C :: Reversing Elements In Array

Write the definition of a function reverse , whose first parameter is an array of integers and whose second parameter is the number of elements in thearray . The function reverses the elements of the array . The function does not return a value .

Code:
void reverse(int a[], int num) {
for ( int i=0; i <= num/2 ; i++){
int temp = a[i];
a[i] = a[num-i-1];
a[num-i-1] = temp;
} }

This is supposed to be the answer but I'm not quite sure why this is. I understand everything up until the actual loop. For one, shouldn't "int i" be declared outside the loop (I thought perhaps this was an error in the solutions)?

The main thing that I do not understand is the conditional statement.

Code: i<=num/2;

I don't understand why the "num/2" is necessary here. Also I can't really remember but is there a command that actually reverses an array?