add SelectionSamplingExample.swift

Author: Moon1102

Selection Sampling/SelectionSamplingExample.swift

@@ -0,0 +1,32 @@
+//
+//  Select.swift
+//  
+//
+//  Created by Cheer on 2017/3/9.
+//
+//
+
+import Foundation
+
+func select<T>(from a: [T], count requested: Int) -> [T] {
+    
+    // if requested > a.count, will throw ==> fatal error: Index out of range
+    
+    if requested < a.count {
+        var arr = a
+        for i in 0..<requested {
+            
+            //arc4random_uniform(n) ==> the range is [0...n-1]
+            //So: arc4random_uniform(UInt32(arr.count - i - 2)) + (i + 1) ==> [0...arr.count - (i + 1) - 1] + (i + 1) ==> the range is [(i + 1)...arr.count - 1]
+            //Why start index is "(i + 1)"?  Because in "swap(&A,&B)", if A's index == B's index, will throw a error: "swapping a ___location with itself is not supported"
+            
+            let nextIndex = Int(arc4random_uniform(UInt32(arr.count - i - 2))) + (i + 1)
+            swap(&arr[nextIndex], &arr[i])
+        }
+        return Array(arr[0..<requested])
+    }
+    
+    if requested == a.count { return a }
+    
+    return [T]()
+}