update book code

eBook/exercises/chapter_14/concurrent_pi.go

@@ -0,0 +1,45 @@
+// Concurrent computation of pi.
+// See http://goo.gl/ZuTZM. - Comparison with Scala!
+//
+// This demonstrates Go's ability to handle
+// large numbers of concurrent processes.
+// It is an unreasonable way to calculate pi.
+package main
+
+import (
+	"fmt"
+	"math"
+	"time"
+)
+
+func main() {
+	start := time.Now()
+	fmt.Println(CalculatePi(5000))
+	end := time.Now()
+	delta := end.Sub(start)
+	fmt.Printf("longCalculation took this amount of time: %s\n", delta) 
+}
+
+// CalculatePi launches n goroutines to compute an
+// series-approximation of pi.
+func CalculatePi(n int) float64 {
+	ch := make(chan float64)
+	for k := 0; k <= n; k++ {
+		// calculate k-th term in the series
+		go term(ch, float64(k))
+	}
+	f := 0.0
+	//wait for all goroutines to complete, get and sum up their results:
+	for k := 0; k <= n; k++ {
+		f += <-ch
+	}
+	return f
+}
+
+func term(ch chan float64, k float64) {
+	ch <- 4 * math.Pow(-1, k) / (2*k + 1)
+}
+/* Output:
+3.14179261359579
+The calculation took this amount of time: 0.028002
+*/