Variadic Functions

A variadic function accepts a variable number of arguments of the same element type. It is useful for totals, aggregation and flexible APIs.

Declare the final parameter as values ...int. Inside the function it behaves like a []int slice. The caller may pass individual numbers or expand an existing slice with numbers....

A variadic parameter must be the last parameter. Calling it with no variadic arguments is valid; the function then sees an empty slice. Standard-library functions such as fmt.Println also accept variadic arguments.

Follow this diagram before executing the example. Check where a value or decision changes what runs next.

This example is a complete program. Save it as main.go or paste it into the Go Playground.

Go code

package main

import "fmt"

func sum(values ...int) int {
	total := 0
	for _, value := range values {
		total += value
	}
	return total
}
func main() {
	fmt.Println("Two:", sum(4, 6))
	numbers := []int{1, 2, 3, 4}
	fmt.Println("Slice:", sum(numbers...))
	fmt.Println("Empty:", sum())
}

Output

Two: 10
Slice: 10
Empty: 0

The first call collects 4 and 6 and returns 10. Expanding numbers... passes its four elements as variadic values and returns 10 again. With no values the total stays zero.

• A variadic parameter must appear at the end of the parameter list.
• Passing a slice as individual variadic arguments requires ... at the call site.

Write a variadic maximum(values ...int) for nonempty inputs, and decide how to handle the empty case.

Tip: Edit the example above, predict the output, and run the modified code to test your reasoning.

You have practiced Variadic Functions in a small, self-contained program. The important habit is to predict what runs, inspect the actual output and distinguish the language rule from the particular example.

Before continuing, make sure you can explain the program to someone who has not seen it, including the line that changes its result.