Understanding Fprint function in Go (Golang) - Welcome To Golang By Example

Table of Contents

Overview

Fprint 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.

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

Below is the function prototype of Fprint

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

Fprint 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

Fprint formats the string using the default format specifier but does not add a new line after the string. Fprint takes a variable number of arguments after the first argument where each argument is an empty interface. 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 Fprint 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

%v

This format specifier only prints the Value part in the struct. There is also one more function provided by fmt package which appends a new line – Fprintln.

Program

Let’s see an example for the same

package main

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

type employee struct {
	Name string
	Age  int
}

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

	fmt.Fprint(os.Stdout, "Name is:", name, "\n")
	fmt.Fprint(os.Stdout, "Age is:", age, "\n")

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

	fmt.Fprint(os.Stdout, e, "\n")

	fmt.Fprint(os.Stdout, "a", 12, "b", 12.0, "\n")

	fmt.Fprint(os.Stdout, 12, 12.0, "\n")

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

Output

Name is:John
Age is:21
{John 21}
a12b12
12 12
Name is: John
14

Some important points to note about the Fprint function

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

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

It does not append a new line at the end. That is why need to use “\n” for adding a new line.

It only adds space between two arguments if each of them is a non-string. That is why

fmt.Fprint(os.Stdout, 12, 12.0, "\n")

prints

12 12

while

fmt.Fprint(os.Stdout, "a", 12, "b", 12.0, "\n")

prints

a12b12

It also returns the number of characters printed and any error if happens

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

will output below

Name is: John
14

The number of bytesPrinted is 14 as 14 characters are outputted

Fprint 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.Fprint(file, "Name is:", name, "\n")
    fmt.Fprint(file, "Age is:", age, "\n")
    e := employee{
        Name: name,
        Age:  age,
    }
    fmt.Fprint(file, e, "\n")
    fmt.Fprint(file, "a", 12, "b", 12.0, "\n")
    fmt.Fprint(file, 12, 12.0, "\n")
}

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}
a12b12
12 12

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