問題十三 Go 讀寫檔案時常犯的錯誤

我之前再打工的時候撰寫 Go 語言的時候，我在讀取、寫入檔案的時候有注意到一個問題，如果你正常的讀取檔案像下面這樣

	file, err := os.Open("test.csv") // For read access.
	if err != nil {
		log.Fatal(err)
	}
	businessFile, err := os.Create("business.txt")
	companyFile, err := os.Create("company.txt")
	subCompanyFile, err := os.Create("subcompany.txt")
	//br := bufio.NewReader(file)
	br := bufio.NewScanner(file)
	writeBusiness := bufio.NewWriter(businessFile)
	writeCompany := bufio.NewWriter(companyFile)
	writeSubCompany := bufio.NewWriter(subCompanyFile)

	for br.Scan() {
		if err != nil {
			break
		}
		line := br.Text()
		stringSlice := strings.Split(line, ",")
		if stringSlice[1] == "商業登記" {
			writeBusiness.WriteString(line + "\n")
		} else if stringSlice[1] == "公司" {
			writeCompany.WriteString(line + "\n")
		} else if stringSlice[1] == "分公司" {
			writeSubCompany.WriteString(line + "\n")
		}
		fmt.Printf("%v\n", line)
	}
	if err := br.Err(); err != nil {
		log.Fatal(err)
	}

這個 Code 看起來很正常，正常的讀取寫入檔案，如果檔案很小大概都沒有問題，但是檔案一大就會發現寫入後的結果不對，會少一些字又或是亂碼，這個是因為沒有 flush 掉舊的記憶體的關係。

所以下面這個才是對的

	file, err := os.Open("test.csv") // For read access.
	if err != nil {
		log.Fatal(err)
	}

	//dataStruct := map[string]string{}
	businessFile, err := os.Create("business.txt")
	companyFile, err := os.Create("company.txt")
	subCompanyFile, err := os.Create("subcompany.txt")
	//br := bufio.NewReader(file)
	br := bufio.NewScanner(file)
	writeBusiness := bufio.NewWriter(businessFile)
	writeCompany := bufio.NewWriter(companyFile)
	writeSubCompany := bufio.NewWriter(subCompanyFile)

	for br.Scan() {

		//line, err := br.ReadString('\n')

		if err != nil {
			break
		}

		//stringSlice []string
		line := br.Text()
		stringSlice := strings.Split(line, ",")
		//fmt.Printf("%#v\n", stringSlice)
		if stringSlice[1] == "商業登記" {
			writeBusiness.WriteString(line + "\n")
            writeBusiness.Flush()
		} else if stringSlice[1] == "公司" {
			writeCompany.WriteString(line + "\n")
            writeCompany.Flush()
		} else if stringSlice[1] == "分公司" {
			writeSubCompany.WriteString(line + "\n")
            writeSubCompany.Flush()
		}
		fmt.Printf("%v\n", line)
	}
	if err := br.Err(); err != nil {
		log.Fatal(err)
	}

寫入後 flush 掉就 ok 了