math/rand: add example / regression test

This makes sure the outputs do not change for a fixed seed.
See also https://golang.org/cl/6905049.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/6907048

src/pkg/math/rand/example_test.go

+// Copyright 2012 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package rand_test
+
+import (
+	"fmt"
+	"math/rand"
+	"os"
+	"text/tabwriter"
+)
+
+// This test serves as an example but also makes sure we don't change
+// the output of the random number generator when given a fixed seed.
+
+// This example shows the use of each of the methods on a *Rand.
+// The use of the global functions is the same, without the receiver.
+func Example() {
+	// Create and seed the generator.
+	// Typically a non-fixed seed should be used, such as time.Now().UnixNano().
+	// Using a fixed seed will produce the same output on every run.
+	r := rand.New(rand.NewSource(99))
+
+	// The tabwriter here helps us generate aligned output.
+	w := tabwriter.NewWriter(os.Stdout, 1, 1, 1, ' ', 0)
+	defer w.Flush()
+	show := func(name string, v1, v2, v3 interface{}) {
+		fmt.Fprintf(w, "%s\t%v\t%v\t%v\n", name, v1, v2, v3)
+	}
+
+	// Float32 and Float64 values are in [0, 1).
+	show("Float32", r.Float32(), r.Float32(), r.Float32())
+	show("Float64", r.Float64(), r.Float64(), r.Float64())
+
+	// ExpFloat64 values have an average of 1 but decay exponentially.
+	show("ExpFloat64", r.ExpFloat64(), r.ExpFloat64(), r.ExpFloat64())
+
+	// NormFloat64 values have an average of 0 and a standard deviation of 1.
+	show("NormFloat64", r.NormFloat64(), r.NormFloat64(), r.NormFloat64())
+
+	// Int31, Int63, and Uint32 generate values of the given width.
+	// The Int method (not shown) is like either Int31 or Int64
+	// depending on the size of 'int'.
+	show("Int31", r.Int31(), r.Int31(), r.Int31())
+	show("Int63", r.Int63(), r.Int63(), r.Int63())
+	show("Uint32", r.Int63(), r.Int63(), r.Int63())
+
+	// Intn, Int31n, and Int63n limit their output to be < n.
+	// They do so more carefully than using r.Int()%n.
+	show("Intn(10)", r.Intn(10), r.Intn(10), r.Intn(10))
+	show("Int31n(10)", r.Int31n(10), r.Int31n(10), r.Int31n(10))
+	show("Int63n(10)", r.Int63n(10), r.Int63n(10), r.Int63n(10))
+
+	// Perm generates a random permutation of the numbers [0, n).
+	show("Perm", r.Perm(5), r.Perm(5), r.Perm(5))
+	// Output:
+	// Float32     0.2635776           0.6358173           0.6718283
+	// Float64     0.628605430454327   0.4504798828572669  0.9562755949377957
+	// ExpFloat64  0.3362240648200941  1.4256072328483647  0.24354758816173044
+	// NormFloat64 0.17233959114940064 1.577014951434847   0.04259129641113857
+	// Int31       1501292890          1486668269          182840835
+	// Int63       3546343826724305832 5724354148158589552 5239846799706671610
+	// Uint32      5927547564735367388 637072299495207830  4128311955958246186
+	// Intn(10)    1                   2                   5
+	// Int31n(10)  4                   7                   8
+	// Int63n(10)  7                   6                   3
+	// Perm        [1 4 2 3 0]         [4 2 1 3 0]         [1 2 4 0 3]
+}