Understanding Fprintln function in Go (Golang)

Overview

Fprintln is defined in the fmt package and is used to format a string using the default format specifier and write it to io.Writer instance passed to it. It also adds a new line.

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

Below is the function prototype of Fprintln

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

Fprintln 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 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. Each of these arguments is formatted using the default specifier

Fprintln formats the string using the default format specifier adds a new line after the string. Fprintln takes a variable number of arguments where each argument is an empty interface. It returns the number of characters printed and any error if happens. Since the argument type is an empty interface we can pass any data type to it. We can pass a string, int, float, struct, or any other data type. Each of the arguments to the Fprintln function is formatted according to the default format specifier of that argument type. For example, the struct will be formatted according to the below specifier

There is also another function Fprint provided by fmt package which is the same as Fprintln. The only difference being

• Fprintln appends a new line while Fprint does not append a new line.

%v

This format specifier only prints the Value part in the struct. Let’s see an example.

Program

package main

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

type employee struct {
	Name string
	Age  int
}

func main() {
	name := "John"
	age := 21
	fmt.Fprintln(os.Stdout, "Name is: ", name)
	fmt.Fprintln(os.Stdout, "Age is: ", age)
	e := employee{
		Name: name,
		Age:  age,
	}
	fmt.Fprintln(os.Stdout, e)
	fmt.Fprintln(os.Stdout, "a", 12, "b", 12.0)

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

Output

Name is:  John
Age is:  21
{John 21}
a 12 b 12
Name is:  John
15

Some important points to note about the FPrintln function

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

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

• It appends a new line at the end. That is why each of the output is on a different line

• Each of the arguments will be separated by space in the output. That is why

fmt.Fprintln(os.Stdout,"Name is: ", name)

prints

Name is: John

Space is introduced automatically between the two arguments.

• It returns the number of characters printed or any error if happens

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

will output below

Name is: John
14

The number of bytesPrinted is 14 as 14 characters are outputted

Fprintln 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() {
	file, err := os.Create("./temp.txt")
	if err != nil {
		log.Fatal(err)
	}

	name := "John"
	age := 21
	fmt.Fprintln(file, "Name is: ", name)
	fmt.Fprintln(file, "Age is: ", age)
	e := employee{
		Name: name,
		Age:  age,
	}
	fmt.Fprintln(file, e)
	fmt.Fprintln(file, "a", 12, "b", 12.0)
}

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
{John 21}
a 12 b 12

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