How to Pass Default Parameter to Golang Function

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To pass the default parameter to a function in Golang, you can use a “variadic function”, “struct with a default value”, or “map with a default value”.

Method 1: Using a variadic function

A variadic is a function that accepts a variable number of arguments. In Go, you can define a variadic function using the … syntax before the last parameter type. This suggests that the function can accept any number of arguments of that type, which will be accessible within the function as a slice.

Example

package main

import (
  "fmt"
)

func Greet(name string, titles ...string) {
  title := "Mr."
  if len(titles) > 0 {
    title = titles[0]
  }
  fmt.Printf("Hello, %s %s!\n", title, name)
}

func main() {
  Greet("Krunal") // Uses the default title
  Greet("KB", "Ms.") // Passes a custom title
  Greet("Niva", "Dr.") // Passes a custom title
}

Output

Hello, Mr. Krunal!
Hello, Ms. KB!
Hello, Dr. Niva!

In this example, we defined a Greet() function with a name parameter and a variadic titles parameter.

Inside the function, we checked if any value was provided for titles. If a value is provided, we use the first value as the title; otherwise, we use the default value “Mr.”.

Method 2: Using a struct with a default value

To implement default parameters in a function using a struct in Go, you can define a struct that holds the function parameters, including the ones you want to have default values. Then, in the function, check if the parameters have been set; assign the default values.

Example

package main

import (
  "fmt"
)

type GreetParams struct {
  Name string
  Title string
}

func GreetWithStruct(params GreetParams) {
  if params.Title == "" {
    params.Title = "Mr."
  }
  fmt.Printf("Hello, %s %s!\n", params.Title, params.Name)
}

func main() {
  GreetWithStruct(GreetParams{Name: "Krunal"})
  GreetWithStruct(GreetParams{Name: "KB", Title: "Ms."})
  GreetWithStruct(GreetParams{Name: "Niva", Title: "Dr."})
}

Output

In this example, we define a GreetParams struct to hold the parameters for the GreetWithStruct() function. The GreetWithStruct() function accepts an instance of GreetParams as its argument.

Inside the function, we checked if a value was provided for the Title field. If a value is provided, we use it; otherwise, we use the default value “Mr.”.

Method 3: Using a map with a default value

In Go, you can use a map to provide default values for function parameters. This can be useful when you have a function that accepts many optional parameters.

Example

package main

import (
  "fmt"
)

func PrintDetails(details map[string]string) {
  defaults := map[string]string{
    "name": "Ankit",
    "age": "30",
    "country": "India",
  }

  for key, defaultValue := range defaults {
    if value, ok := details[key]; ok {
      fmt.Printf("%s: %s\n", key, value)
    } else {
      fmt.Printf("%s: %s\n", key, defaultValue)
    }
  }
}

func main() {
  PrintDetails(map[string]string{
    "name": "Krunal",
    "age": "30",
  })
}

Output

country: India
name: Krunal
age: 30

The PrintDetails() function then checks if the details map has a value for each key. If it does, it uses the value from the details map; otherwise, it uses the default value.

In the main function, we call PrintDetails() with a map that only has values for the “name” and “age” keys. The function will use these provided values for “name” and “age”, and the default value for “country”.

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