::equal but the issue is that I need to find first which vector is of smaller size which leads to extra line of code :

#include <vector>
#include <iostream>

int main()
{
  std::vector<int> a(1000, 3);
  std::vector<int> a1(100, 3);

  if(a.size() > a1.size())
  {
    if(std::equal(a1.begin(), a1.end(), a.begin()))
    {
      std::cout << "Same a gt a1" << std::endl;
    }
  }

  if(a1.size() > a.size())
  {
    if(std::equal(a.begin(), a.end(), a1.begin()))
    {
      std::cout << "Same a1 gt a" << std::endl;
    }
  }

  if(a1.size() == a.size())
  {
    if(std::equal(a.begin(), a.end(), a1.begin()))
    {
       std::cout << "Same a = a1" << std::endl;
    }
  }
}

Can the code to compare two vectors or differ only at the end of smaller vector be improved?

Answer 1

Since C++14, you can use std::mismatch and check the pair of iterators returned against the end of each range:

auto it = std::mismatch(a.begin(), a.end(), a1.begin(), a1.end());
if (it.first == a.end() || it.second == a1.end()) {
    // Equality
}

You also get to know where the elements start to differ, and if they don't, at which point the bigger vector is bigger (the start of the subrange you don't want to compare).