Understanding Fprintf function in Go (Golang)

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3 years ago

Table of Contents

Overview

Fprintf is defined in the fmt package and is used to format a string and write that formatted string to io.Writer instance passed to it as one of the arguments.

https://golang.org/pkg/fmt/#Fprint

Golang provides another similar function Printf. The only difference between Frprintf and Printf is that Fprintf writes to the io.Writer instance passed to it while Printf function writes to the standard output

Below is the function prototype of Fprintf

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

Fprintf is also a variadic function meaning that it can have multiple arguments. Here are the details about its arguments

The first argument is the io.Writer instance to which it writes

The second argument is a format or template string. The string contains the custom specifiers like %s for string, %d for int, etc

The next is a variable number of arguments. Each of the arguments in this list could be string, int, struct, or anything. That is why it is an empty interface.

Fprintf formats the string using custom specifiers. The format or template string contains the actual string that needs to be formatted plus some formating verbs. These formating verbs tell how the trailing arguments will be formatted in the final string. So basically the format string argument contains certain symbols which are replaced by trailing arguments.

Also to note about the Fprintf function is that it also doesn’t add a new line. We need to use the new line identifier to add a new line “\n”.

Formatting a string variable

%s symbol is used

Example

name := "John"
fmt.Fprintf(os.Stdout, "Name is: %s\n", name)

The output will be as below. It will be written to os.Stdout which is standard output. See we pass os.Stdout as the first argument.

Name is John

Formatting an integer

%d symbol is used

Example

age := 21
fmt.Fprintf(os.Stdout, "Age is: %d\n", age)

The output will be as below. It will be written to os.Stdout which is standard output.

Formatting a struct

For example, there are three format specifiers for printing a struct.

%v – It will print only values. The field name will not be printed. This is the default way of printing a struct when using Println
%+v – It will print both field and value.
%#v – It will print the struct, also both field name and value

That is why

fmt.Fprintf(os.Stdout, "Employee is %v\n", e)
fmt.Fprintf(os.Stdout, "Employee is %+v\n", e)
fmt.Fprintf(os.Stdout,"Employee is %#v\n", e)

writes below respectively to the os.Stdout instance

Employee is {John 21}
Employee is {Name:John Age:21}
Employee is main.employee{Name:"John", Age:21}

It is as per the explanation above. Also, note that this function returns the number of characters printed and any error if happens. It does not add a new line. You will have to add “\n” explicitly.

Program

Here is the working program for the same

package main

import (
	"fmt"
	"log"
	"os"
)

type employee struct {
	Name string
	Age  int
}

func main() {
	name := "John"
	age := 21

	fmt.Fprintf(os.Stdout, "Name is: %s\n", name)
	fmt.Fprintf(os.Stdout, "Age is: %d\n", age)

	fmt.Fprintf(os.Stdout, "Name: %s Age: %d\n", name, age)

	e := employee{
		Name: name,
		Age:  age,
	}

	fmt.Fprintf(os.Stdout, "Employee is %v\n", e)
	fmt.Fprintf(os.Stdout, "Employee is %+v\n", e)
	fmt.Fprintf(os.Stdout, "Employee is %#v\n", e)

	bytesPrinted, err := fmt.Fprintf(os.Stdout, "Name is: %s\n", name)
	if err != nil {
		log.Fatalln("Error occured", err)
	}
	fmt.Println(bytesPrinted)
}

Output

Name is: John
Age is: 21
Name: John Age: 21
Employee is {John 21}
Employee is {Name:John Age:21}
Employee is main.employee{Name:"John", Age:21}
Name is: John
14

In all the Fprintf function above we pass it the instance of os.Stdout to it which implements the io.Writer interface. Basically with os.Stdout, Fprintf writes to standard output. This is how os.Stdout defined.

Stdout = NewFile(uintptr(syscall.Stdout), "/dev/stdout")

Notice that in below Fprintf

fmt.Fprintf(os.Stdout, "Name: %s Age: %d\n", name, age)

%s is replaced by name.

%d is replaced by age.

So basically the symbols or verbs in the format string argument are replaced by trailing arguments in order

Fprintf can also be used to write to a file. Since the file instance in golang implements the io.Writer, this is not a problem. Below is the program for the same

package main
import (
    "fmt"
    "log"
    "os"
)
type employee struct {
    Name string
    Age  int
}
func main() {
    name := "John"
    age := 21
    file, err := os.Create("./temp.txt")
    if err != nil {
        log.Fatal(err)
    }
    fmt.Fprintf(file, "Name is: %s\n", name)
    fmt.Fprintf(file, "Age is: %d\n", age)
    fmt.Fprintf(file, "Name: %s Age: %d\n", name, age)
    e := employee{
        Name: name,
        Age:  age,
    }
    fmt.Fprintf(file, "Employee is %v\n", e)
    fmt.Fprintf(file, "Employee is %+v\n", e)
    fmt.Fprintf(file, "Employee is %#v\n", e)
}

Output

It will create file name temp.txt in the current directory with the below contents. In this program we replaced os.Stdout with the file created.

Name is: John
Age is: 21
Name: John Age: 21
Employee is {John 21}
Employee is {Name:John Age:21}
Employee is main.employee{Name:"John", Age:21}

If the number of format specifiers in the format string does not match the number of next variable arguments then the format specifier will be printed as is. For example, in the below code, we have two format specifier

While the next variable number of arguments is only one. Hence when we call it then it will write the second format specifier as is with MISSING as a warning

package main

import (
	"fmt"
	"os"
)

type employee struct {
	Name string
	Age  int
}

func main() {
	name := "John"

	fmt.Fprintf(os.Stdout, "Name is: %s %d\n", name)
}

Output

Name is: John %!d(MISSING)

Also, check out our Golang advance tutorial Series – Golang Advance Tutorial