bufio: add examples for Scanner

Mention Scanner in docs for ReadLine etc.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/7375045

src/pkg/bufio/bufio.go

@@ -278,7 +278,7 @@ func (b *Reader) ReadSlice(delim byte) (line []byte, err error) {
 }
 
 // ReadLine is a low-level line-reading primitive. Most callers should use
-// ReadBytes('\n') or ReadString('\n') instead.
+// ReadBytes('\n') or ReadString('\n') instead or use a Scanner.
 //
 // ReadLine tries to return a single line, not including the end-of-line bytes.
 // If the line was too long for the buffer then isPrefix is set and the
@@ -331,6 +331,7 @@ func (b *Reader) ReadLine() (line []byte, isPrefix bool, err error) {
 // it returns the data read before the error and the error itself (often io.EOF).
 // ReadBytes returns err != nil if and only if the returned data does not end in
 // delim.
+// For simple uses, a Scanner may be more convenient.
 func (b *Reader) ReadBytes(delim byte) (line []byte, err error) {
 	// Use ReadSlice to look for array,
 	// accumulating full buffers.
@@ -378,6 +379,7 @@ func (b *Reader) ReadBytes(delim byte) (line []byte, err error) {
 // it returns the data read before the error and the error itself (often io.EOF).
 // ReadString returns err != nil if and only if the returned data does not end in
 // delim.
+// For simple uses, a Scanner may be more convenient.
 func (b *Reader) ReadString(delim byte) (line string, err error) {
 	bytes, err := b.ReadBytes(delim)
 	return string(bytes), err

src/pkg/bufio/example_test.go

@@ -0,0 +1,74 @@
+// Copyright 2013 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 bufio_test
+
+import (
+	"bufio"
+	"fmt"
+	"os"
+	"strconv"
+	"strings"
+)
+
+// The simplest use of a Scanner, to read standard input as a set of lines.
+func ExampleScanner_lines() {
+	scanner := bufio.NewScanner(os.Stdin)
+	for scanner.Scan() {
+		fmt.Println(scanner.Text()) // Println will add back the final '\n'
+	}
+	if err := scanner.Err(); err != nil {
+		fmt.Fprintln(os.Stdout, "reading standard input:", err)
+	}
+}
+
+// Use a Scanner to implement a simple word-count utility by scanning the
+// input as a sequence of space-delimited tokens.
+func ExampleScanner_words() {
+	// An artificial input source.
+	const input = "Now is the winter of our discontent,\nMade glorious summer by this sun of York.\n"
+	scanner := bufio.NewScanner(strings.NewReader(input))
+	// Set the split function for the scanning operation.
+	scanner.Split(bufio.ScanWords)
+	// Count the words.
+	count := 0
+	for scanner.Scan() {
+		count++
+	}
+	if err := scanner.Err(); err != nil {
+		fmt.Fprintln(os.Stdout, "reading input:", err)
+	}
+	fmt.Printf("%d\n", count)
+	// Output: 15
+}
+
+// Use a Scanner with a custom split function (built by wrapping ScanWords) to validate
+// 32-bit decimal input.
+func ExampleScanner_custom() {
+	// An artificial input source.
+	const input = "1234 5678 1234567901234567890"
+	scanner := bufio.NewScanner(strings.NewReader(input))
+	// Create a custom split function by wrapping the existing ScanWords function.
+	split := func(data []byte, atEOF bool) (advance int, token []byte, err error) {
+		advance, token, err = bufio.ScanWords(data, atEOF)
+		if err == nil && token != nil {
+			_, err = strconv.ParseInt(string(token), 10, 32)
+		}
+		return
+	}
+	// Set the split function for the scanning operation.
+	scanner.Split(split)
+	// Validate the input
+	for scanner.Scan() {
+		fmt.Printf("%s\n", scanner.Text())
+	}
+
+	if err := scanner.Err(); err != nil {
+		fmt.Printf("Invalid input: %s", err)
+	}
+	// Output:
+	// 1234
+	// 5678
+	// Invalid input: strconv.ParseInt: parsing "1234567901234567890": value out of range
+}