June 2018 Update

Task/Averages-Median/Go/averages-median-2.go

@@ -0,0 +1,31 @@
+package main
+
+import "fmt"
+
+func main() {
+    fmt.Println(median([]float64{3, 1, 4, 1}))    // prints 2
+    fmt.Println(median([]float64{3, 1, 4, 1, 5})) // prints 3
+}
+
+func median(a []float64) float64 {
+    half := len(a) / 2
+    med := sel(a, half)
+    if len(a)%2 == 0 {
+        return (med + a[half-1]) / 2
+    }
+    return med
+}
+
+func sel(list []float64, k int) float64 {
+    for i, minValue := range list[:k+1] {
+        minIndex := i
+        for j := i + 1; j < len(list); j++ {
+            if list[j] < minValue {
+                minIndex = j
+                minValue = list[j]
+                list[i], list[minIndex] = minValue, list[i]
+            }
+        }
+    }
+    return list[k]
+}

Task/Averages-Median/Go/averages-median-3.go

@@ -0,0 +1,52 @@
+package main
+
+import (
+    "fmt"
+    "math/rand"
+)
+
+func main() {
+    fmt.Println(median([]float64{3, 1, 4, 1}))    // prints 2
+    fmt.Println(median([]float64{3, 1, 4, 1, 5})) // prints 3
+}
+
+func median(list []float64) float64 {
+    half := len(list) / 2
+    med := qsel(list, half)
+    if len(list)%2 == 0 {
+        return (med + qsel(list, half-1)) / 2
+    }
+    return med
+}
+
+func qsel(a []float64, k int) float64 {
+    for len(a) > 1 {
+        px := rand.Intn(len(a))
+        pv := a[px]
+        last := len(a) - 1
+        a[px], a[last] = a[last], pv
+        px = 0
+        for i, v := range a[:last] {
+            if v < pv {
+                a[px], a[i] = v, a[px]
+                px++
+            }
+        }
+        if px == k {
+            return pv
+        }
+        if k < px {
+            a = a[:px]
+        } else {
+            // swap elements.  simply assigning a[last] would be enough to
+            // allow qsel to return the correct result but it would leave slice
+            // "a" unusable for subsequent use.  we want this full swap so that
+            // we can make two successive qsel calls in the case of median
+            // of an even number of elements.
+            a[px], a[last] = pv, a[px]
+            a = a[px+1:]
+            k -= px + 1
+        }
+    }
+    return a[0]
+}