Go Input and Output

Input and output form the backbone of interactive programs. Output sends information to the screen. Input reads data that a user types into the terminal. Go handles both through the fmt package.

Output Functions in fmt

Using fmt.Println

package main

import "fmt"

func main() {
    fmt.Println("Hello, World!")    // prints and moves to next line
    fmt.Println("Name:", "Alice")   // can print multiple values
    fmt.Println(10 + 5)             // prints: 15
}

Output:

Hello, World!
Name: Alice
15

Using fmt.Printf with Format Verbs

Format verbs are placeholders that tell Go how to display a value. They start with %.

package main

import "fmt"

func main() {
    name := "Alice"
    age := 25
    score := 98.75

    fmt.Printf("Name: %s\n", name)
    fmt.Printf("Age: %d\n", age)
    fmt.Printf("Score: %.2f\n", score)
    fmt.Printf("Type of age: %T\n", age)
}

Output:

Name: Alice
Age: 25
Score: 98.75
Type of age: int

Using fmt.Sprintf to Build Strings

fmt.Sprintf works exactly like fmt.Printf but returns the formatted string instead of printing it. This is useful for building messages to store or pass to other functions.

package main

import "fmt"

func main() {
    name := "Bob"
    message := fmt.Sprintf("Welcome, %s! You have %d new messages.", name, 3)
    fmt.Println(message)
}

Output:

Welcome, Bob! You have 3 new messages.

Reading Input with fmt.Scan

fmt.Scan reads values typed by the user and stores them in variables. Pass the variable address using &.

package main

import "fmt"

func main() {
    var name string
    fmt.Print("Enter your name: ")
    fmt.Scan(&name)
    fmt.Println("Hello,", name)
}

Terminal interaction:

Enter your name: Alice
Hello, Alice

Reading Multiple Inputs

package main

import "fmt"

func main() {
    var name string
    var age int

    fmt.Print("Enter name and age: ")
    fmt.Scan(&name, &age)

    fmt.Printf("Name: %s, Age: %d\n", name, age)
}

Terminal interaction:

Enter name and age: Alice 25
Name: Alice, Age: 25

Reading a Full Line with fmt.Scanln

fmt.Scan stops reading at a space. fmt.Scanln reads until the user presses Enter.

package main

import "fmt"

func main() {
    var city string
    fmt.Print("Enter city name: ")
    fmt.Scanln(&city)
    fmt.Println("City:", city)
}

Input/Output Flow Diagram

User types input
       │
       ▼
fmt.Scan / fmt.Scanln  ──► stores value in variable
                                     │
                                     ▼
                          Program processes the value
                                     │
                                     ▼
fmt.Println / fmt.Printf ──► displays output to screen

Key Points

• fmt.Println adds a new line; fmt.Print does not
• fmt.Printf uses format verbs like %s, %d, %f for precise formatting
• fmt.Sprintf builds and returns a formatted string without printing
• fmt.Scan reads user input — always pass the variable address with &
• fmt.Scanln reads a full line until the user presses Enter