Loops in Go

# CHAPTER 8

Loops in Go #

1. Introduction #

In C, Java, and Python, you have for, while, and do-while loops. Go has only one looping keyword: for. The Go creators designed for to be so flexible that it can replicate all the others, keeping the language simple.

2. Learning Objectives #

• Use the standard for loop (Initialization, Condition, Post).

• Use the for loop as a while loop.

• Create Infinite loops.

• Manipulate execution flow using break and continue.

• Understand the basics of the range keyword.

3. The Standard for Loop #

Syntax: for initialization; condition; post { // code }

package main
import "fmt"

func main() {
    // Prints numbers 1 to 5
    for i := 1; i <= 5; i++ {
        fmt.Printf("Count: %d\n", i)
    }
}

How it works:

1. 1. i := 1 (Initialization): Executes ONCE at the start.

1. 2. i <= 5 (Condition): Checked before every loop. If true, run the code. If false, stop.

1. 3. i++ (Post): Executes *after* the code block runs, then goes back to step 2.

4. The "While" Loop (Using for) #

func main() {
    count := 1

    // This is Go's version of a "while" loop
    for count <= 3 {
        fmt.Println("Hello!")
        count++ // CRITICAL: Don't forget to update the condition!
    }
}

5. The Infinite Loop #

func main() {
    for {
        fmt.Println("I will run forever until you press Ctrl+C!")
        // In a real app, this might be: ListenForNetworkConnections()
    }
}

6. Flow Control: break and continue #

• break: Immediately stops and destroys the loop completely.

• continue: Skips the rest of the *current* iteration and instantly jumps back to the top for the next iteration.

func main() {
    for i := 1; i <= 10; i++ {
        if i == 4 {
            continue // Skips printing 4
        }
        if i == 8 {
            break // Stops the loop entirely at 8
        }
        fmt.Print(i, " ")
    }
    // Output: 1 2 3 5 6 7
}

7. The range Keyword (Preview) #

func main() {
    names := []string{"Alice", "Bob", "Charlie"}

    // range automatically provides the Index and the Value
    for index, name := range names {
        fmt.Printf("Index: %d, Name: %s\n", index, name)
    }
}

8. Mini Project: Multiplication Table Generator #

package main
import "fmt"

func main() {
    var number int
    fmt.Print("Enter a number to generate its table: ")
    fmt.Scan(&number)

    fmt.Printf("\n--- Multiplication Table for %d ---\n", number)
    
    for i := 1; i <= 10; i++ {
        result := number * i
        fmt.Printf("%d x %d = %d\n", number, i, result)
    }
}

9. Common Mistakes #

• Infinite Loops by Accident: Forgetting to include count++ in a while-style loop. The condition never becomes false, freezing the program and maxing out a CPU core.

• Off-by-One Errors: Using < 5 instead of <= 5 when you actually want the loop to run exactly 5 times starting from 1.

10. Best Practices #

• Keep Loops Small: If the code inside your for loop is 100 lines long, extract it into a separate Function. Loops should be clean and easy to read.

11. Exercises #

1. 1. Write a for loop that prints all even numbers from 2 to 20.

1. 2. Write a while-style for loop that creates a countdown timer from 10 to 1, then prints "Liftoff!".

12. MCQs with Answers & Explanations #

13. Interview Preparation #

1. 1. Why did the creators of Go decide to exclude the while keyword from the language? (Answer: Minimalism. for can replicate while perfectly, making the language smaller and easier to read).

1. 2. What happens if you run an infinite loop for {} on a backend server without any blocking code (like listening for network requests)? (Answer: It will consume 100% of a CPU core instantly).

14. Summary #

15. Next Chapter Recommendation #