Functions

Syntax

func name(param1 type1, param2 type2) returnType {
    return value
}

func add(a, b int) int {    // same-type params can share a type
    return a + b
}

func greet(name string) {   // no return value
    fmt.Println("Hello,", name)
}

• Arguments are passed by value — the function receives a copy.
• A variadic parameter (nums ...int) accepts any number of arguments and arrives inside the function as a slice.
• Functions are values: you can store them in variables and write anonymous functions inline.
• Go has no default parameter values and no function overloading — each function name in a package is unique.

Example

package main

import "fmt"

func add(a, b int) int {
    return a + b
}

func sum(nums ...int) int {
    total := 0
    for _, n := range nums {
        total += n
    }
    return total
}

func resetToZero(n int) {
    n = 0 // only changes the local copy
}

func main() {
    fmt.Println(add(2, 3))
    fmt.Println(sum(1, 2, 3, 4))

    values := []int{10, 20, 30}
    fmt.Println(sum(values...)) // spread a slice into a variadic call

    x := 5
    resetToZero(x)
    fmt.Println(x)

    square := func(n int) int {
        return n * n
    }
    fmt.Println(square(6))
}

Expected output

5
10
60
5
36

x is still 5 because resetToZero received a copy. To let a function change a caller's variable you pass a pointer — covered in the Pointers lesson.

Common mistakes

• Declaring a return type and forgetting a return on some path — missing return is a compile error.
• Expecting a function to modify an argument it received by value.
• Naming a function max, min, len, or append. It's legal, but it hides the built-in with the same name inside that scope.

Practice

Write a function isEven(n int) bool and a variadic function average(nums ...float64) float64. Print isEven(7) and average(2, 4, 9).

package main

import "fmt"

func isEven(n int) bool {
    return n%2 == 0
}

func average(nums ...float64) float64 {
    if len(nums) == 0 {
        return 0
    }
    total := 0.0
    for _, n := range nums {
        total += n
    }
    return total / float64(len(nums))
}

func main() {
    fmt.Println(isEven(7))         // false
    fmt.Println(average(2, 4, 9))  // 5
}