Be consistent.

int A[10];
 *or*
vector<int> A (10);

Use address of first element:

MPI_Send (&A[0], ...);

Could even do 

MPI_Send (&A[4], ...);

MPI_Send (A.data(), A.size(), mpiType(A), ... )

==

Watch names. Make them mnemonic. 

vector<float> randomNums;
vector<float> localSums;
float sumOfSums;

Use "my" prefix for a name that depends on the process (local).

int myRank;
vector<float> myRandomNums;

==

Prefer vectors to arrays. Ensure vectors are sized properly.

vector<int> nums;
// Bad
for (size_t i = 0; i < N; ++i)
  // Should use push_back
  nums[i] = 5;

vector<int> nums (N);
// Good
for (size_t i = 0; i < N; ++i)
  nums[i] = 5;

How to convert vector to array:

void
f (int A[], size_t len);

// Call with 
f (&nums[0], nums.size ());

==

Arrays cannot be dynamically sized. 

int N;
cin >> N;
// Error: size must be const integral type
int A[N];