golang逐行处理文件
golang 提供了package bufio。bufio.NewReader()创建一个默认大小的readbuf，当然，也可以bufio.NewReaderSize。
func NewReader(rd io.Reader) *Reader NewReader returns a new Reader whose buffer has the default size(4096). func NewReaderSize(rd io.Reader, size int) *Reader NewReaderSize returns a new Reader whose buffer has at least the specified size. If the argument io.Reader is already a Reader with large enough size, it returns the underlying Reader.
bufio
func (b *Reader) ReadByte() (c byte, err error) ReadByte reads and returns a single byte. If no byte is available, returns an error. func (b *Reader) ReadBytes(delim byte) (line []byte, err error) ReadBytes reads until the first occurrence of delim in the input, returning a slice containing the data up to and including the delimiter. If ReadBytes encounters an error before finding a delimiter, 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) ReadString(delim byte) (line string, err error) ReadString reads until the first occurrence of delim in the input, returning a string containing the data up to and including the delimiter. If ReadString encounters an error before finding a delimiter, 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.
ReadByte这个接口，和C语言中fgetc很接近，每次读取一个字节。ReadBytes和ReadString都可以实现逐行读取，只要delim设置为'\n'.
package main
import "fmt"
import "os"
import "io"
import "flag"
import "bufio"
var num_flag = flag.Bool("n",false,"num each line")
func usage(){
fmt.Printf("%s %s\n",os.Args[0],"filename")
}
func cat(r *bufio.Reader){
i := 1
for {
//buf,err := r.ReadBytes('\n')
buf,err := r.ReadString('\n')
if err == io.EOF{
break
}
if *num_flag{
fmt.Fprintf(os.Stdout,"%5d %s",
i,buf)
i++
}else{
fmt.Fprintf(os.Stdout,"%s",buf)
}
}
return
}
func main(){
flag.Parse()
if(flag.NArg() == 0){
cat(bufio.NewReader(os.Stdin))
}
for i:=0;i<flag.NArg();i++{
f,err := os.OpenFile(flag.Arg(i),os.O_RDONLY,0660)
if err != nil{
fmt.Fprintf(os.Stderr,"%s err read from %s : %s\n",
os.Args[0],flag.Arg(0),err)
continue
}
cat(bufio.NewReader(f))
f.Close()
}
}
用scaner逐行读取
func cat(scanner *bufio.Scanner) error{
for scanner.Scan(){
fmt.Println(scanner.Text())
//fmt.Fprintf(os.Stdout,"%s\n",scanner.Text())
}
return scanner.Err()
}
注意，为啥执行Scan，Text()函数就能返回下一行呢？因为默认的分割函数就是ScanLines.如你有特殊的需求来分割，func (s *Scanner) Split(split SplitFunc)
这个函数可以制定SplitFunc。你可以定制自己的分割函数。
需要注意的是，Scan会将分割符号\n去除，如果Fprintf输出的话，不添加\n打印，会出现没有换行的现象，如下所示
fmt.Fprintf(os.Stdout,"%s",scanner.Text())
manu@manu-hacks:~/code/go/self$ go run mycat_v2.go test.txt this is test file created by goif not existed ,please create this fileif existed, Please write appendhello world,hello gothis is test file created by goif not existed ,please create this fileif existed, Please write appendhello world,hello gomanu@manu-hacks:~/code/go/self$ cat test.txt this is test file created by go if not existed ,please create this file if existed, Please write append hello world,hello go this is test file created by go if not existed ,please create this file if existed, Please write append hello world,hello go
调用部分的代码如下：
f,err := os.OpenFile(flag.Arg(i),os.O_RDONLY,0660)
...
error := cat(bufio.NewScanner(f))
if err != nil{
fmt.Fprintf(os.Stderr,"%s err read from %s : %s\n",
os.Args[0],flag.Arg(i),error)
}