Common pointer errors - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare a pointer to an integer.

C

int [1] ptr;

Drag options to blanks, or click blank then click option'

A*

B&

C->

D%

Attempts:

3 left

💡 Hint

Common Mistakes

Using & instead of * to declare a pointer.

Confusing pointer declaration with address-of operator.

The asterisk (*) is used to declare a pointer variable in C.

2fill in blank

medium

Complete the code to assign the address of variable x to pointer p.

C

int x = 10;
int *p;
p = [1]x;

Drag options to blanks, or click blank then click option'

A*

B&

C%

D->

Attempts:

3 left

💡 Hint

Common Mistakes

Using * instead of & when assigning address to pointer.

Trying to assign the value instead of the address.

The & operator gives the address of a variable, which can be assigned to a pointer.

3fill in blank

hard

Fix the error in the code to correctly access the value pointed by p.

C

int x = 5;
int *p = &x;
int y = [1]; 

Drag options to blanks, or click blank then click option'

Ap*

Bp

C&p

D*p

Attempts:

3 left

💡 Hint

Common Mistakes

Using the pointer variable itself instead of dereferencing it.

Using & operator which gives the address, not the value.

The * operator is used to dereference a pointer and access the value it points to.

4fill in blank

hard

Fill both blanks to create a pointer to an array and access its first element.

C

int arr[3] = {1, 2, 3};
int [1] p = arr;
int first = [2]; 

Drag options to blanks, or click blank then click option'

A*

B&

Cp[0]

D[]

Attempts:

3 left

💡 Hint

Common Mistakes

Using & instead of * to declare pointer.

Trying to dereference pointer incorrectly.

Declaring *p makes p a pointer. Accessing p[0] gets the first element of the array.

5fill in blank

hard

Fill all three blanks to correctly allocate memory, assign value, and free it.

C

#include <stdlib.h>

int *p = (int*)[1](sizeof(int));
*p = [2];
[3](p);

Drag options to blanks, or click blank then click option'

Amalloc

B42

Cfree

Dsizeof

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting to cast malloc's return value.

Not freeing allocated memory causing memory leaks.

Assigning value to pointer instead of dereferenced pointer.

malloc allocates memory, 42 is assigned to the allocated space, and free releases the memory.

Practice

1. What is a common mistake when using pointers in C?

easy

A. Assigning a valid memory address to a pointer

B. Declaring pointers with the correct syntax

C. Using uninitialized pointers before assigning them an address

D. Using pointers to access array elements correctly

Solution

1. Step 1: Understand pointer initialization

   Pointers must be assigned a valid memory address before use to avoid undefined behavior.

2. Step 2: Identify common mistakes

   Using a pointer without initialization can cause crashes or unexpected results.

3. Final Answer:

   Using uninitialized pointers before assigning them an address -> Option C

4. Quick Check:

   Uninitialized pointer use = common error [OK]

Hint: Always initialize pointers before use to avoid errors [OK]

Common Mistakes:

• Dereferencing NULL pointers
• Forgetting to initialize pointers
• Confusing pointer and value assignment

2. Which of the following pointer declarations is syntactically correct in C?

easy

A. *int ptr;

B. int ptr*;

C. int &ptr;

D. int *ptr;

Solution

1. Step 1: Review pointer declaration syntax

   In C, the asterisk (*) goes between the type and the pointer name to declare a pointer.

2. Step 2: Check each option

   int *ptr; correctly declares a pointer to int. Options A, B, and D have invalid syntax.

3. Final Answer:

   int *ptr; -> Option D

4. Quick Check:

   Correct pointer syntax = int *ptr; [OK]

Hint: Pointer syntax: type *name; always asterisk before name [OK]

Common Mistakes:

• Placing * after variable name
• Using & instead of * for pointers
• Incorrect order of type and *

3. What will be the output of the following C code?

int x = 10;
int *p = &x;
*p = 20;
printf("%d", x);

medium

A. 10

B. 20

C. Garbage value

D. Compilation error

Solution

1. Step 1: Understand pointer assignment

   Pointer p points to variable x. Changing *p changes x's value.

2. Step 2: Trace the code execution

   *p = 20 sets x to 20. printf prints x, so output is 20.

3. Final Answer:

   20 -> Option B

4. Quick Check:

   Pointer dereference changes value = 20 [OK]

Hint: Changing *p changes the value pointed to [OK]

Common Mistakes:

• Confusing pointer address and value
• Expecting original value unchanged
• Ignoring pointer dereference effect

4. Identify the error in the following code snippet:

int *p;
*p = 5;
printf("%d", *p);

medium

A. Pointer p is not initialized before dereferencing

B. Syntax error in pointer declaration

C. Incorrect printf format specifier

D. No error, code runs fine

Solution

1. Step 1: Check pointer initialization

   Pointer p is declared but not assigned an address before use.

2. Step 2: Understand dereferencing consequences

   Dereferencing uninitialized pointer causes undefined behavior or crash.

3. Final Answer:

   Pointer p is not initialized before dereferencing -> Option A

4. Quick Check:

   Uninitialized pointer dereference = error [OK]

Hint: Always assign pointer before dereferencing [OK]

Common Mistakes:

• Assuming declaration initializes pointer
• Ignoring runtime crashes from bad pointers
• Misreading error as printf issue

5. Consider the code below. What is the safest way to avoid a common pointer error here?

int *p = NULL;
// Some code
printf("%d", *p);

hard

A. Check if p is NULL before dereferencing

B. Assign p to a random memory address

C. Remove the pointer and use an int variable instead

D. Ignore the NULL and dereference directly

Solution

1. Step 1: Understand NULL pointer risks

   Dereferencing NULL pointer causes runtime crash or undefined behavior.

2. Step 2: Apply safe pointer usage

   Always check if pointer is NULL before dereferencing to avoid errors.

3. Final Answer:

   Check if p is NULL before dereferencing -> Option A

4. Quick Check:

   NULL check before dereference = safe code [OK]

Hint: Always test pointer != NULL before use [OK]

Common Mistakes:

• Dereferencing NULL pointer causing crash
• Assigning invalid addresses
• Ignoring pointer safety checks