From 65e1d62d1577447d8b5987b1b915554bb9913af9 Mon Sep 17 00:00:00 2001
From: yashladha <yashladhapankajladha123@gmail.com>
Date: Wed, 15 Mar 2017 19:11:18 +0530
Subject: [PATCH] Added Quick Select [C]

Closes https://github.com/iiitv/algos/issues/10
---
 README.md                   |  1 +
 quick_select/quick_select.c | 60 +++++++++++++++++++++++++++++++++++++
 2 files changed, 61 insertions(+)
 create mode 100644 quick_select/quick_select.c

diff --git a/README.md b/README.md
index 0539dfd1..a7e928da 100644
--- a/README.md
+++ b/README.md
@@ -14,6 +14,7 @@ Community (college) maintained list of algorithm implementations and codes.
 | [Euclidean GCD](https://en.wikipedia.org/wiki/Euclidean_algorithm) | [\[X\]](euclidean_gcd/euclidean_gcd.c) | | [\[X\]](euclidean_gcd/EuclideanGCD.java) | [\[X\]](euclidean_gcd/euclidean_gcd.py) |
 | [Linear Search](https://en.wikipedia.org/wiki/Linear_search) | | | | [\[X\]](linear_search/linear_search.py) |
 | [Longest Common Subsequence](http://www.geeksforgeeks.org/dynamic-programming-set-4-longest-common-subsequence) | [\[X\]](longest_common_subsequence/longestCommonSubsequence.c) | | | |
+| [Quick Select](https://en.wikipedia.org/wiki/Quickselect) | [\[X\]](quick_select/quick_select.c) | | | |
 | [Quicksort](https://en.wikipedia.org/wiki/Quicksort) | [\[X\]](quicksort/quicksort.c) | | | |
 
 
diff --git a/quick_select/quick_select.c b/quick_select/quick_select.c
new file mode 100644
index 00000000..ee39ee3f
--- /dev/null
+++ b/quick_select/quick_select.c
@@ -0,0 +1,60 @@
+#include <stdio.h>
+
+/*
+ * partition function
+ * ar        : array on which partitioning has to be done
+ * left      : left index of the partitioning subarray
+ * right     : right index of the partitioning subarray
+ * pivot_idx : pivot index from which partition has to done
+ * store_idx : index of the last element of the left subarray  
+ */
+int partition(int *ar, int left, int right, int pivot_idx) {
+   int pivot_value = ar[pivot_idx];
+   int temp = ar[right];
+   ar[right] = ar[pivot_idx];
+   ar[pivot_idx] = temp;
+
+   int store_idx = left;
+   while (left < right) {
+      if (ar[left] < pivot_value) {
+         temp = ar[store_idx];
+         ar[store_idx] = ar[left];
+         ar[left] = temp;
+         store_idx++;
+      }
+      left++;
+   }
+   temp = ar[right];
+   ar[right] = ar[store_idx];
+   ar[store_idx] = temp;
+   return store_idx;
+}
+
+/*
+ * Quick Select function
+ * left        : left index of the subarray
+ * right       : right index of the subarray
+ * pos         : position to find the element using quick sort
+ * pivot_index : pivot index 
+ */
+int quick_select(int *ar, int left, int right, int pos) {
+   int pivot_index;
+   if (left == right)
+      return ar[left];
+   pivot_index = right - 1;
+   pivot_index = partition(ar, left, right, pivot_index);
+   if (pos == pivot_index) {
+      return ar[pivot_index];
+   } else if (pos < pivot_index) {
+      return quick_select(ar, left, pivot_index - 1, pos);
+   } else {
+      return quick_select(ar, pivot_index + 1, right, pos);
+   }
+}
+
+int main() {
+   int ar[] = {10, 5, 1, 6, 7, 3, 2, 4, 8, 9};
+   printf("%d\n", quick_select(ar, 0, 9, 3));
+   return 0;
+}
+