Pointers and arrays in C - 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.

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

2fill in blank

medium

Complete the code to access the first element of the array using a pointer.

C

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

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

Aarr[1]

Bp[1]

C&arr[0]

D*p

Attempts:

3 left

💡 Hint

Common Mistakes

Using p[1] which accesses the second element, not the first.

The expression *p dereferences the pointer to get the first element of the array.

3fill in blank

hard

Fix the error in the pointer arithmetic to access the third element.

C

int arr[4] = {10, 20, 30, 40};
int *p = arr;
int val = *[1];

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

Ap - 2

Bp + 2

Carr + 3

D&p + 2

Attempts:

3 left

💡 Hint

Common Mistakes

Using p - 2 which moves pointer backwards.

Adding 2 to pointer p moves it to the third element, then dereferencing gets its value.

4fill in blank

hard

Fill both blanks to create a pointer to the second element and get its value.

C

int nums[5] = {5, 10, 15, 20, 25};
int *ptr = nums [1] 1;
int value = *[2];

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

A+

B-

Cptr

Dnums

Attempts:

3 left

💡 Hint

Common Mistakes

Using '-' instead of '+' to move forward in the array.

Adding 1 to the array name points to the second element. Dereferencing ptr gets its value.

5fill in blank

hard

Fill all three blanks to create a pointer, move it, and access the value correctly.

C

int data[4] = {2, 4, 6, 8};
int *p = [1];
p = p [2] 3;
int val = *[3];

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

Adata

B+

Cp

D&data

Attempts:

3 left

💡 Hint

Common Mistakes

Using '&data' which is pointer to the whole array, not first element.

Pointer p starts at data, moves forward 3 elements, then dereferenced to get the value.

Practice

1. What does a pointer to an array in C typically store?

easy

A. The address of the first element of the array

B. The total number of elements in the array

C. The sum of all elements in the array

D. The value of the last element in the array

Solution

1. Step 1: Understand pointer basics

   A pointer stores a memory address, not values or counts.

2. Step 2: Pointer to array points to first element

   In C, a pointer to an array points to the first element's address.

3. Final Answer:

   The address of the first element of the array -> Option A

4. Quick Check:

   Pointer = first element address [OK]

Hint: Pointers hold addresses, not values or counts [OK]

Common Mistakes:

• Thinking pointer stores array size
• Confusing pointer with array value
• Assuming pointer stores last element

2. Which of the following is the correct way to declare a pointer to an integer array in C?

easy

A. int ptr[];

B. int *ptr;

C. int &ptr;

D. int ptr*;

Solution

1. Step 1: Recall pointer declaration syntax

   In C, a pointer to int is declared as int *ptr;

2. Step 2: Check other options for syntax errors

   int ptr[] declares an array, not a pointer; int &ptr is invalid in C; int ptr* is incorrect syntax.

3. Final Answer:

   int *ptr; -> Option B

4. Quick Check:

   Pointer syntax uses * before variable [OK]

Hint: Pointer declaration uses * before variable name [OK]

Common Mistakes:

• Using array syntax instead of pointer
• Confusing C++ reference (&) with pointer
• Placing * after variable name incorrectly

3. What is the output of the following C code?

int arr[] = {10, 20, 30};
int *p = arr;
printf("%d", *(p + 1));

medium

A. 10

B. 30

C. Garbage value

D. 20

Solution

1. Step 1: Understand pointer arithmetic

   p points to arr[0], so p + 1 points to arr[1].

2. Step 2: Dereference pointer at p + 1

   *(p + 1) gives value at arr[1], which is 20.

3. Final Answer:

   20 -> Option D

4. Quick Check:

   *(p+1) = arr[1] = 20 [OK]

Hint: Pointer + 1 moves to next array element [OK]

Common Mistakes:

• Confusing *(p+1) with *p
• Assuming pointer arithmetic adds 1 byte
• Expecting arr[0] value instead

4. Identify the error in this C code snippet:

int arr[3] = {1, 2, 3};
int *p = arr + 3;
printf("%d", *p);

medium

A. Missing semicolon after array declaration

B. Array size is incorrect

C. Pointer points outside array bounds causing undefined behavior

D. Pointer declaration syntax is wrong

Solution

1. Step 1: Analyze pointer assignment

   arr + 3 points one element past the last array element (out of bounds).

2. Step 2: Dereferencing out-of-bounds pointer

   *p accesses invalid memory, causing undefined behavior.

3. Final Answer:

   Pointer points outside array bounds causing undefined behavior -> Option C

4. Quick Check:

   Pointer out of array range = error [OK]

Hint: Pointer must stay within array limits [OK]

Common Mistakes:

• Ignoring pointer out-of-bounds access
• Thinking array size is wrong
• Confusing pointer syntax errors

5. Given the code below, what will be printed?

int arr[] = {5, 10, 15, 20};
int *p = arr;
for(int i = 0; i < 4; i++) {
    printf("%d ", *(p + i));
}

hard

A. 5 10 15 20

B. 20 15 10 5

C. 5 5 5 5

D. Compilation error

Solution

1. Step 1: Understand the loop and pointer arithmetic

   Loop runs i from 0 to 3, *(p + i) accesses arr[i].

2. Step 2: Print each element in order

   Values printed are arr[0]=5, arr[1]=10, arr[2]=15, arr[3]=20.

3. Final Answer:

   5 10 15 20 -> Option A

4. Quick Check:

   Pointer + i accesses arr[i] in order [OK]

Hint: Pointer + i accesses array element i [OK]

Common Mistakes:

• Assuming reverse order print
• Printing same element repeatedly
• Expecting syntax error