Zero values in Go - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of uninitialized variables

What is the output of this Go program?

Go

package main
import "fmt"
func main() {
    var a int
    var b string
    var c bool
    fmt.Printf("%d %q %t", a, b, c)
}

A 0 "" false

B 0 "0" false

C 0 "nil" false

D 0 "" true

💡 Hint

Think about what Go assigns to variables when you don't set them.

In Go, uninitialized variables get their zero values. For int it's 0, for string it's an empty string "", and for bool it's false.

❓ Predict Output

intermediate

Zero value of a slice

What will this Go program print?

Go

package main
import "fmt"
func main() {
    var s []int
    fmt.Println(s == nil)
    fmt.Println(len(s))
}

A

false
0

B

true
-1

C

true
0

D

false
-1

💡 Hint

Slices have a zero value of nil and length zero.

The zero value of a slice is nil. So s == nil is true. The length of a nil slice is 0.

❓ Predict Output

advanced

Zero value of a map and its behavior

What is the output of this Go program?

Go

package main
import "fmt"
func main() {
    var m map[string]int
    fmt.Println(m == nil)
    fmt.Println(m["key"])
    m["key"] = 10
}

A

false
0
panic: assignment to entry in nil map

B

false
0
no panic

C

true
0
no panic

D

true
0
panic: assignment to entry in nil map

💡 Hint

Maps have zero value nil, reading from nil map is allowed, writing causes panic.

The zero value of a map is nil. Reading from a nil map returns the zero value of the element type (0 here). But writing to a nil map causes a runtime panic.

❓ Predict Output

advanced

Zero value of a struct with embedded fields

What will this Go program print?

Go

package main
import "fmt"
type Inner struct {
    X int
}
type Outer struct {
    Inner
    Y string
}
func main() {
    var o Outer
    fmt.Printf("%d %q", o.X, o.Y)
}

A 0 ""

B 0 "nil"

C 0 "0"

D panic at runtime

💡 Hint

Embedded structs get zero values too.

The embedded struct Inner is zero-initialized, so o.X is 0. The string Y is empty string "".

❓ Predict Output

expert

Zero value of interface and type assertion

What is the output of this Go program?

Go

package main
import "fmt"
func main() {
    var i interface{}
    fmt.Println(i == nil)
    v, ok := i.(int)
    fmt.Println(v, ok)
}

A

false
0 false

B

true
0 false

C

true
0 true

D

false
0 true

💡 Hint

An uninitialized interface is nil; type assertion fails safely.

The zero value of an interface is nil. So i == nil is true. The type assertion i.(int) fails, so ok is false and v is zero value of int (0).

Practice

1. What is the zero value of an int variable in Go?

easy

A. "" (empty string)

B. nil

C. 0

D. false

Solution

1. Step 1: Understand zero values for basic types

   In Go, each type has a default zero value when not initialized.

2. Step 2: Identify zero value for int type

   The zero value for int is the number 0.

3. Final Answer:

   0 -> Option C

4. Quick Check:

   int zero value = 0 [OK]

Hint: Remember: int zero value is always 0 [OK]

Common Mistakes:

• Confusing int zero value with nil
• Thinking zero value is empty string
• Mixing boolean zero value with int

2. Which of the following is the correct way to declare a variable with its zero value in Go?

easy

A. var x int = false

B. var x int = nil

C. var x int = ""

D. var x int

Solution

1. Step 1: Recall Go variable declaration syntax

   Declaring a variable without initialization sets it to its zero value.

2. Step 2: Check each option for correctness

   var x int declares x as int without initialization, so it gets zero value 0. Other options assign invalid values for int.

3. Final Answer:

   var x int -> Option D

4. Quick Check:

   Declare without init = zero value [OK]

Hint: Declare without assignment to get zero value [OK]

Common Mistakes:

• Assigning nil to int
• Assigning string to int
• Assigning boolean to int

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

package main
import "fmt"
func main() {
    var b bool
    fmt.Println(b)
}

medium

A. false

B. Compilation error

C. nil

D. true

Solution

1. Step 1: Identify the variable type and initialization

   Variable b is declared as bool without explicit initialization.

2. Step 2: Recall zero value for bool type

   The zero value for bool is false, so b is false by default.

3. Final Answer:

   false -> Option A

4. Quick Check:

   bool zero value = false [OK]

Hint: Uninitialized bool defaults to false [OK]

Common Mistakes:

• Expecting true as default
• Thinking nil is valid bool
• Assuming compilation error

4. Identify the error in this Go code snippet related to zero values:

package main
func main() {
    var s string = nil
}

medium

A. Assigning nil to string causes a compile error

B. String zero value should be 0

C. Variable s should be declared as int

D. No error, code runs fine

Solution

1. Step 1: Understand zero value for string type

   Zero value for string is an empty string "", not nil.

2. Step 2: Check assignment validity

   Assigning nil to a string variable causes a compile-time error because nil is not a valid string value.

3. Final Answer:

   Assigning nil to string causes a compile error -> Option A

4. Quick Check:

   string zero value = "" not nil [OK]

Hint: Strings cannot be nil, use empty string "" [OK]

Common Mistakes:

• Using nil for string
• Confusing zero value with nil
• Ignoring compile errors

5. You want to create a map in Go that stores string keys and integer values. You declare it as:

var m map[string]int

What is the zero value of m and what must you do before adding elements?

hard

A. m is zero; no initialization needed, just assign values

B. m is nil; you must initialize it with make before adding elements

C. m is an empty map; you can add elements immediately

D. m is a map with default zero values for keys and values

Solution

1. Step 1: Identify zero value of map type

   Maps declared without initialization have zero value nil.

2. Step 2: Understand map usage requirements

   You cannot add elements to a nil map; you must initialize it using make(map[string]int) before adding keys and values.

3. Final Answer:

   m is nil; you must initialize it with make before adding elements -> Option B

4. Quick Check:

   Maps zero value = nil; initialize with make [OK]

Hint: Maps start nil; use make() before adding keys [OK]

Common Mistakes:

• Trying to add to nil map
• Assuming map is empty but usable
• Confusing zero value with empty map