Go — Loops

For loop

Go has only one loop construct — for:

package main

import "fmt"

func main() {
    // Traditional for loop
    for i := 0; i < 5; i++ {
        fmt.Println(i)
    }

    // While-style
    count := 0
    for count < 5 {
        fmt.Println(count)
        count++
    }

    // Infinite loop
    for {
        break
    }
}

Range

Iterate over slices, arrays, maps, and strings:

// Slice
nums := []int{10, 20, 30, 40, 50}
for i, v := range nums {
    fmt.Printf("Index %d: %d\n", i, v)
}

// Map
ages := map[string]int{"Alice": 30, "Bob": 25}
for name, age := range ages {
    fmt.Printf("%s: %d\n", name, age)
}

// String (iterates by rune)
for i, r := range "Hello" {
    fmt.Printf("Index %d: %c\n", i, r)
}

// Ignore index
for _, v := range nums {
    fmt.Println(v)
}

// Ignore value
for i := range nums {
    fmt.Println(i)
}

break and continue

// break: exit the loop
for i := 0; i < 100; i++ {
    if i == 5 {
        break
    }
    fmt.Println(i)
}

// continue: skip to next iteration
for i := 0; i < 10; i++ {
    if i%2 == 0 {
        continue
    }
    fmt.Println(i)
}

Labeled break/continue

outer:
for i := 0; i < 5; i++ {
    for j := 0; j < 5; j++ {
        if i*j > 10 {
            break outer
        }
        fmt.Printf("(%d,%d) ", i, j)
    }
    fmt.Println()
}

Common patterns

// Sum
sum := 0
for i := 1; i <= 100; i++ {
    sum += i
}

// Find
for _, v := range nums {
    if v == target {
        fmt.Println("Found:", v)
        break
    }
}

// Filter
var evens []int
for _, v := range nums {
    if v%2 == 0 {
        evens = append(evens, v)
    }
}

Mini Practice

Write Go code that:

1. Uses for to print numbers 1-20
2. Uses range to iterate a map
3. Uses labeled break to exit nested loops
4. Filters a slice using a loop

Up Next

In the next lesson, you'll learn about Functions — defining and using functions in Go.