Golang基础13问

 Posted on August 25, 2019

最近看见一篇关于go基础的问题，算是给自己的一个小考验，很多基础的东西当你回过头来看看也许有新的认识，当然这些题也适合出面试题。

Quiz1

package main

import (
	"fmt"
)

func hello() []string {
	return nil
}

func main() {
	h := hello // 这里注意区分 hello()
	if h == nil {
		fmt.Println("nil")
	} else {
		fmt.Println("not nil")
	}

	hs := hello() // 这里注意区分 hello
	if hs == nil {
		fmt.Println("nil")
	} else {
		fmt.Println("not nil")
	}
}

// Answer:
// not nil
// nil

// We assign the function hello() to the variable h in line no. 12 and
// hence h is not nil and the program will print not nil.

Quiz2

package main

import (
	"fmt"
	"strconv"
)

func main() {
	i := 2
	s := "1000"
	if len(s) > 1 { // 注意 变量的作用域
		i, _ := strconv.Atoi(s) // 注意这里的是 := 而不是 =
		i = i + 5
	}
	fmt.Println(i)
}

// Answer
// 2

// The tricky part of the above quiz is line no. 12. i, _ := strconv.Atoi(s) creates a new variable i whose scope is only within the if statement.
// The i which is being printed in line no. 15 is actually the one defined in line no. 9
// and not the one defined in line no. 12. Hence this program will print 2.

Quiz3

package main

import (
	"fmt"
)

func hello(num ...int) {
	num[0] = 18
}

func main() {
	i := []int{5, 6, 7}
	hello(i...) // go 中函数参数都是值传递，但对于slice也把地址传入，所谓的引用类型。
	fmt.Println(i[0])
}

// Answer
// 18

Quiz4

package main

import (
	"fmt"
)

func main() {
	a := [2]int{5, 6}
	b := [2]int{5, 6}
	if a == b {
		fmt.Println("equal")
	} else {
		fmt.Println("not equal")
	}
}

// Answer
// equal

这道题目中对比的是数组，如果换成切片呢？

提示：切片不能用 == 进行比较，可以 进行判断 == nil

Quiz5 ⭐️

package main

import "fmt"

type rect struct {
	len, wid int
}

func (r rect) area() {
	fmt.Println(r.len * r.wid)
	r.len = r.len + 1
}

func (r *rect) add() {
	fmt.Println(r.len + r.wid)
	r.len = r.len + 1
}

func main() {
	r := &rect{len: 5, wid: 6}
	r.area()
	fmt.Println(r.len) // 5 没变

	a := rect{len: 5, wid: 6}
	a.add()
	fmt.Println(a.len) // 6 改变

}

// Answer
// 30
// 5
// 11
// 6

// You might be wondering why the program worked when we didn't use (*r).area() in line no. 21. Since area() has a value receiver,
// Go is intelligent enough to interpret r.area() as (*r).area() and hence this program works :).
// 但只有当方法的接受者为指针形式时，才能够在方法内部进行属性操作生效从而改变源数据。反之则不能

Quiz6

package main

import (
	"fmt"
)

func main() {
	a := [5]int{1, 2, 3, 4, 5} // 这里是 array
	t := a[3:4:4]              // 这里就成了 slice len == 4-3 cap == 4-3
	fmt.Println(t[0])
	fmt.Println(len(t), cap(t))
}

// Answer
// 4
// 1 1

// a[low : high : max]
// constructs a slice of the same type,
// and with the same length and elements as the simple slice expression a[low : high].
// Additionally, it controls the resulting slice's capacity by setting it to max - low.
// Hence, the slice t in line no.9 has one element 4 and is of capacity 1.

Quiz7 ⭐️

package main

import (
	"fmt"
	"safeuem/common/log"
)

type person struct {
	name string
}

func main() {
	var m map[person]int // 注意这里用 var  和 make 的区别
	p := person{"mike"}
	fmt.Println(m[p])
	v, ok := m[p]
	log.Error(v, ok)
	log.Error(m == nil)
	log.Errorf("%p", m)

}

// Answer
// 0
// 0 false
// true
//  0x0
// var 并没有给变量分配控制内存地址，但是对于的值已经初始化为类型零值

Quiz8

package main

import (
	"fmt"
	"safeuem/common/log"
	"strconv"
)

func main() {
	i := 65
	fmt.Println(string(i))

	s := strconv.FormatInt(int64(i), 10) // string与其他类型之间的转换 应该使用 strconv 包
	log.Error(s)
}

// Answer
// A
// 65
// The unicode value of A is 65.
// Hence when i is type casted to string in line no. 9, A is printed.

Quiz9 ⭐️

package main

import (
	"fmt"
)

func main() {
	var i interface{} // 注意 var 声明方式 不会分配内存地址 但会初始化对于类型的零值
	if i == nil {
		fmt.Println("nil")
		return
	}
	fmt.Println("not nil")

}

// Answer
// nil
// An empty interface has both its underlying value and concrete type as nil. Hence i is nil

这道题，请自行查看interface 与 nil 的知识点

Quiz10 ⭐️

package main

import (
	"fmt"
)

func hello(i int) {
	fmt.Println(i)
}
func main() {
	i := 5
	defer hello(i) // defer 函数的参数 是在声明时进行读入值，而不是在实际调用时才读入
	i = i + 10
}

// Answer
// 5
// The arguments of a deferred function are evaluated
// when the defer statement is executed and not when the actual function call is done.
// Hence when the defer statement is encountered in line no. 12, the value of i is 5.
// So this program will print 5.

这道题关于defer其中一个知识点，关于defer的知识点还有其他，自行查看吧。

Quiz11

package main

import (
	"fmt"
)

func main() {
	fmt.Printf("%%") // 输出 % 本体
}

// Answer
// %
// The format specifier %% prints a literal % sign. Hence the program prints %.

这里直接补充下：

动 词 功 能
%v 按值的本来值输出
%+v 在 %v 基础上，对结构体字段名和值进行展开
%#v 输出 Go 语言语法格式的值
%T 输出 Go 语言语法格式的类型和值
%% 输出 % 本体
%b 整型以二进制方式显示
%o 整型以八进制方式显示
%d 整型以十进制方式显示
%+d 整型以十进制方式显示并显示数字的符号
%x 整型以十六进制方式显示
%X 整型以十六进制、字母大写方式显示
%U Unicode 字符
%f 浮点数
%p 指针，十六进制方式显示

Quiz12

package main

import (
	"fmt"
)

func main() {
	s := make(map[string]int)
	delete(s, "h")
	fmt.Println(s["h"])
}

// Answer
// 0
// The delete function in line no. 9 doesn't return anything and will do nothing if the specified key doesn't exist.
// Since the key h doesn't exist,
// the delete function will not do anything. In line no. 10, we are trying to print s["h"].
// Since the map s doesn't have the key h, it will return the default value of int. Hence 0 will be printed.

Quiz13

package main

import (
	"fmt"
)

func main() {
	i := -5
	j := +5
	fmt.Printf("%+d %+d", i, j) // +d 表示始终打出数字的符号
}

// Answer
// -5 +5
// The + flag in the format specifier %+d is used to always print a sign for numeric values.
// Hence this program outputs -5 +5.
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