How to set up a partition without sorting? - c++

Question

I am having trouble creating a partition for set inputs. I have tried two things, the first is commented out and it is just some basic pseudo code for a partition algorithm and the second is what I interpreted from the class notes. First off the main function is fine, that was given to us. I only need to finish coding the partition function. Everything commented in the partition function is my first attempt, everything not commented is my second attempt.
Here is some inputs we are supposed to test along with the proper results:
1 2 3 4 5 6 7  pivot is 4; elements less than pivot are in the left section
8 7 6 5 4 3 2 1  pivot is 5; elements less than pivot are in the left section
5 1 4 3 6 7 8 2  pivot is 3; elements less than pivot are in the left section
4 3 1 2 7 5  pivot is 1; the left section is empty
2 6 8 4 1 7  pivot is 8; the right section is empty
int partition(int a[], int first, int last) {
/* int pivot = a[last / 2];
//cout << pivot << endl;
int i = (first - 1);
for (int j = first; j <= last; j++) {
if (a[j] < pivot) {
i++;
//cout << pivot << endl << i << endl << j << endl;
swap(a[i], a[j]);
}
}
cout << pivot << endl << i << endl << first << endl << last << endl;
//swap(a[i + 1], a[last]);
return (i + 1);
}*/
swap(a[first],a[(first+last)/2]);
int pivot = a[first];
int i = a[first];
int j = a[last];
while (i <= j)
{
if (pivot > a[i])
{i++;}
if (pivot < a[j])
{j--;}
if (i <= j)
{swap(a[i],a[j]);}
}
swap(pivot, a[j]);
return i;
}
int main()
{
int x; // number of elements
int nums[10];
cout << "How many elements would you like to enter into the array? (must
be less than 10): ";
cin >> x;
cout << "Enter the elements one per line" << endl;
for (int i = 0; i < x; i++)
{ cin >> nums[i]; }
int pivot_index = partition(nums, 0, x-1);
cout << "The array was partitioned" << endl;
// display up to the pivot without endl
for (int i = 0; i < pivot_index; i++)
cout << nums[i] << " ";
// display the pivot
cout << "| " << nums[pivot_index] << " | " ;
// display from the pivot without endl
for (int i = pivot_index+1; i < x; i++)
cout << nums[i] << " ";
cout << endl;
}