Implementing Merge Sort - c++

Question

So, I am trying to implement merge sort, which I conceptually understand but am obviously having great difficulty programming (I'm quite inexperienced). I have looked up previous posts which has helped, but I can't get past this point. I am currently having the following errors :
13:20: error: âSizeâ was not declared in this scope
mergesort(array[Size], low, mid);
41:20: error: invalid conversion from âintâ to âint*â [-fpermissive]
mergesort(data[size], 0, size-1);
6:6: error: initializing argument 1 of âvoid mergesort(int*, int,
int)â [-fpermissive] void mergesort(int array[], int low, int high)
I am also very confused on how to proceed in general. I really don't know how to merge everything back together either. This did not seem like it would be nearly this difficult, but recursion really confuses me :/ Thanks for your help in advance.
#include <iostream>
using namespace std;
void merge(int[], int, int, int);
void mergesort(int array[], int low, int high)
{
int mid;
if(low < high)
{
mid = low + (high-low)/2;
mergesort(array[Size], low, mid);
mergesort(array[Size], mid+1, high);
merge(array[Size], low, mid, high);
}
}
void merge(int array, int low, int mid, int high)
{
}
int main()
{
int size;
cin >> size;
int data[size];
for(int i = 0; i < size; i++)
{
cin >> data[i];
}
mergesort(data[size], 0, size-1);
}

Answer 1

data[Size] attempts to get the Sizeth value from the array data. Size doesn't exist, and I don't think this is what you want. If you want to refer to the array itself, just use it's name: data.
You have the same problem later on with data[size], except size does exist in this case. The error message is telling you that it can't convert an int to an int*. When you declare a function parameter like int array[], it is actually just syntactic sugar for int* array. You are passing data[size] to this parameter, which attempts to access an int from data (although size is outside the bounds). Hence, the compiler does not know how to convert an int to a int* - they're different types.
Note that the following code is not standard C++, because variable-length arrays are not supported:
int size;
cin >> size;
int data[size];
That is, the size of an array needs to be known at compile-time, which size is not. You could instead use std::vector<int> data(size);.
Also, your declaration and definition for merge don't match - one takes an int as its first parameter, while the other takes int[].

Answer 2

It seems you are having difficulties understanding static arrays vs dynamic arrays.
I would suggest using std::vector instead of your data[size] declaration .
Regarding your errors :
Note that inside your mergesort function you are referring to Size which isn't defined . If you want to go with static array, I would suggest the following :
#define SIZE 200
...
int data[SIZE];
This will allow you to use the same SIZE thorughout the code.
However your array won't be the size of your input .
If you want to allocate an array at runtime , you need to change your code from
int size;
cin >> size;
int data[size];
To
int size;
cin >> size;
int* data = new int[size];
Then , in your mergesort function, you will have to pass the size as parameter.