Chapter 3 · Lesson 1

If, For, and Switch

Control Flow in Go

Go keeps control flow simple: there is only one loop keyword, and if/switch have helpful extras.

if with init statement Go's if can run a short statement before the condition. Variables declared there are scoped to the if/else block:

if err := doWork(); err != nil {
    log.Fatal(err)
}

Go's only loop: for Go unifies all loop forms with for:

Classic: for i := 0; i < 10; i++ { }
While-style: for condition { }
Infinite: for { }
Range: for i, v := range slice { }

range Iterates over arrays, slices, maps, strings (runes), and channels. Returns index and value.

switch Go's switch does not fall through by default. Cases can list multiple values. A switch without a condition behaves like if/else if:

switch {
case x < 0: fmt.Println("negative")
case x == 0: fmt.Println("zero")
default: fmt.Println("positive")
}

Key points:

No parentheses around conditions in if/for/switch.
Use fallthrough to explicitly fall through in switch.
break exits a loop; continue skips to the next iteration.

Code Examples

if with init and for rangego

package main

import (
	"fmt"
	"strconv"
)

func main() {
	// if with init statement
	inputs := []string{"42", "abc", "100"}
	for _, s := range inputs {
		if n, err := strconv.Atoi(s); err == nil {
			fmt.Printf("%s -> %d\n", s, n*2)
		} else {
			fmt.Printf("%s -> invalid\n", s)
		}
	}

	// range over map
	scores := map[string]int{"Alice": 95, "Bob": 80}
	for name, score := range scores {
		fmt.Printf("%s: %d\n", name, score)
	}
}

The init statement in if scopes err to the if/else block. range over a map returns key and value in each iteration.

switch without conditiongo

package main

import "fmt"

func classify(n int) string {
	switch {
	case n < 0:
		return "negative"
	case n == 0:
		return "zero"
	case n < 10:
		return "small"
	case n < 100:
		return "medium"
	default:
		return "large"
	}
}

func dayType(day string) string {
	switch day {
	case "Saturday", "Sunday":
		return "weekend"
	default:
		return "weekday"
	}
}

func main() {
	for _, n := range []int{-5, 0, 7, 42, 200} {
		fmt.Printf("%d: %s\n", n, classify(n))
	}
	fmt.Println(dayType("Saturday"))
	fmt.Println(dayType("Monday"))
}

A switch without a condition is like if/else if — cases evaluate boolean expressions. Multiple values in one case use commas.

Quick Quiz

1. How do you write a while-style loop in Go?

2. Does Go's switch fall through by default?

Was this lesson helpful?

Control Flow in Go

Go keeps control flow simple: there is only one loop keyword, and if/switch have helpful extras.

if with init statement Go's if can run a short statement before the condition. Variables declared there are scoped to the if/else block:

if err := doWork(); err != nil {
    log.Fatal(err)
}

Go's only loop: for Go unifies all loop forms with for:

Classic: for i := 0; i < 10; i++ { }

While-style: for condition { }

Infinite: for { }

Range: for i, v := range slice { }