Pointer declaration - Time & Space Complexity

Time Complexity: Pointer declaration

O(1)

Understanding Time Complexity

When we declare pointers in C, it's important to understand how this affects the program's steps.

We want to see how the program's work changes as we use pointers.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

int main() {
    int x = 10;
    int *p = &x;
    *p = 20;
    return 0;
}
    

This code declares a pointer to an integer, assigns it the address of a variable, and changes the variable's value through the pointer.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Simple variable assignment and pointer usage.
• How many times: Each operation runs only once, no loops or repeated steps.

How Execution Grows With Input

Since there are no loops or repeated steps, the work stays the same no matter the input size.

Input Size (n)	Approx. Operations
10	4
100	4
1000	4

Pattern observation: The number of steps does not increase as input size grows.

Final Time Complexity

Time Complexity: O(1)

This means the program does a fixed amount of work regardless of input size.

Common Mistake

[X] Wrong: "Using pointers makes the program slower because it adds extra steps."

[OK] Correct: Declaring and using a pointer is just like using a variable; it does not add repeated work or loops.

Interview Connect

Understanding how pointers work and their time cost helps you write clear and efficient code, a skill valued in many programming tasks.

Self-Check

"What if we added a loop that uses the pointer to change multiple values? How would the time complexity change?"

Practice

1. What does the following declaration mean in C?
int *ptr;

easy

A. ptr is a pointer to a character variable

B. ptr is a pointer to an integer variable

C. ptr is an integer variable

D. ptr is a function returning an integer

Solution

1. Step 1: Understand the * symbol in declaration

   The * before ptr means ptr is a pointer variable, not a normal integer.

2. Step 2: Check the type before *

   The type before * is int, so ptr points to an integer variable.

3. Final Answer:

   ptr is a pointer to an integer variable -> Option B

4. Quick Check:

   int *ptr means pointer to int [OK]

Hint: Look for * before variable name to identify pointer [OK]

Common Mistakes:

• Thinking * means multiplication here
• Confusing pointer with normal variable
• Ignoring the type before *

2. Which of the following is the correct way to declare a pointer to a float variable in C?

easy

A. ptr float*;

B. float ptr*;

C. *float ptr;

D. float *ptr;

Solution

1. Step 1: Recall pointer declaration syntax

   Pointer declaration syntax is: type *name; so the * goes between type and variable name.

2. Step 2: Check each option

   float *ptr; matches correct syntax: float *ptr; others have wrong order or missing parts.

3. Final Answer:

   float *ptr; -> Option D

4. Quick Check:

   Pointer syntax = type *name; [OK]

Hint: Always put * between type and variable name [OK]

Common Mistakes:

• Placing * after variable name
• Putting * before type
• Missing * entirely

3. What is the output of this C code?

int x = 10;
int *p = &x;
printf("%d", *p);

medium

A. 10

B. Compilation error

C. Garbage value

D. Address of x

Solution

1. Step 1: Understand pointer initialization

   p is assigned the address of x using &x, so p points to x.

2. Step 2: Dereference pointer to get value

   *p means value at address p, which is x's value 10.

3. Final Answer:

   10 -> Option A

4. Quick Check:

   *p gives value at pointer address [OK]

Hint: Use * to get value from pointer address [OK]

Common Mistakes:

• Printing pointer p instead of *p
• Confusing address and value
• Forgetting & to get address

4. Identify the error in this pointer declaration:

int *p;
p = 5;

medium

A. Error: assigning integer directly to pointer

B. Correct: p points to integer 5

C. Error: missing * in assignment

D. Error: pointer must be initialized with & operator

Solution

1. Step 1: Check pointer assignment rules

   A pointer must hold a memory address, not a direct integer value.

2. Step 2: Analyze given assignment

   Assigning 5 directly to pointer p is invalid; it should be an address like &variable.

3. Final Answer:

   Error: assigning integer directly to pointer -> Option A

4. Quick Check:

   Pointer = address, not direct value [OK]

Hint: Pointers store addresses, never assign raw values directly [OK]

Common Mistakes:

• Assigning integer instead of address
• Forgetting & operator for address
• Confusing pointer and normal variable

5. Given the code below, which pointer declaration correctly allows modifying the value of num through the pointer?

int num = 20;
// Which declaration is correct?

hard

A. int *ptr = num;

B. int ptr = #

C. int *ptr = #

D. int &ptr = num;

Solution

1. Step 1: Understand pointer declaration for modifying variable

   To modify num via pointer, pointer must hold address of num, declared as int *ptr.

2. Step 2: Check each option

   int *ptr = # correctly declares pointer to int and assigns address of num. Others are invalid syntax or wrong types.

3. Final Answer:

   int *ptr = # -> Option C

4. Quick Check:

   Pointer to int = &variable for modification [OK]

Hint: Use int *ptr = &var to modify var via pointer [OK]

Common Mistakes:

• Assigning value instead of address
• Using wrong type for pointer
• Confusing references with pointers