Creating custom packages in Go - Performance & Efficiency

Time Complexity: Creating custom packages

O(n)

Understanding Time Complexity

When we create custom packages in Go, we want to know how the time it takes to run our code changes as we use these packages more.

We ask: How does adding package code affect the speed of our program?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

package mathutils

func Sum(numbers []int) int {
    total := 0
    for _, num := range numbers {
        total += num
    }
    return total
}

This package function sums all numbers in a list by looping through each one once.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Looping through each number in the input list.
• How many times: Exactly once for each number in the list.

How Execution Grows With Input

As the list gets bigger, the time to add all numbers grows in a straight line.

Input Size (n)	Approx. Operations
10	10 additions
100	100 additions
1000	1000 additions

Pattern observation: Doubling the list size doubles the work needed.

Final Time Complexity

Time Complexity: O(n)

This means the time to run grows directly with the number of items we add.

Common Mistake

[X] Wrong: "Using a package always makes code slower because it adds extra steps."

[OK] Correct: The package code runs just like any other code, so its speed depends on what it does, not just that it is a package.

Interview Connect

Understanding how your package code runs helps you write clear and efficient programs, a skill that shows you know how to build good software.

Self-Check

"What if the Sum function used recursion instead of a loop? How would the time complexity change?"

Practice

1. What is the main purpose of creating a custom package in Go?

easy

A. To organize code into reusable parts

B. To make the program run faster

C. To avoid using functions

D. To write code without imports

Solution

1. Step 1: Understand the role of packages

   Packages help group related code together for reuse and clarity.

2. Step 2: Identify the main benefit

   Custom packages allow you to reuse code in different parts of your program or in other programs.

3. Final Answer:

   To organize code into reusable parts -> Option A

4. Quick Check:

   Custom packages = reusable code [OK]

Hint: Packages group code for reuse and clarity [OK]

Common Mistakes:

• Thinking packages speed up code execution
• Believing packages remove the need for functions
• Assuming packages eliminate imports

2. Which of the following is the correct way to export a function from a custom package in Go?

easy

A. func MyFunction() {}

B. func my_function() {}

C. func _MyFunction() {}

D. func myFunction() {}

Solution

1. Step 1: Recall Go export rules

   In Go, only names starting with an uppercase letter are exported (visible outside the package).

2. Step 2: Check function names

   Only 'MyFunction' starts with uppercase, so it is exported.

3. Final Answer:

   func MyFunction() {} -> Option A

4. Quick Check:

   Uppercase start = exported function [OK]

Hint: Exported names start with uppercase letters [OK]

Common Mistakes:

• Using lowercase to export functions
• Using underscores to export
• Assuming snake_case exports functions

3. Given a package named mathutil with an exported function Square, what is the output of this code?

package main

import (
  "fmt"
  "mathutil"
)

func main() {
  fmt.Println(mathutil.Square(4))
}

medium

A. Square(4)

B. 16

C. Error: undefined mathutil

D. 0

Solution

1. Step 1: Understand the function call

   The code calls mathutil.Square(4), which should return 4 squared (4*4).

2. Step 2: Calculate the expected output

   4 multiplied by 4 equals 16, so the output is 16.

3. Final Answer:

   16 -> Option B

4. Quick Check:

   Square(4) = 16 [OK]

Hint: Square means multiply number by itself [OK]

Common Mistakes:

• Assuming function name prints as string
• Thinking import fails without path
• Expecting zero as default output

4. What is wrong with this custom package usage?

package main

import (
  "fmt"
  "mypkg"
)

func main() {
  fmt.Println(mypkg.helper())
}

Assuming helper is a function in mypkg.

medium

A. Package name must be Main to import

B. fmt.Println cannot print function results

C. Import path must be a URL

D. Function name should start with uppercase to be accessible

Solution

1. Step 1: Check function visibility rules

   In Go, only functions starting with uppercase letters are exported and accessible outside their package.

2. Step 2: Identify the function name issue

   The function is named 'helper' (lowercase), so it is not accessible from main package.

3. Final Answer:

   Function name should start with uppercase to be accessible -> Option D

4. Quick Check:

   Lowercase function = not exported [OK]

Hint: Exported functions start uppercase [OK]

Common Mistakes:

• Thinking package name must be Main
• Believing import path must be URL
• Assuming fmt.Println can't print function output

5. You want to create a custom package stringutil with a function Reverse that reverses a string. Which of the following is the correct way to define and use it in your main.go?

hard

A. In stringutil package: func Reverse(s string) string { /* code */ }In main.go: import "stringutil" and call Reverse("hello")

B. In stringutil package: func reverse(s string) string { /* code */ }In main.go: import "stringutil" and call stringutil.reverse("hello")

C. In stringutil package: func Reverse(s string) string { /* code */ }In main.go: import "stringutil" and call stringutil.Reverse("hello")

D. In stringutil package: func Reverse(s string) string { /* code */ }In main.go: no import needed, call Reverse("hello")

Solution

1. Step 1: Define exported function in package

   The function must start with uppercase 'Reverse' to be accessible outside the package.

2. Step 2: Import package and call function with package name

   In main.go, import "stringutil" and call the function as stringutil.Reverse("hello").

3. Final Answer:

   Define func Reverse and call stringutil.Reverse("hello") after import -> Option C

4. Quick Check:

   Uppercase function + import + package prefix = correct usage [OK]

Hint: Exported func uppercase + import + package.func call [OK]

Common Mistakes:

• Using lowercase function name and expecting export
• Calling function without package prefix after import
• Not importing the custom package