math/rand: mention Seed in overview, add another top-level example

Fixes #5937.

R=golang-dev, mirtchovski, r
CC=golang-dev
https://golang.org/cl/11705043

src/pkg/math/rand/example_test.go

@@ -11,12 +11,40 @@ import (
 	"text/tabwriter"
 )
 
-// This test serves as an example but also makes sure we don't change
+// These tests serve as an example but also make sure we don't change
 // the output of the random number generator when given a fixed seed.
 
+func Example() {
+	rand.Seed(42) // Try changing this number!
+	answers := []string{
+		"It is certain",
+		"It is decidedly so",
+		"Without a doubt",
+		"Yes definitely",
+		"You may rely on it",
+		"As I see it yes",
+		"Most likely",
+		"Outlook good",
+		"Yes",
+		"Signs point to yes",
+		"Reply hazy try again",
+		"Ask again later",
+		"Better not tell you now",
+		"Cannot predict now",
+		"Concentrate and ask again",
+		"Don't count on it",
+		"My reply is no",
+		"My sources say no",
+		"Outlook not so good",
+		"Very doubtful",
+	}
+	fmt.Println("Magic 8-Ball says:", answers[rand.Intn(len(answers))])
+	// Output: Magic 8-Ball says: As I see it yes
+}
+
 // 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() {
+func Example_rand() {
 	// 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.

src/pkg/math/rand/rand.go

@@ -3,6 +3,11 @@
 // license that can be found in the LICENSE file.
 
 // Package rand implements pseudo-random number generators.
+//
+// Random numbers are generated by a Source. Top-level functions, such as
+// Float64 and Int, use a default shared Source that produces a deterministic
+// sequence of values each time a program is run. Use the Seed function to
+// initialize the default Source if different behavior is required for each run.
 package rand
 
 import "sync"
@@ -113,47 +118,57 @@ func (r *Rand) Perm(n int) []int {
 
 var globalRand = New(&lockedSource{src: NewSource(1)})
 
-// Seed uses the provided seed value to initialize the generator to a
+// Seed uses the provided seed value to initialize the default Source to a
 // deterministic state. If Seed is not called, the generator behaves as
 // if seeded by Seed(1).
 func Seed(seed int64) { globalRand.Seed(seed) }
 
-// Int63 returns a non-negative pseudo-random 63-bit integer as an int64.
+// Int63 returns a non-negative pseudo-random 63-bit integer as an int64
+// from the default Source.
 func Int63() int64 { return globalRand.Int63() }
 
-// Uint32 returns a pseudo-random 32-bit value as a uint32.
+// Uint32 returns a pseudo-random 32-bit value as a uint32
+// from the default Source.
 func Uint32() uint32 { return globalRand.Uint32() }
 
-// Int31 returns a non-negative pseudo-random 31-bit integer as an int32.
+// Int31 returns a non-negative pseudo-random 31-bit integer as an int32
+// from the default Source.
 func Int31() int32 { return globalRand.Int31() }
 
-// Int returns a non-negative pseudo-random int.
+// Int returns a non-negative pseudo-random int from the default Source.
 func Int() int { return globalRand.Int() }
 
-// Int63n returns, as an int64, a non-negative pseudo-random number in [0,n).
+// Int63n returns, as an int64, a non-negative pseudo-random number in [0,n)
+// from the default Source.
 // It panics if n <= 0.
 func Int63n(n int64) int64 { return globalRand.Int63n(n) }
 
-// Int31n returns, as an int32, a non-negative pseudo-random number in [0,n).
+// Int31n returns, as an int32, a non-negative pseudo-random number in [0,n)
+// from the default Source.
 // It panics if n <= 0.
 func Int31n(n int32) int32 { return globalRand.Int31n(n) }
 
-// Intn returns, as an int, a non-negative pseudo-random number in [0,n).
+// Intn returns, as an int, a non-negative pseudo-random number in [0,n)
+// from the default Source.
 // It panics if n <= 0.
 func Intn(n int) int { return globalRand.Intn(n) }
 
-// Float64 returns, as a float64, a pseudo-random number in [0.0,1.0).
+// Float64 returns, as a float64, a pseudo-random number in [0.0,1.0)
+// from the default Source.
 func Float64() float64 { return globalRand.Float64() }
 
-// Float32 returns, as a float32, a pseudo-random number in [0.0,1.0).
+// Float32 returns, as a float32, a pseudo-random number in [0.0,1.0)
+// from the default Source.
 func Float32() float32 { return globalRand.Float32() }
 
-// Perm returns, as a slice of n ints, a pseudo-random permutation of the integers [0,n).
+// Perm returns, as a slice of n ints, a pseudo-random permutation of the integers [0,n)
+// from the default Source.
 func Perm(n int) []int { return globalRand.Perm(n) }
 
 // NormFloat64 returns a normally distributed float64 in the range
 // [-math.MaxFloat64, +math.MaxFloat64] with
-// standard normal distribution (mean = 0, stddev = 1).
+// standard normal distribution (mean = 0, stddev = 1)
+// from the default Source.
 // To produce a different normal distribution, callers can
 // adjust the output using:
 //
@@ -163,7 +178,7 @@ func NormFloat64() float64 { return globalRand.NormFloat64() }
 
 // ExpFloat64 returns an exponentially distributed float64 in the range
 // (0, +math.MaxFloat64] with an exponential distribution whose rate parameter
-// (lambda) is 1 and whose mean is 1/lambda (1).
+// (lambda) is 1 and whose mean is 1/lambda (1) from the default Source.
 // To produce a distribution with a different rate parameter,
 // callers can adjust the output using:
 //