Modifying iterator inside for loop - c++

Question

I have a problem like this:
I got size of array and the array itself (all the elements) and i need to sort the array in the way that first item in array (lets call it key item) is placed so all items smaller then key item go left from key item and all items larger then key item go right form key item, but the items need to be i a same order as they was in declaration like so:
array input:
4 7 3 5 6 2 9 1 10 8
array output:
3 2 1 4 7 5 6 9 10 8
so i thought i can just go one time through array and if value is smaller then key item swap them and if value is larger then key item, place that value (item ) at the end (if there is n items in array place it on n+1 place) and then to shift all items from current index one place to the left like:
| 4 | 7 | 3 | 5 | 6 | 2 | 9 | 1 | 10 | 8 |
| 4 | | 3 | 5 | 6 | 2 | 9 | 1 | 10 | 8 | 7 |
| 4 | 3 | 5 | 6 | 2 | 9 | 1 | 10 | 8 | 7 |
but like that i would need to after every shifting make iterator stay same (so it can check that place again.
I have a code like this :
int u=1;
for (int i=1;i<n;i++){
if (x[i]<x[u]){
swap(x[i],x[u]);
u+=1;
}else {
x[i]=x[n];
for (int k=i;k<n-1;k++){x[k]=x[k+1];}
i--;
}
}
i think i made a mistake in the swapping part but i cant figure it out.

Answer 1

What you are looking for can be done using the STL's std::stable_partition() algorithm:
divides elements into two groups while preserving their relative order
Let it do the hard work for you, eg:
#include <iostream>
#include <algorithm>
struct doPartitionOn
{
int m_key;
doPartitionOn(int key) : m_key(key) {}
bool operator()(int a)
{
return (a < m_key);
}
};
int main()
{
int arr[] = {4, 7, 3, 5, 6, 2, 9, 1, 10, 8};
for(int i = 0; i < 10; ++i)
std::cout << arr[i] << ' ';
std::cout << std::endl;
std::stable_partition(arr, arr+10, doPartitionOn(arr[0]));
for(int i = 0; i < 10; ++i)
std::cout << arr[i] << ' ';
std::cout << std::endl;
std::cin.get();
return 0;
}
4 7 3 5 6 2 9 1 10 8
3 2 1 4 7 5 6 9 10 8
Live demo
Or, if using C++11 or later, you can use a lambda instead of a manual functor:
#include <iostream>
#include <algorithm>
int main()
{
int arr[] = {4, 7, 3, 5, 6, 2, 9, 1, 10, 8};
for(int value: arr)
std::cout << value << ' ';
std::cout << std::endl;
std::stable_partition(arr, arr+10, [&arr](int a){ return (a < arr[0]); });
for(int value: arr)
std::cout << value << ' ';
std::cout << std::endl;
std::cin.get();
return 0;
}
4 7 3 5 6 2 9 1 10 8
3 2 1 4 7 5 6 9 10 8
Live demo