Add all the A tasks

Task/Averages-Arithmetic-mean/Go/averages-arithmetic-mean.go

@@ -0,0 +1,65 @@
+package main
+
+import (
+    "fmt"
+    "math"
+)
+
+func mean(v []float64) (m float64, ok bool) {
+    if len(v) == 0 {
+        return
+    }
+    // an algorithm that attempts to retain accuracy
+    // with widely different values.
+    var parts []float64
+    for _, x := range v {
+        var i int
+        for _, p := range parts {
+            sum := p + x
+            var err float64
+            switch ax, ap := math.Abs(x), math.Abs(p); {
+            case ax < ap:
+                err = x - (sum - p)
+            case ap < ax:
+                err = p - (sum - x)
+            }
+            if err != 0 {
+                parts[i] = err
+                i++
+            }
+            x = sum
+        }
+        parts = append(parts[:i], x)
+    }
+    var sum float64
+    for _, x := range parts {
+        sum += x
+    }
+    return sum / float64(len(v)), true
+}
+
+func main() {
+    for _, v := range [][]float64{
+        []float64{},                         // mean returns ok = false
+        []float64{math.Inf(1), math.Inf(1)}, // answer is +Inf
+
+        // answer is NaN, and mean returns ok = true, indicating NaN
+        // is the correct result
+        []float64{math.Inf(1), math.Inf(-1)},
+
+        []float64{3, 1, 4, 1, 5, 9},
+
+        // large magnitude numbers cancel. answer is mean of small numbers.
+        []float64{1e20, 3, 1, 4, 1, 5, 9, -1e20},
+
+        []float64{10, 9, 8, 7, 6, 5, 4, 3, 2, 1, 0, 0, 0, 0, .11},
+        []float64{10, 20, 30, 40, 50, -100, 4.7, -11e2},
+    } {
+        fmt.Println("Vector:", v)
+        if m, ok := mean(v); ok {
+            fmt.Printf("Mean of %d numbers is %g\n\n", len(v), m)
+        } else {
+            fmt.Println("Mean undefined\n")
+        }
+    }
+}