User Input and Output

# CHAPTER 6

User Input and Output #

1. Introduction #

2. Learning Objectives #

• Use fmt.Print and fmt.Println effectively.

• Master formatted output using fmt.Printf (String Interpolation).

• Read user input using fmt.Scan and fmt.Scanln.

• Understand memory addresses when scanning data.

3. Formatting Output (fmt.Printf) #

Instead of ugly concatenation like fmt.Println("User " + name + " is " + age + " years old"), Go uses fmt.Printf (Print Format).

It uses "Verbs" (placeholders starting with %) that are replaced by variables.

package main
import "fmt"

func main() {
    name := "Alice"
    age := 28
    score := 95.5

    // %s is for strings, %d is for base-10 integers, %f is for floats
    fmt.Printf("User: %s | Age: %d | Score: %f\n", name, age, score)
    
    // Formatting a float to only show 2 decimal places: %.2f
    fmt.Printf("Formatted Score: %.2f\n", score)
}

Common Formatting Verbs:

• %v (Value): Prints the value in its default format (A great catch-all!).

• %T (Type): Prints the data type of the variable (e.g., int, string).

• %s: String.

• %d: Integer.

• %f: Float.

4. Reading User Input (fmt.Scan) #

Memory Concept: When you use Scan, you must tell Go exactly *where in memory* to put the typed data. You do this by placing an ampersand (&) before the variable name. This is called a Pointer. (We will dive deep into pointers in Chapter 13).

package main
import "fmt"

func main() {
    var firstName string
    var age int

    fmt.Print("Enter your first name: ")
    fmt.Scan(&firstName) // The & tells Go where to store the typed text

    fmt.Print("Enter your age: ")
    fmt.Scan(&age)

    fmt.Printf("\nWelcome %s! You are %d years old.\n", firstName, age)
}

5. fmt.Scan vs fmt.Scanln #

• fmt.Scan reads text separated by spaces. If you type "Alice Smith", it will only store "Alice" in the first variable, and save "Smith" for the next Scan call.

• fmt.Scanln is similar, but it explicitly stops reading the moment the user presses the Enter key.

6. Mini Project: User Profile CLI #

package main
import "fmt"

func main() {
    var user string
    var balance float64

    fmt.Println("--- BANK REGISTRATION ---")
    
    fmt.Print("Enter Username: ")
    fmt.Scanln(&user)
    
    fmt.Print("Enter Initial Deposit ($): ")
    fmt.Scanln(&balance)
    
    fmt.Println("-------------------------")
    // Use %v as an easy placeholder, and %.2f for currency
    fmt.Printf("Success! Account '%v' created with a balance of $%.2f\n", user, balance)
}

7. Common Mistakes #

• Forgetting the Ampersand (&): Writing fmt.Scan(firstName) instead of fmt.Scan(&firstName). The code will compile, but the variable won't update because Go didn't know the memory address.

• Using Printf without \n: fmt.Printf does NOT automatically add a new line at the end like Println. If you forget the \n, your next print statement will jam onto the same line.

8. Best Practices #

• Use %v for debugging: If you aren't sure what type a variable is, %v is incredibly useful as it dynamically adapts to print almost anything.

• Use %T for type checking: If Go inferred a type using := and you are confused, fmt.Printf("Type is %T", myVar) will tell you exactly what it is.

9. Exercises #

1. 1. Write a program that asks the user for the radius of a circle as a float64.

1. 2. Calculate the area (3.14159 * radius * radius).

1. 3. Print the area strictly formatted to 2 decimal places using fmt.Printf.

10. MCQs with Answers & Explanations #

11. Interview Preparation #

1. 1. Why must we use pointers (&) when reading input with fmt.Scan?

1. 2. Explain the difference between %v, %s, and %T in fmt.Printf.

Debugging Task: A junior developer writes fmt.Printf("User is %d years old", "Twenty") and it prints garbled formatting errors. Why? *Answer: %d requires an integer, but a string was provided. Use %s for strings.*

12. Summary #

13. Next Chapter Recommendation #