reading-files.go

package main

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

// Reading files requires checking most calls for errors. This helper will streamline our error checks below.
func check(e error) {
	if e != nil {
		panic(e)
	}
}

func main() {
	// Perhaps the most basic file reading task is slurping a file’s entire contents into memory.
	dat, err := ioutil.ReadFile("/tmp/dat")
	check(err)
	fmt.Print(string(dat))
	// You’ll often want more control over how and what parts of a file are read. For these tasks, start by Opening a file to obtain an os.File value.
	f, err := os.Open("/tmp/dat")
	check(err)
	// Read some bytes from the beginning of the file. Allow up to 5 to be read but also note how many actually were read.
	b1 := make([]byte, 5)
	n1, err := f.Read(b1)
	check(err)
	fmt.Printf("%d bytes: %s\n", n1, string(b1))
	// You can also Seek to a known location in the file and Read from there.
	o2, err := f.Seek(6, 0)
	check(err)
	b2 := make([]byte, 2)
	n2, err := f.Read(b2)
	check(err)
	fmt.Printf("%d bytes @ %d: %s\n", n2, o2, string(b2))

	// The io package provides some functions that may be helpful for file reading. For example, reads like the ones above can be more robustly implemented with ReadAtLeast
	o3, err := f.Seek(6, 0)
	check(err)
	b3 := make([]byte, 4)
	n3, err := io.ReadAtLeast(f, b3, 4)
	check(err)
	fmt.Printf("%d bytes @ %d: %s\n", n3, o3, string(b3))
	// The bufio package implements a buffered reader that may be useful both for its efficiency with many small reads and because of the additional reading methods it provides./
	r4 := bufio.NewReader(f)
	b4, err := r4.Peek(5)
	check(err)
	fmt.Printf("5 bytes: %s\n", string(b4))
	defer f.Close()
}