## Abstract

This article aims to record how to code for quick sort algorithm with C++. Although a sort function has been provided in STL library, it is still useful to understand how to achieve the sort algorithm with basic loops.

## Principle

To understand the quick sort easily, visit the website https://visualgo.net/.

## C++ Code

```
#include <iostream>
#include <vector>
#include <stdio.h>
using namespace std;
void quick_sort(vector<int> *serial, int left_pos, int right_pos);
int main(int argc, char* argv[])
{
vector<int> serial;
int temp = 0;
//Hint sentence to ask user input numbers.
cout << "Please input numbers with space to seperate and finished with any non-number character:" << endl;
//Use a vector as dynamic array to store numbers.
while(cin >> temp) serial.push_back(temp);
//Call quick_sort function.
quick_sort(&serial, 0, serial.size()-1);
//Print the result.
cout << endl << "From min to max:" << endl;
for(int i = 0; i < serial.size(); i++)
cout << serial[i] << " ";
cout << endl;
return 0;
}
void quick_sort(vector<int> *serial, int left, int right)
{
int temp = 0, base, base_pos;
int left_pos, right_pos;
//Flag and condition for recursive finished.
if(left > right) return;
//The first number on the left is the base number by default.
base = (*serial)[left];
base_pos = left;
//Specifies the numbers at both ends of this sort.
left_pos = left;
right_pos = right;
//When the left and right are not search to the same number.
while(left_pos != right_pos)
{
//Search from right one by one until the searched number < base number.
while(((*serial)[right_pos] >= base) && (left_pos < right_pos)) right_pos--;
//Search from left one by one until the searched number > base number.
while(((*serial)[left_pos] <= base) && (left_pos < right_pos)) left_pos++;
//Swap the two searched number.
if(left_pos != right_pos)
{
temp = (*serial)[left_pos];
(*serial)[left_pos] = (*serial)[right_pos];
(*serial)[right_pos] = temp;
}
}
/* When it become the same number for searching from both sides, swap the current
number and the base number to confirm the position of base number. */
(*serial)[base_pos] = (*serial)[left_pos];
(*serial)[left_pos] = base;
//Recursive to continue sorting for the left and right parts.
quick_sort(serial, left, left_pos-1);
quick_sort(serial, left_pos+1, right);
}
```