Passing values vs pointers in Go - Performance Comparison

Time Complexity: Passing values vs pointers

O(n)

Understanding Time Complexity

When we pass values or pointers in Go, it affects how much work the program does.

We want to see how the time to run changes as the data size grows.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

func processValue(data [1000]int) {
    // Simulate some processing
    for _, v := range data {
        _ = v * 2
    }
}

func processPointer(data *[1000]int) {
    // Simulate some processing
    for _, v := range *data {
        _ = v * 2
    }
}

This code shows two functions: one takes a large array by value, the other by pointer, both loop over the array.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Looping over 1000 elements in the array.
• How many times: Exactly 1000 times for each function call.
• Additional operation for value passing: Copying the entire array once when calling the function.

How Execution Grows With Input

Explain the growth pattern intuitively.

Input Size (n)	Approx. Operations
10	Copy 10 items + loop 10 times
100	Copy 100 items + loop 100 times
1000	Copy 1000 items + loop 1000 times

Pattern observation: Passing by value adds a copying step that grows with input size, while passing by pointer avoids this copy.

Final Time Complexity

Time Complexity: O(n)

This means the time grows linearly with the size of the data, mainly due to looping and copying when passing by value.

Common Mistake

[X] Wrong: "Passing by pointer always makes the code faster regardless of what happens inside the function."

[OK] Correct: Sometimes the function does more work that dominates time, so the cost of copying might be small compared to processing.

Interview Connect

Understanding how passing values or pointers affects performance helps you write efficient code and explain your choices clearly.

Self-Check

"What if the array size doubled? How would passing by value vs pointer affect the time complexity then?"

Practice

1. What happens when you pass a variable to a function by value in Go?

easy

A. The function receives a copy; changes inside do not affect the original variable.

B. The function receives the original variable; changes inside affect the original.

C. The function cannot modify the variable at all.

D. The function receives a reference but cannot change the original variable.

Solution

1. Step 1: Understand passing by value

   Passing by value means the function gets a copy of the variable, not the original.

2. Step 2: Effect of changes inside the function

   Changes to the copy do not affect the original variable outside the function.

3. Final Answer:

   The function receives a copy; changes inside do not affect the original variable. -> Option A

4. Quick Check:

   Passing by value = copy, no original change [OK]

Hint: Passing by value means function works on a copy [OK]

Common Mistakes:

• Thinking changes inside function affect original variable
• Confusing pointers with values
• Assuming function cannot modify any data

2. Which of the following is the correct way to declare a function parameter as a pointer in Go?

easy

A. func updateValue(val *int) {}

B. func updateValue(&val int) {}

C. func updateValue(val int) {}

D. func updateValue(*val int) {}

Solution

1. Step 1: Recall pointer syntax in Go function parameters

   In Go, a pointer parameter is declared by placing * before the type, e.g., *int.

2. Step 2: Identify correct syntax

   func updateValue(val *int) {} is correct syntax, with * before the type. func updateValue(val int) {} declares a value parameter. func updateValue(&val int) {} and func updateValue(*val int) {} have invalid syntax.

3. Final Answer:

   func updateValue(val *int) {} -> Option A

4. Quick Check:

   Pointer parameter syntax = *type [OK]

Hint: Pointer parameter uses * before type, not variable name [OK]

Common Mistakes:

• Putting * before variable name instead of type
• Using & in parameter declaration
• Confusing pointer syntax with value parameter

3. What is the output of this Go program?

package main
import "fmt"
func changeValue(val int) {
    val = 10
}
func main() {
    x := 5
    changeValue(x)
    fmt.Println(x)
}

medium

A. 10

B. 5

C. 0

D. Compilation error

Solution

1. Step 1: Analyze function parameter passing

   The function changeValue receives x by value, so val is a copy of x.

2. Step 2: Check if original variable changes

   Inside changeValue, val is set to 10, but this does not affect x in main.

3. Final Answer:

   5 -> Option B

4. Quick Check:

   Passing by value means original x stays 5 [OK]

Hint: Passing by value means original variable unchanged [OK]

Common Mistakes:

• Assuming x changes to 10 after function call
• Confusing pointer and value passing
• Expecting compilation error due to assignment

4. Identify the error in this Go code snippet:

func increment(num *int) {
    num = num + 1
}

func main() {
    x := 5
    increment(&x)
    fmt.Println(x)
}

medium

A. Missing & operator when calling increment.

B. x is not initialized before use.

C. Function parameter should not be a pointer.

D. Cannot add 1 to a pointer directly; need to dereference first.

Solution

1. Step 1: Understand pointer arithmetic in Go

   Go does not support arithmetic on pointers like 'num + 1'. You must dereference the pointer to change the value.

2. Step 2: Correct way to increment value pointed by pointer

   Use '*num = *num + 1' to increment the value at the pointer address.

3. Final Answer:

   Cannot add 1 to a pointer directly; need to dereference first. -> Option D

4. Quick Check:

   Pointer arithmetic invalid; dereference to modify [OK]

Hint: Use * to access pointer value before arithmetic [OK]

Common Mistakes:

• Trying to add directly to pointer variable
• Forgetting to dereference pointer
• Confusing pointer and value types

5. You want to write a function in Go that doubles the value of an integer variable passed to it. Which approach correctly modifies the original variable?

hard

A. Return the doubled value and assign it outside the function.

B. Pass the integer by value and assign double inside the function.

C. Pass a pointer to the integer and modify the value via dereferencing.

D. Use a global variable instead of passing parameters.

Solution

1. Step 1: Understand how to modify original variable

   Passing by value copies the variable, so changes inside function won't affect original.

2. Step 2: Use pointer to modify original variable

   Passing a pointer allows the function to access and change the original variable's value via dereferencing.

3. Step 3: Consider alternatives

   Returning doubled value requires assignment outside; using global variables is bad practice.

4. Final Answer:

   Pass a pointer to the integer and modify the value via dereferencing. -> Option C

5. Quick Check:

   Pointer passing modifies original variable [OK]

Hint: Use pointer to change original variable inside function [OK]

Common Mistakes:

• Assuming passing by value changes original variable
• Relying on return value without assignment
• Using global variables unnecessarily