Split an array at a specific value C++ - c++

Question

Say I have an array like this:
int arr [9] = {2,1,5,8,9,4,10,15,20}
How can you split the array at a certain value threshold? So say int 8 is our splitting value, the end result would be two separate arrays (or a 2d array if you want to give that a shot) that in this example would be arr1 [4] = {1,2,4,5} and arr2 [5] = {8,9,10,15,20}. arr1 stores all the values in arr that are below 8 and and arr2 stores all the values in arr that are 8 and above.
I haven't been able to locate sufficient documentation or examples of this being done and I think array manipulation and splitting is worth having examples of.

Answer 1

Use std::partition, or if you want to maintain the relative order and not sort the data, std::stable_partition.
#include <algorithm>
#include <iostream>
#include <vector>
int main()
{
int pivot = 8;
int arr [9] = {2,1,5,8,9,4,10,15,20};
// get partition point
int *pt = std::stable_partition(arr, std::end(arr), [&](int n) {return n < pivot;});
// create two vectors consisting of left and right hand side
// of partition
std::vector<int> a1(arr, pt);
std::vector<int> a2(pt, std::end(arr));
// output results
for (auto& i : a1)
std::cout << i << " ";
std::cout << '\n';
for (auto& i : a2)
std::cout << i << " ";
}
Live Example

Answer 2

If you can use C++11 then this is one way of using the standard library:
Using a partition_point: (edited the example from the link)
#include <algorithm>
#include <array>
#include <iostream>
#include <iterator>
#include <vector>
int main()
{
std::array<int, 9> v = {2,1,5,8,9,4,10,15,20};
auto is_lower_than_8 = [](int i){ return i < 8; };
std::partition(v.begin(), v.end(), is_lower_than_8 );
auto p = std::partition_point(v.begin(), v.end(), is_lower_than_8 );
std::cout << "Before partition:\n ";
std::vector<int> p1(v.begin(), p);
std::sort(p1.begin(), p1.end());
std::copy(p1.begin(), p1.end(), std::ostream_iterator<int>(std::cout, " "));
std::cout << "\nAfter partition:\n ";
std::vector<int> p2(p, v.end());
std::sort(p2.begin(), p2.end());
std::copy(p2.begin(), p2.end(), std::ostream_iterator<int>(std::cout, " "));
}
Which prints:
Before partition:
1 2 4 5
After partition:
8 9 10 15 20

Answer 3

I'm working on a solution with loops. This is a work in progress. Let me know what you think.
void splitarr(int arr[], int length) {
int accu = 0;
int accu2 = 0;
int splitter = rand() % 20;
for (int i = 0; i < length; i++) {
if (i != splitter) {
accu++;
}
}
int arr1[accu];
for (int i = 0; i < length; i++) {
if (i != splitter) {
arr1[i] = i;
}
}
for (int i = 0; i < length; i++) {
if (i == splitter) {
accu2++;
}
}
int arr2[accu2];
for (int i = 0; i < length; i++) {
if (i == splitter) {
arr2[i] = i;
}
}
}