Pointer arithmetic - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is pointer arithmetic in C?

Pointer arithmetic means adding or subtracting integers to or from pointers to move through memory locations of the data type the pointer points to.

beginner

How does adding 1 to a pointer affect its value?

Adding 1 to a pointer moves it to the next element of its type in memory. For example, if it points to an int, it moves forward by sizeof(int) bytes.

intermediate

What happens when you subtract two pointers?

Subtracting two pointers gives the number of elements between them, not the byte difference. Both pointers must point to elements of the same array.

beginner

Why can't you add two pointers together?

Adding two pointers is not allowed because it doesn't make sense to combine two memory addresses. Pointer arithmetic only supports adding or subtracting integers to pointers.

intermediate

Explain how pointer arithmetic relates to array indexing.

Array indexing like arr[i] is actually pointer arithmetic under the hood. It means *(arr + i), moving the pointer i elements forward and accessing that value.

If p is an int* pointing to address 1000 and sizeof(int) is 4, what is the address of p + 2?

A 1002

B 1008

C 1004

D 1006

Adding 2 to p moves it 2 elements forward. Each int is 4 bytes, so 2 * 4 = 8 bytes. 1000 + 8 = 1008.

What does the expression (p2 - p1) return if p1 and p2 are pointers to elements in the same array?

A The byte difference between p2 and p1

B The sum of the addresses

C An error

D The number of elements between p1 and p2

Subtracting pointers returns how many elements apart they are, not the byte difference.

Which of these operations is NOT valid in pointer arithmetic?

A p1 + p2

B p - 1

C p1 - p2

D p + 3

You cannot add two pointers together. Only add or subtract integers to/from pointers.

What does arr[i] mean in terms of pointers?

A arr - i

B arr + i

C *(arr + i)

D *(i + arr - 1)

Array indexing is pointer arithmetic: arr[i] is the same as *(arr + i).

If p points to a char and p = p + 5, how many bytes does p move forward?

A 5 bytes

B 1 byte

C Depends on the system

D 10 bytes

A char is 1 byte, so moving 5 elements forward moves 5 bytes.

Describe what happens when you add an integer to a pointer in C.

Think about how arrays and pointers relate.

Explain why subtracting two pointers gives the number of elements between them, not the byte difference.

Consider how pointer arithmetic is designed for array navigation.

Practice

1. What does adding 1 to a pointer in C do?
int *p;

easy

A. Causes a syntax error

B. Moves the pointer by 1 byte

C. Moves the pointer to the previous element

D. Moves the pointer to the next element of the type it points to

Solution

1. Step 1: Understand pointer arithmetic basics

   In C, adding 1 to a pointer moves it to the next element of the type it points to, not just 1 byte.

2. Step 2: Apply to the given pointer

   Since p is an int*, p + 1 points to the next int in memory.

3. Final Answer:

   Moves the pointer to the next element of the type it points to -> Option D

4. Quick Check:

   Pointer + 1 moves by element size [OK]

Hint: Pointer + 1 moves to next element, not next byte [OK]

Common Mistakes:

• Thinking pointer arithmetic moves by 1 byte
• Confusing pointer increment with value increment
• Assuming pointer addition causes errors

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

int arr[5];
int *p = arr;

easy

A. p + 2

B. p ++ 2

C. p + ++ 2

D. p +* 2

Solution

1. Step 1: Review pointer arithmetic syntax

   Valid pointer arithmetic uses p + n or p - n where n is an integer.

2. Step 2: Check each option

   p + 2 is valid syntax. Options A, C, and D contain invalid operators or spacing causing syntax errors.

3. Final Answer:

   p + 2 -> Option A

4. Quick Check:

   Correct pointer addition syntax is p + n [OK]

Hint: Pointer arithmetic uses '+' or '-' with integers only [OK]

Common Mistakes:

• Using invalid operators like '++' incorrectly
• Adding spaces that change meaning
• Confusing pointer increment with addition

3. What is the output of this code?

int arr[] = {10, 20, 30, 40};
int *p = arr;
printf("%d", *(p + 2));

medium

A. 10

B. 20

C. 30

D. 40

Solution

1. Step 1: Understand pointer and array relation

   Pointer p points to arr[0], which is 10.

2. Step 2: Calculate pointer offset

   *(p + 2) accesses the element 2 positions after arr[0], which is arr[2] = 30.

3. Final Answer:

   30 -> Option C

4. Quick Check:

   Pointer + 2 points to third element = 30 [OK]

Hint: *(p + n) accesses nth element after pointer [OK]

Common Mistakes:

• Confusing index starting at 1 instead of 0
• Dereferencing pointer without offset
• Mixing pointer arithmetic with array indexing

4. Identify the error in this code snippet:

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

medium

A. Syntax error in pointer declaration

B. Pointer points beyond array bounds causing undefined behavior

C. Missing semicolon after array initialization

D. Incorrect printf format specifier

Solution

1. Step 1: Analyze pointer assignment

   p = arr + 3 points to one element past the last valid element (index 3 in a 3-element array).

2. Step 2: Understand dereferencing invalid pointer

   Dereferencing *p here causes undefined behavior because it accesses memory outside the array.

3. Final Answer:

   Pointer points beyond array bounds causing undefined behavior -> Option B

4. Quick Check:

   Pointer must not dereference beyond array end [OK]

Hint: Pointer + array size points just past array, not valid to dereference [OK]

Common Mistakes:

• Assuming pointer can safely dereference past array end
• Confusing pointer arithmetic with array length
• Ignoring undefined behavior risks

5. Given two pointers int *p1 and int *p2 pointing inside the same array, what does p2 - p1 represent?

hard

A. The number of elements between the two pointers

B. The difference in memory addresses as a raw number

C. The number of bytes between the two pointers

D. A syntax error because pointers cannot be subtracted

Solution

1. Step 1: Understand pointer subtraction in C

   Subtracting two pointers gives the number of elements of the pointer type between them, not bytes.

2. Step 2: Apply to pointers inside the same array

   Since p1 and p2 point inside the same array, p2 - p1 counts how many elements separate them.

3. Final Answer:

   The number of elements between the two pointers -> Option A

4. Quick Check:

   Pointer subtraction = element count difference [OK]

Hint: Pointer subtraction counts elements, not bytes [OK]

Common Mistakes:

• Thinking subtraction returns byte difference
• Assuming pointer subtraction is invalid
• Confusing pointer subtraction with pointer addition