reading-files.go

package main

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

/*
Reading and writing files are basic tasks needed for many Go programs.
First we’ll look at some examples of reading files.
*/

func checkRead(e error) {
	if e != nil {
		panic(e)
	}
}

// ReadFiles function to illustrate reading files in go.
func ReadFiles() {

	// Perhaps the most basic file reading task is
	// slurping a file’s entire contents into memory.
	dat, err := ioutil.ReadFile("/tmp/dat")
	checkRead(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")
	defer f.Close()
	checkRead(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)
	checkRead(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)
	checkRead(err)
	b2 := make([]byte, 2)
	n2, err := f.Read(b2)
	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)
	checkRead(err)
	b3 := make([]byte, 2)
	n3, err := io.ReadAtLeast(f, b3, 2)
	checkRead(err)
	fmt.Printf("%d bytes @ %d: %s\n", n3, o3, string(b3))

	// There is no built-in rewind, but Seek(0, 0) accomplishes this.
	// sets the file position to the beginning of the file of the given stream.
	_, err = f.Seek(0, 0)
	checkRead(err)

	// 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)
	checkRead(err)
	fmt.Printf("5 bytes: %s\n", string(b4))
}