[Author: @mdimamhosen Date: 2025-04-19 Category: interview-qa/constants Tags: [go, constants, beginner] ]

The const keyword declares the variable as "constant", which means that it is unchangeable and read-only.

package main
import ("fmt")

const user = "admin" // cannot be changed

func main() {
  fmt.Println("admin")
}

Constant Rules

1. Constant names follow the same naming rules as variables

2. Constant names are usually written in uppercase letters

3. Constants can be declared both inside and outside of a function

Frequently Asked Questions

1. What is a constant in Go?

Answer: A constant is a variable whose value cannot be changed once it is assigned. Constants are declared using the const keyword.

Code Example:

package main
import "fmt"

const PI = 3.14

func main() {
    fmt.Println("The value of PI is:", PI)
}

2. Can constants be declared inside a function?

Answer: Yes, constants can be declared both inside and outside of a function.

Code Example:

package main
import "fmt"

func main() {
    const GREETING = "Hello, World!"
    fmt.Println(GREETING)
}

3. Can constants hold values other than numbers?

Answer: Yes, constants can hold string, boolean, or even character values.

Code Example:

package main
import "fmt"

const IS_ACTIVE = true
const MESSAGE = "Welcome to Go!"

func main() {
    fmt.Println("Is Active:", IS_ACTIVE)
    fmt.Println("Message:", MESSAGE)
}

4. Can constants be computed at runtime?

Answer: No, constants must be assigned a value that can be determined at compile time.

Code Example:

package main
import "fmt"

const VALUE = 10 * 2 // Valid

func main() {
    fmt.Println("Value:", VALUE)
}

5. What happens if you try to change a constant's value?

Answer: The compiler will throw an error if you try to change the value of a constant.

Code Example:

package main
import "fmt"

const NAME = "John"

func main() {
    // NAME = "Doe" // Uncommenting this line will cause a compilation error
    fmt.Println(NAME)
}

6. Can constants be used in expressions?

Answer: Yes, constants can be used in expressions to compute other constants.

Code Example:

package main
import "fmt"

const A = 5
const B = 10
const SUM = A + B

func main() {
    fmt.Println("Sum:", SUM)
}

7. What is the difference between const and var in Go?

Answer: const is used for values that do not change, while var is used for variables whose values can change.

Code Example:

package main
import "fmt"

const FIXED = 100
var changeable = 200

func main() {
    fmt.Println("Fixed:", FIXED)
    fmt.Println("Changeable:", changeable)
    changeable = 300
    fmt.Println("Updated Changeable:", changeable)
}

8. Can constants be of type array or slice?

Answer: No, constants cannot be of type array, slice, or map.

Code Example:

package main
import "fmt"

func main() {
    // const ARR = [3]int{1, 2, 3} // This will cause a compilation error
    fmt.Println("Constants cannot be arrays or slices.")
}

9. Can constants be exported in Go?

Answer: Yes, constants can be exported if their names start with an uppercase letter.

Code Example:

package main
import "fmt"

const ExportedConstant = "I am exported!"

func main() {
    fmt.Println(ExportedConstant)
}

10. What are untyped constants in Go?

Answer: Untyped constants do not have a specific type until they are assigned to a variable.

Code Example:

package main
import "fmt"

const VALUE = 42

func main() {
    var x int = VALUE
    var y float64 = VALUE
    fmt.Println("x:", x, "y:", y)
}