Use:
reader.ReadString('\n')
\n at the end of the string returned.reader.ReadLine()
I tested the various solutions suggested by writing a program to test the scenarios which are identified as problems in other answers:
I found that:
Scanner solution does not handle long lines.ReadLine solution is complex to implement.ReadString solution is the simplest and works for long lines.Here is code which demonstrates each solution, it can be run via go run main.go, or at https://play.golang.org/p/RAW3sGblbas
package main
import (
"bufio"
"bytes"
"fmt"
"io"
"os"
)
func readFileWithReadString(fn string) (err error) {
fmt.Println("readFileWithReadString")
file, err := os.Open(fn)
if err != nil {
return err
}
defer file.Close()
// Start reading from the file with a reader.
reader := bufio.NewReader(file)
var line string
for {
line, err = reader.ReadString('\n')
if err != nil && err != io.EOF {
break
}
// Process the line here.
fmt.Printf(" > Read %d characters\n", len(line))
fmt.Printf(" > > %s\n", limitLength(line, 50))
if err != nil {
break
}
}
if err != io.EOF {
fmt.Printf(" > Failed with error: %v\n", err)
return err
}
return
}
func readFileWithScanner(fn string) (err error) {
fmt.Println("readFileWithScanner (scanner fails with long lines)")
// Don't use this, it doesn't work with long lines...
file, err := os.Open(fn)
if err != nil {
return err
}
defer file.Close()
// Start reading from the file using a scanner.
scanner := bufio.NewScanner(file)
for scanner.Scan() {
line := scanner.Text()
// Process the line here.
fmt.Printf(" > Read %d characters\n", len(line))
fmt.Printf(" > > %s\n", limitLength(line, 50))
}
if scanner.Err() != nil {
fmt.Printf(" > Failed with error %v\n", scanner.Err())
return scanner.Err()
}
return
}
func readFileWithReadLine(fn string) (err error) {
fmt.Println("readFileWithReadLine")
file, err := os.Open(fn)
if err != nil {
return err
}
defer file.Close()
// Start reading from the file with a reader.
reader := bufio.NewReader(file)
for {
var buffer bytes.Buffer
var l []byte
var isPrefix bool
for {
l, isPrefix, err = reader.ReadLine()
buffer.Write(l)
// If we've reached the end of the line, stop reading.
if !isPrefix {
break
}
// If we're at the EOF, break.
if err != nil {
if err != io.EOF {
return err
}
break
}
}
line := buffer.String()
// Process the line here.
fmt.Printf(" > Read %d characters\n", len(line))
fmt.Printf(" > > %s\n", limitLength(line, 50))
if err == io.EOF {
break
}
}
if err != io.EOF {
fmt.Printf(" > Failed with error: %v\n", err)
return err
}
return
}
func main() {
testLongLines()
testLinesThatDoNotFinishWithALinebreak()
}
func testLongLines() {
fmt.Println("Long lines")
fmt.Println()
createFileWithLongLine("longline.txt")
readFileWithReadString("longline.txt")
fmt.Println()
readFileWithScanner("longline.txt")
fmt.Println()
readFileWithReadLine("longline.txt")
fmt.Println()
}
func testLinesThatDoNotFinishWithALinebreak() {
fmt.Println("No linebreak")
fmt.Println()
createFileThatDoesNotEndWithALineBreak("nolinebreak.txt")
readFileWithReadString("nolinebreak.txt")
fmt.Println()
readFileWithScanner("nolinebreak.txt")
fmt.Println()
readFileWithReadLine("nolinebreak.txt")
fmt.Println()
}
func createFileThatDoesNotEndWithALineBreak(fn string) (err error) {
file, err := os.Create(fn)
if err != nil {
return err
}
defer file.Close()
w := bufio.NewWriter(file)
w.WriteString("Does not end with linebreak.")
w.Flush()
return
}
func createFileWithLongLine(fn string) (err error) {
file, err := os.Create(fn)
if err != nil {
return err
}
defer file.Close()
w := bufio.NewWriter(file)
fs := 1024 * 1024 * 4 // 4MB
// Create a 4MB long line consisting of the letter a.
for i := 0; i < fs; i++ {
w.WriteRune('a')
}
// Terminate the line with a break.
w.WriteRune('\n')
// Put in a second line, which doesn't have a linebreak.
w.WriteString("Second line.")
w.Flush()
return
}
func limitLength(s string, length int) string {
if len(s) < length {
return s
}
return s[:length]
}
I tested on:
The test program outputs:
Long lines
readFileWithReadString
> Read 4194305 characters
> > aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
> Read 12 characters
> > Second line.
readFileWithScanner (scanner fails with long lines)
> Failed with error bufio.Scanner: token too long
readFileWithReadLine
> Read 4194304 characters
> > aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa
> Read 12 characters
> > Second line.
> Read 0 characters
> >
No linebreak
readFileWithReadString
> Read 28 characters
> > Does not end with linebreak.
readFileWithScanner (scanner fails with long lines)
> Read 28 characters
> > Does not end with linebreak.
readFileWithReadLine
> Read 28 characters
> > Does not end with linebreak.
> Read 0 characters
> >
Another method is to use the io/ioutil and strings libraries to read the entire file's bytes, convert them into a string and split them using a "\n" (newline) character as the delimiter, for example:
import (
"io/ioutil"
"strings"
)
func main() {
bytesRead, _ := ioutil.ReadFile("something.txt")
file_content := string(bytesRead)
lines := strings.Split(file_content, "\n")
}
Technically you're not reading the file line-by-line, however you are able to parse each line using this technique. This method is applicable to smaller files. If you're attempting to parse a massive file use one of the techniques that reads line-by-line.
Example from this gist
func readLine(path string) {
inFile, err := os.Open(path)
if err != nil {
fmt.Println(err.Error() + `: ` + path)
return
}
defer inFile.Close()
scanner := bufio.NewScanner(inFile)
for scanner.Scan() {
fmt.Println(scanner.Text()) // the line
}
}
but this gives an error when there is a line that larger than Scanner's buffer.
When that happened, what I do is use reader := bufio.NewReader(inFile) create and concat my own buffer either using ch, err := reader.ReadByte() or len, err := reader.Read(myBuffer)
Another way that I use (replace os.Stdin with file like above), this one concats when lines are long (isPrefix) and ignores empty lines:
func readLines() []string {
r := bufio.NewReader(os.Stdin)
bytes := []byte{}
lines := []string{}
for {
line, isPrefix, err := r.ReadLine()
if err != nil {
break
}
bytes = append(bytes, line...)
if !isPrefix {
str := strings.TrimSpace(string(bytes))
if len(str) > 0 {
lines = append(lines, str)
bytes = []byte{}
}
}
}
if len(bytes) > 0 {
lines = append(lines, string(bytes))
}
return lines
}
In the code bellow, I read the interests from the CLI until the user hits enter and I'm using Readline:
interests := make([]string, 1)
r := bufio.NewReader(os.Stdin)
for true {
fmt.Print("Give me an interest:")
t, _, _ := r.ReadLine()
interests = append(interests, string(t))
if len(t) == 0 {
break;
}
}
fmt.Println(interests)
You can also use ReadString with \n as a separator:
f, err := os.Open(filename)
if err != nil {
fmt.Println("error opening file ", err)
os.Exit(1)
}
defer f.Close()
r := bufio.NewReader(f)
for {
path, err := r.ReadString(10) // 0x0A separator = newline
if err == io.EOF {
// do something here
break
} else if err != nil {
return err // if you return error
}
}
I wrote up a way to easily read each line from a file. The Readln(*bufio.Reader) function returns a line (sans \n) from the underlying bufio.Reader struct.
// Readln returns a single line (without the ending \n)
// from the input buffered reader.
// An error is returned iff there is an error with the
// buffered reader.
func Readln(r *bufio.Reader) (string, error) {
var (isPrefix bool = true
err error = nil
line, ln []byte
)
for isPrefix && err == nil {
line, isPrefix, err = r.ReadLine()
ln = append(ln, line...)
}
return string(ln),err
}
You can use Readln to read every line from a file. The following code reads every line in a file and outputs each line to stdout.
f, err := os.Open(fi)
if err != nil {
fmt.Printf("error opening file: %v\n",err)
os.Exit(1)
}
r := bufio.NewReader(f)
s, e := Readln(r)
for e == nil {
fmt.Println(s)
s,e = Readln(r)
}
Cheers!
In Go 1.1 and newer the most simple way to do this is with a bufio.Scanner. Here is a simple example that reads lines from a file:
package main
import (
"bufio"
"fmt"
"log"
"os"
)
func main() {
file, err := os.Open("/path/to/file.txt")
if err != nil {
log.Fatal(err)
}
defer file.Close()
scanner := bufio.NewScanner(file)
for scanner.Scan() {
fmt.Println(scanner.Text())
}
if err := scanner.Err(); err != nil {
log.Fatal(err)
}
}
This is the cleanest way to read from a Reader line by line.
There is one caveat: Scanner does not deal well with lines longer than 65536 characters. If that is an issue for you then then you should probably roll your own on top of Reader.Read().
import (
"bufio"
"os"
)
var (
reader = bufio.NewReader(os.Stdin)
)
func ReadFromStdin() string{
result, _ := reader.ReadString('\n')
witl := result[:len(result)-1]
return witl
}
Here is an example with function ReadFromStdin() it's like fmt.Scan(&name) but its takes all strings with blank spaces like: "Hello My Name Is ..."
var name string = ReadFromStdin()
println(name)
There two common way to read file line by line.
In my testcase, ~250MB, ~2,500,000 lines, bufio.Scanner(time used: 0.395491384s) is faster than bufio.Reader.ReadString(time_used: 0.446867622s).
Source code: https://github.com/xpzouying/go-practice/tree/master/read_file_line_by_line
Read file use bufio.Scanner,
func scanFile() {
f, err := os.OpenFile(logfile, os.O_RDONLY, os.ModePerm)
if err != nil {
log.Fatalf("open file error: %v", err)
return
}
defer f.Close()
sc := bufio.NewScanner(f)
for sc.Scan() {
_ = sc.Text() // GET the line string
}
if err := sc.Err(); err != nil {
log.Fatalf("scan file error: %v", err)
return
}
}
Read file use bufio.Reader,
func readFileLines() {
f, err := os.OpenFile(logfile, os.O_RDONLY, os.ModePerm)
if err != nil {
log.Fatalf("open file error: %v", err)
return
}
defer f.Close()
rd := bufio.NewReader(f)
for {
line, err := rd.ReadString('\n')
if err != nil {
if err == io.EOF {
break
}
log.Fatalf("read file line error: %v", err)
return
}
_ = line // GET the line string
}
}
bufio.Reader.ReadLine() works well. But if you want to read each line by a string, try to use ReadString('\n'). It doesn't need to reinvent the wheel.
// strip '\n' or read until EOF, return error if read error
func readline(reader io.Reader) (line []byte, err error) {
line = make([]byte, 0, 100)
for {
b := make([]byte, 1)
n, er := reader.Read(b)
if n > 0 {
c := b[0]
if c == '\n' { // end of line
break
}
line = append(line, c)
}
if er != nil {
err = er
return
}
}
return
}
In the new version of Go 1.16 we can use package embed to read the file contents as shown below.
package main
import _"embed"
func main() {
//go:embed "hello.txt"
var s string
print(s)
//go:embed "hello.txt"
var b []byte
print(string(b))
//go:embed hello.txt
var f embed.FS
data, _ := f.ReadFile("hello.txt")
print(string(data))
}
For more details go through https://tip.golang.org/pkg/embed/ And https://golangtutorial.dev/tips/embed-files-in-go/
Source: Stackoverflow.com