C++ :: Write A Function That Return Maximum Value Of Array Of Doubles?

I'm trying to get correct answers from my exam to study for a midterm:

Write a function that returns the maximum value of an array of doubles.

Declaration:

double maximum(array[],int a_size);

So far I've done:

double maximum(array[], int a_size) {
double maxVal;
maxVal = array[0];
for(int i=1; i < size; i++) {
if(array[i] > max
maxVal=array[i]
}
return maxVal;
}

I just need any corrections.

Second, I need to write a function definition that merges two vectors containing integers, alternating elements from both vectors.

Prototype:

vector<int> merge(const vector<int> a, const vector<int> b);

This is my definition so far:

vector<int>merge(const vector<int> a, const vector<int> b){
vector<int> merged;
for(int i=0; i<a.size+b.size; i++) {
if(flag == true)
merged.push_back(a);
flag = false;
else
merged.push_back(b)
flag = true;}

I think I'm lost in the second part...