math/big: refine Fibonacci example

Change-Id: Id9e8c3f89e021b9f389ab3c8403e6a8450fa9f5f
Reviewed-on: https://go-review.googlesource.com/11231
Reviewed-by: Robert Griesemer <gri@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>

diff --git a/src/math/big/example_test.go b/src/math/big/example_test.go
index 8cbd379b92810a16a20f6c04c43a10d8bebf6a5b..ac7955219d71af73b832841c7983fb0b9a265543 100644 (file)
--- a/src/math/big/example_test.go
+++ b/src/math/big/example_test.go
@@ -51,34 +51,30 @@ func ExampleInt_Scan() {
        // Output: 18446744073709551617
 }
 
-// Example_fibonacci demonstrates how to use big.Int to compute the smallest
-// Fibonacci number with 100 decimal digits, and find out whether it is prime.
+// This example demonstrates how to use big.Int to compute the smallest
+// Fibonacci number with 100 decimal digits and to test whether it is prime.
 func Example_fibonacci() {
-       // create and initialize big.Ints from int64s
-       fib1 := big.NewInt(0)
-       fib2 := big.NewInt(1)
+       // Initialize two big ints with the first two numbers in the sequence.
+       a := big.NewInt(0)
+       b := big.NewInt(1)
 
-       // initialize limit as 10^99 (the smallest integer with 100 digits)
+       // Initialize limit as 10^99, the smallest integer with 100 digits.
        var limit big.Int
        limit.Exp(big.NewInt(10), big.NewInt(99), nil)
 
-       // loop while fib1 is smaller than 1e100
-       for fib1.Cmp(&limit) < 0 {
-               // Compute the next Fibonacci number:
-               //    t1 := fib2
-               //    t2 := fib1.Add(fib1, fib2) // Note that Add "assigns" to fib1!
-               //    fib1 = t1
-               //    fib2 = t2
-               // Using Go's multi-value ("parallel") assignment, we can simply write:
-               fib1, fib2 = fib2, fib1.Add(fib1, fib2)
+       // Loop while a is smaller than 1e100.
+       for a.Cmp(&limit) < 0 {
+               // Compute the next Fibonacci number, storing it in a.
+               a.Add(a, b)
+               // Swap a and b so that b is the next number in the sequence.
+               a, b = b, a
        }
+       fmt.Println(a) // 100-digit Fibonacci number
 
-       fmt.Println(fib1) // 100-digit Fibonacci number
-
-       // Test fib1 for primality. The ProbablyPrimes parameter sets the number
-       // of Miller-Rabin rounds to be performed. 20 is a good value.
-       isPrime := fib1.ProbablyPrime(20)
-       fmt.Println(isPrime)
+       // Test a for primality.
+       // (ProbablyPrimes' argument sets the number of Miller-Rabin
+       // rounds to be performed. 20 is a good value.)
+       fmt.Println(a.ProbablyPrime(20))
 
        // Output:
        // 1344719667586153181419716641724567886890850696275767987106294472017884974410332069524504824747437757