Constants in Go - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a constant in Go?

A constant is a fixed value that cannot be changed during the program's execution. It is declared using the keyword const.

beginner

How do you declare a constant in Go?

Use the <code>const</code> keyword followed by the name, optional type, and value. Example: <code>const Pi = 3.14</code>

beginner

Can you change the value of a constant after it is declared?

No, constants cannot be changed once declared. Trying to assign a new value will cause a compile-time error.

intermediate

What types of values can constants hold in Go?

Constants can hold boolean, numeric (integer, float, complex), and string values.

beginner

What is the difference between a constant and a variable in Go?

A constant's value is fixed and cannot change, while a variable's value can be changed during program execution.

Which keyword is used to declare a constant in Go?

A const

B var

C let

D static

The const keyword is used to declare constants in Go.

What happens if you try to change the value of a constant after declaration?

A The value changes successfully

B Compile-time error occurs

C The program compiles and runs normally

D Runtime error occurs

Constants cannot be changed after declaration; trying to do so causes a compile-time error.

Which of these can be a constant value in Go?

A A slice

B A struct

C A map

D An integer

Constants can hold numeric values like integers, but not slices, maps, or structs.

How do you declare multiple constants at once in Go?

A Using multiple <code>const</code> statements

B Using <code>var</code> with <code>const</code>

C Using a <code>const</code> block with parentheses

D Using <code>let</code> keyword

You can declare multiple constants inside a const block using parentheses.

Which of the following is a valid constant declaration in Go?

A const greeting = "Hello"

B const number = []int{1,2,3}

C const pi = math.Pi

D const flag = true

A (string literal) and D (boolean literal) are valid constant declarations. Slices (B) and variables like math.Pi (C) cannot be used as constant initializers.

Explain what a constant is in Go and how it differs from a variable.

Think about whether the value can change after declaration.

Describe how to declare multiple constants together in Go and give an example.

Use parentheses after the const keyword.

Practice

1. What is the purpose of using const in Go programming?

easy

A. To declare a value that cannot be changed

B. To declare a variable that can change

C. To create a function

D. To start a loop

Solution

1. Step 1: Understand the role of const

   The const keyword is used to declare constants, which are fixed values that cannot be changed after being set.

2. Step 2: Compare options with the definition

   To declare a value that cannot be changed correctly states that const declares a value that cannot be changed, while other options describe unrelated concepts.

3. Final Answer:

   To declare a value that cannot be changed -> Option A

4. Quick Check:

   Constants = fixed values [OK]

Hint: Constants hold fixed values, variables do not change [OK]

Common Mistakes:

• Confusing constants with variables
• Thinking const declares functions
• Assuming const starts loops

2. Which of the following is the correct way to declare a constant named Pi with value 3.14 in Go?

easy

A. const Pi = 3.14

B. let Pi = 3.14

C. Pi := 3.14

D. var Pi = 3.14

Solution

1. Step 1: Recall Go syntax for constants

   In Go, constants are declared using the const keyword followed by the name and value.

2. Step 2: Identify the correct syntax

   const Pi = 3.14 uses const Pi = 3.14, which is the correct syntax. Options A and C declare variables, and B uses invalid Go syntax.

3. Final Answer:

   const Pi = 3.14 -> Option A

4. Quick Check:

   const keyword = correct declaration [OK]

Hint: Use const keyword for fixed values, not var or := [OK]

Common Mistakes:

• Using var or := instead of const
• Using let which is not Go syntax
• Missing the const keyword

3. What will be the output of this Go program?

package main
import "fmt"
const greeting = "Hello"
func main() {
    fmt.Println(greeting)
}

medium

A. greeting

B. Compilation error

C. Hello\n

D. Hello

Solution

1. Step 1: Understand constant usage in the program

   The constant greeting is declared with value "Hello" and printed in main.

2. Step 2: Predict the output of fmt.Println

   fmt.Println(greeting) prints the string value of the constant, which is "Hello" without quotes.

3. Final Answer:

   Hello -> Option D

4. Quick Check:

   Printing constant value = Hello [OK]

Hint: Print constant name to get its fixed value output [OK]

Common Mistakes:

• Expecting the name 'greeting' to print
• Thinking it prints with quotes
• Assuming a compile error

4. Identify the error in this Go code snippet:

package main
import "fmt"
const x = 10
func main() {
    x = 20
    fmt.Println(x)
}

medium

A. Variable 'x' not declared

B. Cannot assign to constant 'x' after declaration

C. Missing import of fmt package

D. No error, code runs fine

Solution

1. Step 1: Check constant reassignment

   The code declares x as a constant with value 10, then tries to assign 20 to it, which is not allowed.

2. Step 2: Verify other possible errors

   There are no other compilation errors; the only issue is reassignment of the constant.

3. Final Answer:

   Cannot assign to constant 'x' after declaration -> Option B

4. Quick Check:

   Constants cannot be reassigned [OK]

Hint: Constants cannot be changed after declaration [OK]

Common Mistakes:

• Trying to assign new value to a constant
• Ignoring missing imports
• Confusing constants with variables

5. Given the following Go code, what will be the output?

package main
import "fmt"
const (
    A = iota
    B
    C = 5
    D
)
func main() {
    fmt.Println(A, B, C, D)
}

hard

A. 0 1 2 3

B. 0 1 5 5

C. 0 1 5 3

D. Compilation error due to iota usage

Solution

1. Step 1: Understand iota behavior in constant blocks

   iota starts at 0 and increments by 1 for each line unless overridden.

2. Step 2: Analyze each constant value

   A = iota (0), iota increments to 1; B = iota (1), iota to 2; C = 5 (explicit), iota to 3; D = iota (3).

3. Step 3: Confirm output

   Printing A, B, C, D outputs: 0 1 5 3

4. Final Answer:

   0 1 5 3 -> Option C

5. Quick Check:

   iota increments except when overridden [OK]

Hint: iota increments unless constant value is explicitly set [OK]

Common Mistakes:

• Assuming D equals C's value (5)
• Thinking iota resets after explicit value
• Expecting compilation error