How can i read desired line from the file

[favorman]

Hello Everybody

I just wanna learn how can i read desired line from the file ?
e.g
I wanna read third line from the file
How can i do ?
Please help me

[luk4z7 (Lucas Alves)]

arquivo.txt

First line
Second line
Third line

main.go

package main

import (
	"os"
	"io/ioutil"
	"strings"
	"fmt"
)

func main() {
	fileIO, err := os.OpenFile("arquivo.txt", os.O_RDWR, 0600)
	if err != nil {
		panic(err)
	}
	defer fileIO.Close()
	rawBytes, err := ioutil.ReadAll(fileIO)
	if err != nil {
		panic(err)
	}
	lines := strings.Split(string(rawBytes), "\n")
	for i, line := range lines {
		if i == 2 {
			fmt.Println(line)
		}
	}
}

[favorman]

@luk4z7

fileIO, err := os.OpenFile(“arquivo.txt”, os.O_RDWR, 0600)

Why we use (os.O_RDWR,0600)
Sorry for this question because i am new at this languege
Thank for helping

[luk4z7 (Lucas Alves)]

constant O_RDWR and 0600 permission of file, as the constants below

// Flags to OpenFile wrapping those of the underlying system. Not all
// flags may be implemented on a given system.
const (
	O_RDONLY int = syscall.O_RDONLY // open the file read-only.
	O_WRONLY int = syscall.O_WRONLY // open the file write-only.
	O_RDWR   int = syscall.O_RDWR   // open the file read-write.
	O_APPEND int = syscall.O_APPEND // append data to the file when writing.
	O_CREATE int = syscall.O_CREAT  // create a new file if none exists.
	O_EXCL   int = syscall.O_EXCL   // used with O_CREATE, file must not exist
	O_SYNC   int = syscall.O_SYNC   // open for synchronous I/O.
	O_TRUNC  int = syscall.O_TRUNC  // if possible, truncate file when opened.
)

[favorman]

Thank you for helping
Have a good day

[luk4z7 (Lucas Alves)]

[dfc (Dave Cheney)]

Use os.Open instead

[radovskyb (Benjamin Radovsky)]

It would be preferable to read a file using a buffered scanner and reading line by line, rather than using ioutil.ReadAll and reading the whole file into memory just to read a single line.

Also as @dfc has suggested, definitely use os.Open rather than os.OpenFile for this case.

Anyway here’s a simple example to print the 3rd line from a file:

package main

import (
	"bufio"
	"fmt"
	"log"
	"os"
)

func main() {
	f, err := os.Open("file.txt")
	if err != nil {
		log.Fatalln(err)
	}
	defer f.Close()

	scanner := bufio.NewScanner(f)
	var line int
	for scanner.Scan() {
		if line == 2 {
			fmt.Println(scanner.Text())
		}
		line++
	}
	if err := scanner.Err(); err != nil {
		log.Fatalln(err)
	}
}

Although keeping in mind that for only reading the third line in a file, if you know that it’s going to be a small file, then reading it all into memory might be better, but for reading say the third line from a large file, it wouldn’t be ideal.

[favorman]

Thank you for helping

[radovskyb (Benjamin Radovsky)]

No worries