###
**What is Quick Select Algorithm? How to implement in C++?**

- The QuickSelect algorithm quickly finds the k-th smallest element of an unsorted array of n elements.
- It is an
*O*(*n*), worst-case linear time, selection algorithm. A typical selection by sorting method would need atleast O(*n* log *n*) time.
- This algorithm is identical to quick sort but it does only a partial sort, since we already know which partition our desired element lies as the pivot is in final sorted position.

###
Quick select implementation in C++

```
#include <iostream>
using namespace std;
// A simple print function
void print(int *input)
{
for ( int i = 0; i < 5; i++ )
cout << input[i] << " ";
cout << endl;
}
int partition(int* input, int p, int r)
{
int pivot = input[r];
while ( p < r )
{
while ( input[p] < pivot )
p++;
while ( input[r] > pivot )
r--;
if ( input[p] == input[r] )
p++;
else if ( p < r ) {
int tmp = input[p];
input[p] = input[r];
input[r] = tmp;
}
}
return r;
}
int quick_select(int* input, int p, int r, int k)
{
if ( p == r ) return input[p];
int j = partition(input, p, r);
int length = j - p + 1;
if ( length == k ) return input[j];
else if ( k < length ) return quick_select(input, p, j - 1, k);
else return quick_select(input, j + 1, r, k - length);
}
int main()
{
int A1[] = { 100, 400, 300, 500, 200 };
cout << "1st order element " << quick_select(A1, 0, 4, 1) << endl;
int A2[] = { 100, 400, 300, 500, 200 };
cout << "2nd order element " << quick_select(A2, 0, 4, 2) << endl;
int A3[] = { 100, 400, 300, 500, 200 };
cout << "3rd order element " << quick_select(A3, 0, 4, 3) << endl;
int A4[] = { 100, 400, 300, 500, 200 };
cout << "4th order element " << quick_select(A4, 0, 4, 4) << endl;
int A5[] = { 100, 400, 300, 500, 200 };
cout << "5th order element " << quick_select(A5, 0, 4, 5) << endl;
}
```

OUTPUT:-

1st order element 100
2nd order element 200
3rd order element 300
4th order element 400
5th order element 500

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ReplyDeleteConcise and informational!!!

ReplyDeleteGood One. But I think if you choose the pivot from the last element of the array, it would not be a linearized algorithm. Instead I think you should do a randomized partition where you choose a random pivot and track where it ends up after you partition and then do recursion. Even by using Randomized method, the worst case is O(n2). http://pine.cs.yale.edu/pinewiki/QuickSelect

ReplyDeleteTo minimize the worst case complexity, median of medians is used to choose the pivot.

http://www.ics.uci.edu/~eppstein/161/960130.html

good and informative post. very nice and well organie. thanks for sharing.

ReplyDeletehelpful for me....

the time complexity is not O(n).

ReplyDeletehow to implement it in c please help i m newcommer to programming

ReplyDeleteall this on the condition there is no multiple values...

ReplyDeletei Think the time complexity is not O(n)

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