Practical use cases in Go - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Go code using goroutines?

Consider the following Go program that launches goroutines to print numbers. What will it print?

Go

package main

import (
	"fmt"
	"time"
)

func main() {
	for i := 0; i < 3; i++ {
		go func(i int) {
			fmt.Print(i, " ")
		}(i)
	}
	time.Sleep(100 * time.Millisecond)
}

A 3 3 3

B Compilation error

C 0 0 0

D 0 1 2

💡 Hint

Think about when the goroutines capture the variable i.

The goroutines capture the variable i by reference, not by value. By the time they run, the loop has finished and i is 3, so all print 3.

❓ Predict Output

intermediate

What does this Go code print when using channels?

Examine this Go program that uses channels to communicate between goroutines. What is printed?

Go

package main

import "fmt"

func main() {
	ch := make(chan int, 2)
	ch <- 1
	ch <- 2
	fmt.Println(<-ch)
	fmt.Println(<-ch)
}

A 2\n1

B 1\n2

C Compilation error: channel buffer size mismatch

D Deadlock error at runtime

💡 Hint

Think about how buffered channels work and the order of sends and receives.

The channel has a buffer size of 2, so both sends succeed immediately. The receives happen in the same order, so it prints 1 then 2.

🔧 Debug

advanced

Why does this Go code cause a deadlock?

Look at this Go program. Why does it cause a deadlock at runtime?

Go

package main

func main() {
	ch := make(chan int)
	ch <- 5
	<-ch
}

A Because the channel is unbuffered and no goroutine is receiving when sending

B Because the channel buffer is full

C Because the receive happens before the send

D Because the channel is closed before sending

💡 Hint

Think about how unbuffered channels block on send and receive.

An unbuffered channel blocks the sender until a receiver is ready. Here, the send happens in main goroutine but no other goroutine receives, so it deadlocks.

❓ Predict Output

advanced

What is the output of this Go code using select with multiple channels?

Consider this Go program using select to receive from two channels. What will it print?

Go

package main

import (
	"fmt"
	"time"
)

func main() {
	ch1 := make(chan string)
	ch2 := make(chan string)

	go func() {
		time.Sleep(50 * time.Millisecond)
		ch1 <- "first"
	}()

	go func() {
		time.Sleep(10 * time.Millisecond)
		ch2 <- "second"
	}()

	select {
	case msg1 := <-ch1:
		fmt.Println(msg1)
	case msg2 := <-ch2:
		fmt.Println(msg2)
	}
}

A Compilation error: select cases invalid

B first

C second

D Deadlock error at runtime

💡 Hint

Which channel sends first? select picks a ready channel.

The goroutine sending "second" sleeps less, so ch2 is ready first. The select receives from ch2 and prints "second".

🧠 Conceptual

expert

What is the main advantage of using Go's context package in concurrent programs?

Why do Go programmers use the context package when working with concurrent operations?

A To manage cancellation, deadlines, and passing request-scoped values across goroutines

B To improve the speed of goroutines by parallelizing CPU instructions

C To automatically recover from panics in goroutines without crashing the program

D To serialize data structures for network communication

💡 Hint

Think about how to stop or control multiple goroutines working on the same task.

The context package helps control cancellation signals, deadlines, and passing values safely between goroutines, improving coordination.

Practice

1. Which Go feature is commonly used to create a simple web server that listens on a port?

easy

A. The net/http package with http.ListenAndServe

B. The fmt package with Println

C. The os package with Open

D. The math package with Pow

Solution

1. Step 1: Identify the package for web servers

   The net/http package provides tools to create web servers in Go.

2. Step 2: Recognize the function to start the server

   http.ListenAndServe starts a server listening on a specified port.

3. Final Answer:

   The net/http package with http.ListenAndServe -> Option A

4. Quick Check:

   Web server = net/http.ListenAndServe [OK]

Hint: Web servers use net/http.ListenAndServe to listen on ports [OK]

Common Mistakes:

• Confusing printing functions with server functions
• Using file operations instead of network functions
• Thinking math package handles networking

2. Which of the following is the correct syntax to open a file named "data.txt" for reading in Go?

easy

A. file, err := os.Open("data.txt")

B. file := os.OpenFile("data.txt", os.O_WRONLY)

C. file, err := ioutil.ReadFile("data.txt")

D. file := os.Read("data.txt")

Solution

1. Step 1: Identify the function to open a file for reading

   os.Open opens a file in read-only mode and returns a file and error.

2. Step 2: Check the syntax correctness

   The syntax file, err := os.Open("data.txt") correctly assigns both file and error.

3. Final Answer:

   file, err := os.Open("data.txt") -> Option A

4. Quick Check:

   Open file for reading = os.Open [OK]

Hint: Use os.Open with two variables to open files safely [OK]

Common Mistakes:

• Using os.Read which does not exist
• Using ioutil.ReadFile which reads whole file, not opens
• Opening file with wrong flags for reading

3. What will be the output of this Go code snippet?

package main
import "fmt"
func main() {
    nums := []int{2, 4, 6}
    squares := make(map[int]int)
    for _, n := range nums {
        squares[n] = n * n
    }
    fmt.Println(squares)
}

medium

A. map[2:4 4:8 6:12]

B. map[2:4 4:16 6:36]

C. map[2:8 4:16 6:36]

D. map[2:2 4:4 6:6]

Solution

1. Step 1: Understand the loop and map assignment

   The loop goes through each number in nums and assigns squares[n] = n * n.

2. Step 2: Calculate each square value

   2*2=4, 4*4=16, 6*6=36, so the map stores these pairs.

3. Final Answer:

   map[2:4 4:16 6:36] -> Option B

4. Quick Check:

   Squares map = map with keys and squared values [OK]

Hint: Map keys with squares store n:n*n pairs [OK]

Common Mistakes:

• Confusing keys and values in map
• Multiplying incorrectly
• Expecting slice output instead of map

4. This Go code tries to read a file but has an error. What is the problem?

package main
import (
    "fmt"
    "os"
)
func main() {
    file := os.Open("file.txt")
    defer file.Close()
    fmt.Println("File opened")
}

medium

A. File name should be absolute path

B. Using defer incorrectly

C. Missing error check after os.Open

D. fmt.Println cannot print strings

Solution

1. Step 1: Check os.Open usage

   os.Open returns two values: file and error. The code only assigns one variable.

2. Step 2: Identify missing error handling

   Without checking error, the program may panic if file does not open.

3. Final Answer:

   Missing error check after os.Open -> Option C

4. Quick Check:

   Always check error from os.Open [OK]

Hint: Always assign and check error from os.Open [OK]

Common Mistakes:

• Ignoring error returned by os.Open
• Assuming defer is wrong here
• Thinking file path must be absolute always

5. You want to create a Go program that reads a list of numbers from a file, squares each number, and writes the results to a new file. Which approach best combines Go features for this task?

hard

A. Use fmt.Println to read and write files directly

B. Use net/http to read files and fmt.Scan to write

C. Use math.Pow to read files and os.Remove to write

D. Use os.Open to read, loop to square, and os.Create with WriteString to save

Solution

1. Step 1: Reading numbers from a file

   os.Open opens the file for reading; then you can scan or read lines.

2. Step 2: Processing and writing results

   Loop through numbers, square them, then use os.Create to create output file and WriteString or similar to write results.

3. Final Answer:

   Use os.Open to read, loop to square, and os.Create with WriteString to save -> Option D

4. Quick Check:

   File read + process + write = os.Open + loop + os.Create [OK]

Hint: Read with os.Open, write with os.Create, process in loop [OK]

Common Mistakes:

• Using fmt.Println for file I/O
• Confusing math.Pow with file reading
• Using network packages for file tasks