Golang Tips

1. Go的数组只能有固定的长度，传入变量作为数组长度时，只能创建为定义了size的切片

length := 5
array := [length]int  // error: non-constant array bound length
array := make([]int, length)

2. Go没有内置的比较整数大小的方法，需要自己实现

func max(a int, b int) int {
    if a >= b { 
        return a
    } else {
        return b
    }
}

3. Go没有内置的判断array或map是否存在某一个元素的方法，需要通过判断普通的get的error实现

// implement array/list contains
func contains(array []int, val int) bool {
    contains := false
    for _, value := range array {
        if val == value {
            contains = true
            break
        }
    }
    return contains
}

// implement map containsKey
func containsKey(myMap map[int]int, key int) bool {
    if _, ok := myMap[key]; ok {
        return true
    } else {
        return false
    }
}

4. Golang使用动态切片长度作为for loop上限
   如果写作

searchSet = getKeySet()
for i :=0; i < len(searchSet); i++ {
    searchSet = append(searchSet, generateRandomKey())
}

可能会出现i无法察觉上限，无限循环

Generate random key for i = 208944
Enter generate random key
Generate random key for i = 208945
Enter generate random key
Generate random key for i = 208946
Enter generate random key
Generate random key for i = 208947

应该写作

searchSet = getKeySet()
size := len(searchSet)
for i :=0; i < size; i++ {
    searchSet = append(searchSet, generateRandomKey())
}

5. Golang channel如果不close，遍历会出deadlock error

ans := make(chan string)
for i := 0; i<5; i++ {
    ans <- "hello"
}
// fatal error: all goroutines are asleep - deadlock!
// need to close it before for loop, or read out fixed numbers of items
// close(ans)
for n, ok := range ans {
    fmt.Println(n)
}

6. Golang I/O Sample

package main

import (
    "os"
    "fmt"
    "context"
    "bufio"
)

func read() {
    textfile := "input.txt"
    file, err := os.Open(textfile)
    if err != nil {
        fmt.Printf("Cannot open text file: %s, err: [%v]", textfile, err)
    }
    defer file.Close()

    scanner := bufio.NewScanner(file)
    for scanner.Scan() {
        line := scanner.Text()
        write(line)
    }

    if err := scanner.Err(); err != nil {
        fmt.Printf("Cannot scanner text file: %s, err: [%v]", textfile, err)
    }
}

func write(line string) {
    textfile := "output.txt"
    if file, err := os.OpenFile(textfile,os.O_RDWR|os.O_CREATE|os.O_APPEND,0644); err == nil {
        defer file.Close()
        writer := bufio.NewWriterSize(file)
        content := "My name is " + line
        if _,err := writer.WriteString(content); err == nil {
            fmt.Println("Successful appending buffer and flush to file with bufio's Writer obj WriteString method",content)
        }
    }
}

OpenFile flags

const (
        O_RDONLY int = syscall.O_RDONLY // 只读打开文件和os.Open()同义
        O_WRONLY int = syscall.O_WRONLY // 只写打开文件
        O_RDWR   int = syscall.O_RDWR   // 读写方式打开文件
        O_APPEND int = syscall.O_APPEND // 当写的时候使用追加模式到文件末尾
        O_CREATE int = syscall.O_CREAT  // 如果文件不存在，此案创建
        O_EXCL   int = syscall.O_EXCL   // 和O_CREATE一起使用, 只有当文件不存在时才创建
        O_SYNC   int = syscall.O_SYNC   // 以同步I/O方式打开文件，直接写入硬盘.
        O_TRUNC  int = syscall.O_TRUNC  // 如果可以的话，当打开文件时先清空文件
)