Function declaration in Go - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of a function returning multiple values

What is the output of this Go program?

Go

package main
import "fmt"

func divideAndRemainder(a, b int) (int, int) {
    return a / b, a % b
}

func main() {
    q, r := divideAndRemainder(17, 5)
    fmt.Println(q, r)
}

A 3 1

B 4 2

C 3 2

D Error: multiple return values not handled

💡 Hint

Remember that 17 divided by 5 is 3 with remainder 2.

The function divides 17 by 5 returning quotient 3 and remainder 2. The main function prints these values.

❓ Predict Output

intermediate

Output of a function with named return values

What does this Go program print?

Go

package main
import "fmt"

func swap(x, y string) (a, b string) {
    a = y
    b = x
    return
}

func main() {
    a, b := swap("hello", "world")
    fmt.Println(a, b)
}

A world world

B hello world

C hello hello

D world hello

💡 Hint

Named return values are assigned inside the function before return.

The function swaps the two strings by assigning named return variables and returns them. So output is 'world hello'.

❓ Predict Output

advanced

Output of recursive function calculating factorial

What is the output of this Go program?

Go

package main
import "fmt"

func factorial(n int) int {
    if n <= 1 {
        return 1
    }
    return n * factorial(n-1)
}

func main() {
    fmt.Println(factorial(5))
}

A 120

B 24

C Error: stack overflow

D 5

💡 Hint

Factorial of 5 is 5*4*3*2*1.

The recursive function correctly calculates factorial by multiplying n by factorial of n-1 until base case 1.

❓ Predict Output

advanced

Output of function with variadic parameters

What does this Go program print?

Go

package main
import "fmt"

func sum(nums ...int) int {
    total := 0
    for _, n := range nums {
        total += n
    }
    return total
}

func main() {
    fmt.Println(sum(1, 2, 3, 4))
}

A 10

B 24

C Error: cannot use ... with int

D 0

💡 Hint

Sum all numbers passed as arguments.

The variadic function sums all integers passed and returns the total 10.

❓ Predict Output

expert

Output of function returning a closure capturing variable

What is the output of this Go program?

Go

package main
import "fmt"

func makeCounter() func() int {
    count := 0
    return func() int {
        count++
        return count
    }
}

func main() {
    counter := makeCounter()
    fmt.Println(counter())
    fmt.Println(counter())
    fmt.Println(counter())
}

A Error: cannot return closure

B

1
2
3

C

1
1
1

D

0
1
2

💡 Hint

Closures remember the variable state between calls.

The returned function increments and returns count each time, producing 1, 2, 3.

Practice

1. What keyword is used to declare a function in Go?

easy

A. func

B. function

C. def

D. fun

Solution

1. Step 1: Recall Go function syntax

   In Go, functions are declared using the keyword func.

2. Step 2: Compare options

   Only func uses func, which is correct for Go.

3. Final Answer:

   func -> Option A

4. Quick Check:

   Function declaration keyword = func [OK]

Hint: Remember: Go uses 'func' to start a function [OK]

Common Mistakes:

• Using 'function' like JavaScript
• Using 'def' like Python
• Using 'fun' which is invalid

2. Which of the following is the correct syntax to declare a function named add that takes two integers and returns an integer?

easy

A. function add(a int, b int) int {}

B. func add(int a, int b) int {}

C. func add(a, b int) {}

D. func add(a int, b int) int {}

Solution

1. Step 1: Understand Go function parameter syntax

   In Go, parameters are declared as name type. So a int, b int is correct.

2. Step 2: Check return type and function keyword

   Function must start with func and specify return type int. func add(a int, b int) int {} matches this exactly.

3. Final Answer:

   func add(a int, b int) int {} -> Option D

4. Quick Check:

   Parameter syntax = name type [OK]

Hint: Parameters: name then type, return type after parentheses [OK]

Common Mistakes:

• Putting type before name in parameters
• Using 'function' keyword instead of 'func'
• Omitting return type when needed

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

package main
import "fmt"
func multiply(x int, y int) int {
    return x * y
}
func main() {
    fmt.Println(multiply(3, 4))
}

medium

A. 12

B. 7

C. 34

D. Error

Solution

1. Step 1: Understand function call and return

   The function multiply takes two integers and returns their product.

2. Step 2: Calculate multiply(3, 4)

   3 * 4 = 12, so the function returns 12, which is printed.

3. Final Answer:

   12 -> Option A

4. Quick Check:

   3 times 4 = 12 [OK]

Hint: Multiply parameters and print result [OK]

Common Mistakes:

• Adding instead of multiplying
• Concatenating numbers as strings
• Expecting syntax error due to function

4. Identify the error in this Go function declaration:

func greet(name string) string {
    return "Hello, " + name

medium

A. Incorrect function keyword

B. Wrong parameter type

C. Missing closing brace '}'

D. Missing return type

Solution

1. Step 1: Check function syntax

   The function starts with func, has parameter and return type correctly.

2. Step 2: Look for missing parts

   The function body is missing the closing brace } to end the function.

3. Final Answer:

   Missing closing brace '}' -> Option C

4. Quick Check:

   Every function must end with '}' [OK]

Hint: Count opening and closing braces carefully [OK]

Common Mistakes:

• Forgetting to close function body with '}'
• Confusing parameter types
• Omitting return type

5. How would you declare a Go function named filterEven that takes a slice of integers and returns a new slice containing only even numbers?

hard

A. func filterEven(nums int) int {
var evens int
for _, n := range nums {
if n%2 == 0 {
evens += n
}
}
return evens
}

B. func filterEven(nums []int) []int {
var evens []int
for _, n := range nums {
if n%2 == 0 {
evens = append(evens, n)
}
}
return evens
}

C. func filterEven(nums []int) {
for _, n := range nums {
if n%2 == 0 {
fmt.Println(n)
}
}
}

D. func filterEven(nums []int) []int {
for n := range nums {
if n%2 == 0 {
return nums
}
}
return nil
}

Solution

1. Step 1: Check parameter and return types

   The function must take a slice of integers []int and return a slice of integers []int. func filterEven(nums []int) []int { var evens []int for _, n := range nums { if n%2 == 0 { evens = append(evens, n) } } return evens } matches this.

2. Step 2: Verify logic to filter even numbers

   func filterEven(nums []int) []int { var evens []int for _, n := range nums { if n%2 == 0 { evens = append(evens, n) } } return evens } creates a new slice, loops over input, appends even numbers, and returns the new slice. This is correct.

3. Final Answer:

   func filterEven(nums []int) []int { var evens []int for _, n := range nums { if n%2 == 0 { evens = append(evens, n) } } return evens } -> Option B

4. Quick Check:

   Filter slice by appending evens [OK]

Hint: Use slice, loop, append evens, return new slice [OK]

Common Mistakes:

• Wrong parameter or return types
• Returning inside loop prematurely
• Printing instead of returning filtered slice