FMT package usage of go and string formatting

Catalog

1. Standard library fmt Several outputs are available / Input correlation function .

2. Format data placeholder

3. Use fmt Problems encountered in formatting

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fmt The package provides something similar to C Of print and scan Function to implement formatting I/O. fmt It mainly provides output and input functions and string formatting methods .

1. Standard library fmt Several outputs are available / Input correlation function .

1. Print A series of functions will output the content to the standard output of the system

func Print(a ...interface{}) (n int, err error)

2.Printf The function supports formatting the output string to the standard output of the system

func Printf(format string, a ...interface{}) (n int, err error)

3. Println Function adds a newline at the end of the output , Output to the standard output of the system

func Println(a ...interface{}) (n int, err error)

4. Fprint A series of functions will output the content to a io.Writer Variables of interface type w in , We usually use this function to write to a file .

func Fprint(w io.Writer, a ...interface{}) (n int, err error)
func Fprintf(w io.Writer, format string, a ...interface{}) (n int, err error)
func Fprintln(w io.Writer, a ...interface{}) (n int, err error)

5. Sprint A series of functions will generate the incoming data and return a string .

func Sprint(a ...interface{}) string
func Sprintf(format string, a ...interface{}) string
func Sprintln(a ...interface{}) string

6. Errorf Functions are based on format Parameter generates a formatted string and returns an error containing the string .

func Errorf(format string, a ...interface{}) error

be relative to fmt Output ,fmt  Corresponding input functions are provided ：

1. Get user input from standard input .

func Scan(a ...interface{}) (n int, err error)

2. according to format The format specified by the parameter , Read data from standard input

func Scanf(format string, a ...interface{}) (n int, err error)

3. Getting data from standard input , Until the newline character is obtained

func Scanln(a ...interface{}) (n int, err error)

4. and Fpritn Corresponding Fscan, from io.Reader Read data from

func Fscan(r io.Reader, a ...interface{}) (n int, err error)
func Fscanln(r io.Reader, a ...interface{}) (n int, err error)
func Fscanf(r io.Reader, format string, a ...interface{}) (n int, err error)

5. and Sprint Corresponding Sscan, Read from a string

func Sscan(str string, a ...interface{}) (n int, err error)
func Sscanln(str string, a ...interface{}) (n int, err error)
func Sscanf(str string, format string, a ...interface{}) (n int, err error)

The standard provides input from the standard 、 Files are read in many forms , And output to standard input 、 Documents, etc. . Some of these functions also provide the ability to output formatted data as placeholders .

2. Format data placeholder

*printf A series of functions support format Format parameters , The actual value of the variable that the placeholder will be replaced , Here we divide the data by format data type

Here we first define a Person Structure , As a follow-up example ：

type Person struct {
	Name string
	Age  int
}

person := &Person{
    Name: "Xiao Ming",
	Age:  18,
}

1. Universal

Place holder	explain	example
%v	go Default print format for	Print person structure ：&{Xiao Ming 18}
%+v	When printing structures , Add field name	Print person structure ：&{Name:Xiao Ming Age:18}
%#v	It's worth it go Grammar said	Print person structure ： &main.Person{Name:"Xiao Ming", Age:18}
%T	go The type of median	String slice formatting ：[]string
%%	Print the original % Symbol

2. bool type

Place holder	explain	example
%t	format bool Type variable	true/false

3. integer

Place holder	explain	example
%b	int The format is binary data	for example 65 The corresponding value is ：1000001
%c	int The type corresponds to Unicode The character represented by the code point	for example 65 Corresponding to letters A
%d	Format as decimal integer
%o	Format as octal integer	for example 65 Format output ：101
%O	Format as band 0o Prefix octal integer	for example 65 Format output ：0o101
%q	The value is enclosed in single quotes go Syntax character literal , Secure escape representations are used when necessary
%x	Expressed in hexadecimal , Use a-f	for example 255 Output is ff
%X	Expressed in hexadecimal , Use A-F	for example 255 Output is FF
%U	Expressed as Unicode Format ：U+1234, Equivalent to "U+%04X"

4. Floating point and complex number

Place holder	explain	example
%b	format bool type
%e	Scientific enumeration ,e Express	123.321 The formatted output of ： 1.233210e+02
%E	Scientific enumeration ,E Express	123.321 The formatted output of ： 1.233210E+02
%f	There is a decimal format , No exponential representation	123.321 The formatted output of ：123.321
%F	Same as %f
%g	According to the actual situation %e or %f Output , If the number is large %e Output
%G	According to the actual situation %E or %f Output , If the number is large %E Output
%x	Each byte is represented by two hexadecimal bytes ,a-f Express	123.321 Format output ：0x1.ed48b43958106p+06
%X	Each byte is represented by two hexadecimal bytes ,A-F Express	123.321 Format output ：0X1.ED48B43958106P+06

 5. Slicing of strings and bytes

Place holder	explain	example
%s	Direct output string or []byte	“A1b Add 2c3” Format output ：A1b Add 2c3
%q	A string in double quotation marks	“A1b Add 2c3” Format output ："A1b Add 2c3"
%x	Each byte is represented by two hexadecimal bytes ,a-f Express	“A1b Add 2c3” Format output ：413162e58aa0326333
%X	Each byte is represented by two hexadecimal bytes ,A-F Express	“A1b Add 2c3” Format output ：413162E58AA0326333

6. The pointer

Place holder	explain	example
%p	0x The first hexadecimal number indicates	person Format output ：0xc0000a7248

7. Width identifier

The width is followed by a % The following decimal number specifies , If the width is not specified , The value is not filled unless it is required . Precision is specified by the decimal number following the dot , No indication precision is specified 0. Examples are as follows

Place holder	explain	example
%f	Default width , Default precision
%9f	Width 9, Default precision	123.321 Format output ：123.321000
%.2f	Default width , precision 2	123.321 Format output ：123.32
%9.2f	Width 9, precision 2	123.321 Format output ：   123.32, Front complement 4 Empty
%9.f	Width 9, precision 0	123.321 Format output ：      123, Front complement 6 Empty

8. Other placeholders

3. Use fmt Problems encountered in formatting

1.  String() Method to format infinite recursion

type X string
func (x X) String() string {
	return fmt.Sprintf("<%s>", x)
}


func main (){
    var x X
	x = "hello"
	fmt.Print(x.String())
}

When calling x.String() When the method is used ,X Of String() Method call fmt.Sprintf when , That's when it calls x Of String() Method to format a string, resulting in an infinite loop recursion . Here we need to talk about String() The method is modified as follows ：

func (x X) String() string {
	return fmt.Sprintf("<%s>", string(x))
}

  2.  If an invalid argument is supplied to the placeholder , The generated string will contain a description of the problem , Not the result you want . for example ：

foo := "bar"
fmt.Printf("%d", foo)    

//  Print out ：%!d(string=bar)