Loops in Go (for, range, labeled loops)

Go provides the for loop as its primary looping construct. Understanding different loop patterns is essential for effective Go programming.

Basic For Loop

Traditional For Loop

package main

import "fmt"

func main() {
    // Traditional C-style for loop
    for i := 0; i < 5; i++ {
        fmt.Println(i)
    }
    // Output: 0 1 2 3 4
}

While-Style Loop

package main

import "fmt"

func main() {
    // While-style loop (condition only)
    i := 0
    for i < 5 {
        fmt.Println(i)
        i++
    }
}

Infinite Loop

package main

import "fmt"

func main() {
    // Infinite loop (must break manually)
    i := 0
    for {
        fmt.Println(i)
        i++
        if i >= 5 {
            break
        }
    }
}

Range Iteration

Range Over Slices

package main

import "fmt"

func main() {
    numbers := []int{10, 20, 30, 40, 50}

    // Range with index and value
    for i, v := range numbers {
        fmt.Printf("Index: %d, Value: %d\n", i, v)
    }

    // Range with index only
    for i := range numbers {
        fmt.Printf("Index: %d\n", i)
    }

    // Range with value only
    for _, v := range numbers {
        fmt.Printf("Value: %d\n", v)
    }
}

Range Over Maps

package main

import "fmt"

func main() {
    person := map[string]int{
        "Alice": 30,
        "Bob":   25,
        "Charlie": 35,
    }

    // Range over map
    for name, age := range person {
        fmt.Printf("%s: %d\n", name, age)
    }

    // Range with key only
    for name := range person {
        fmt.Printf("Name: %s\n", name)
    }
}

Range Over Strings

package main

import "fmt"

func main() {
    str := "Hello"

    // Range over string (returns rune)
    for i, ch := range str {
        fmt.Printf("Index: %d, Char: %c, Code: %d\n", i, ch, ch)
    }
}

Range Over Channels

package main

import "fmt"

func main() {
    ch := make(chan int, 3)
    ch <- 1
    ch <- 2
    ch <- 3
    close(ch)

    // Range over channel
    for value := range ch {
        fmt.Println(value)
    }
}

Break and Continue

Break Statement

package main

import "fmt"

func main() {
    for i := 0; i < 10; i++ {
        if i == 5 {
            break  // Exit loop
        }
        fmt.Println(i)
    }
    // Output: 0 1 2 3 4
}

Continue Statement

package main

import "fmt"

func main() {
    for i := 0; i < 5; i++ {
        if i == 2 {
            continue  // Skip to next iteration
        }
        fmt.Println(i)
    }
    // Output: 0 1 3 4
}

Labeled Loops

Break with Label

package main

import "fmt"

func main() {
    // Labeled loop
OuterLoop:
    for i := 0; i < 3; i++ {
        for j := 0; j < 3; j++ {
            if i == 1 && j == 1 {
                break OuterLoop  // Break outer loop
            }
            fmt.Printf("i=%d, j=%d\n", i, j)
        }
    }
}

Continue with Label

package main

import "fmt"

func main() {
    // Labeled loop with continue
OuterLoop:
    for i := 0; i < 3; i++ {
        for j := 0; j < 3; j++ {
            if j == 1 {
                continue OuterLoop  // Continue outer loop
            }
            fmt.Printf("i=%d, j=%d\n", i, j)
        }
    }
}

Loop Patterns

Iterating with Index

package main

import "fmt"

func main() {
    items := []string{"apple", "banana", "cherry"}

    for i := 0; i < len(items); i++ {
        fmt.Printf("%d: %s\n", i, items[i])
    }
}

Iterating Backwards

package main

import "fmt"

func main() {
    items := []string{"apple", "banana", "cherry"}

    for i := len(items) - 1; i >= 0; i-- {
        fmt.Printf("%d: %s\n", i, items[i])
    }
}

Filtering During Iteration

package main

import "fmt"

func main() {
    numbers := []int{1, 2, 3, 4, 5, 6, 7, 8, 9, 10}

    for _, n := range numbers {
        if n%2 == 0 {
            fmt.Println(n)  // Print even numbers
        }
    }
}

Best Practices

✅ Good: Use Range for Iteration

// DO: Use range for cleaner iteration
for i, v := range items {
    fmt.Println(i, v)
}

❌ Bad: Manual Index Management

// DON'T: Manually manage indices
for i := 0; i < len(items); i++ {
    fmt.Println(i, items[i])
}

✅ Good: Use Blank Identifier

// DO: Use blank identifier for unused values
for _, v := range items {
    fmt.Println(v)
}

❌ Bad: Unused Variables

// DON'T: Declare unused variables
for i, v := range items {
    fmt.Println(v)  // i is unused
}

✅ Good: Clear Loop Conditions

// DO: Use clear, readable conditions
for i := 0; i < len(items); i++ {
    // ...
}

Summary

Go’s loop constructs provide:

For loops in multiple styles
Range iteration for collections
Break and continue for loop control
Labeled loops for nested loop control
Clean syntax for common patterns

These tools enable efficient and readable iteration in Go programs.