Understanding Println function in Go (Golang)

Overview

Println is defined in the fmt package and is used to format a string and write to standard output

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

Below is the function prototype of Println

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

Println formats the string using the default format specifier adds a new line after the string. Println 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 Println 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. Let’s see an example

Program

package main
import "fmt"
type employee struct {
Name string
Age int
}
func main() {
name := "John"
age := 21
fmt.Println("Name is: ", name)
fmt.Println("Age is: ", age)
e := employee{
Name: name,
Age: age,
}
fmt.Println(e)
fmt.Println("a", 12, "b", 12.0)
bytesPrinted, err := fmt.Println("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
14

Some important points to note about the Println function

• It appends a newline 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.Println("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.Println("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

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