Pointers and arrays in C++ - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a pointer in C++?

A pointer is a variable that stores the memory address of another variable. It 'points' to the location where data is stored.

beginner

How are arrays and pointers related in C++?

The name of an array acts like a pointer to its first element. You can use pointers to access array elements by moving through memory addresses.

intermediate

What does the expression *(arr + i) mean?

It means accessing the element at index i of the array arr by moving i positions from the start address and dereferencing the pointer.

beginner

How do you declare a pointer to an integer in C++?

You declare it like this: int* ptr; where ptr can store the address of an integer variable.

intermediate

What happens if you increment a pointer pointing to an array element?

Incrementing the pointer moves it to the next element in the array, because it increases the address by the size of the element type.

What does the pointer variable store?

A Size of a variable

B Value of another variable

C Type of a variable

D Memory address of another variable

Pointers store the memory address where data is located, not the data itself.

If int arr[5];, what does arr represent?

A Pointer to the last element of arr

B Pointer to the first element of arr

C Value of the first element

D Size of the array

The array name acts as a pointer to its first element.

What does *(arr + 2) access?

A Third element of arr

B Second element of arr

C First element of arr

D Fourth element of arr

Pointer arithmetic moves by element size, so adding 2 moves to the third element (index 2).

How do you declare a pointer to a float variable?

A float* ptr;

B ptr* float;

C float ptr*;

D pointer float ptr;

The correct syntax is float* ptr;.

What happens when you do ptr++ if ptr points to an int array element?

A Pointer value stays the same

B Pointer moves to the next byte

C Pointer moves to the next int element

D Pointer moves to the previous element

Incrementing a pointer moves it by the size of the type it points to, so to the next int element.

Explain how pointers and arrays are connected in C++.

Think about how the array name can be used like a pointer to the first element.

Describe what happens when you increment a pointer that points to an array element.

Consider how memory addresses change when moving through an array.

Practice

1. What does the name of an array represent in C++?

easy

A. A copy of the entire array

B. The total size of the array

C. The last element of the array

D. A pointer to the first element of the array

Solution

1. Step 1: Understand array name behavior

   In C++, the name of an array acts as a pointer to its first element, not the whole array or size.

2. Step 2: Confirm pointer usage

   This pointer can be used to access elements by pointer arithmetic or indexing.

3. Final Answer:

   A pointer to the first element of the array -> Option D

4. Quick Check:

   Array name = pointer to first element [OK]

Hint: Remember array name points to element zero [OK]

Common Mistakes:

• Thinking array name stores size
• Confusing array name with last element
• Assuming array name copies data

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

easy

A. int *ptr = &arr;

B. int ptr = &arr;

C. int *ptr = arr;

D. int ptr = arr;

Solution

1. Step 1: Understand pointer declaration

   To declare a pointer to an integer, use int *ptr. The array name arr acts as a pointer to its first element.

2. Step 2: Assign pointer correctly

   Assigning ptr = arr; sets the pointer to the first element's address. Using &arr gives the address of the whole array, which has a different type.

3. Final Answer:

   int *ptr = arr; -> Option C

4. Quick Check:

   Pointer to first element = int *ptr = arr [OK]

Hint: Use int *ptr = arr; to point to array start [OK]

Common Mistakes:

• Using &arr instead of arr for pointer
• Declaring pointer as int instead of int*
• Assigning pointer without *

3. What is the output of the following code?

int arr[] = {10, 20, 30};
int *p = arr;
std::cout << *(p + 1) << std::endl;

medium

A. 20

B. 10

C. 30

D. Compilation error

Solution

1. Step 1: Understand pointer arithmetic

   Pointer p points to arr[0]. Adding 1 moves pointer to arr[1].

2. Step 2: Dereference pointer at new position

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

3. Final Answer:

   20 -> Option A

4. Quick Check:

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

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

Common Mistakes:

• Confusing pointer arithmetic with addition of values
• Dereferencing pointer without offset
• Expecting compilation error

4. Identify the error in the following code snippet:

int arr[3] = {1, 2, 3};
int *p = &arr;
std::cout << *(p + 2) << std::endl;

medium

A. Pointer declaration is correct, no error

B. Incorrect pointer assignment: &arr should be arr

C. Dereferencing pointer is invalid syntax

D. Array size is too small for pointer arithmetic

Solution

1. Step 1: Analyze pointer assignment

   arr is an array name acting as pointer to first element. &arr is address of whole array, type int (*)[3], incompatible with int *.

2. Step 2: Correct pointer assignment

   Assigning int *p = arr; is correct to point to first element. Using &arr causes type mismatch and incorrect pointer arithmetic.

3. Final Answer:

   Incorrect pointer assignment: &arr should be arr -> Option B

4. Quick Check:

   Use arr, not &arr, for int* pointer [OK]

Hint: Use arr, not &arr, when assigning int* pointer [OK]

Common Mistakes:

• Using &arr instead of arr for int*
• Ignoring pointer type mismatch warnings
• Assuming &arr points to first element

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

int arr[] = {5, 10, 15, 20};
int *p = arr + 1;
std::cout << p[2] << std::endl;

hard

A. 20

B. 15

C. Undefined behavior

D. 10

Solution

1. Step 1: Understand pointer initialization

   p = arr + 1; points to arr[1], which is 10.

2. Step 2: Access element using pointer indexing

   p[2] means *(p + 2), which points to arr[3] (1 + 2 = 3), value 20.

3. Final Answer:

   20 -> Option A

4. Quick Check:

   p[2] = arr[3] = 20 [OK]

Hint: Pointer[index] = *(pointer + index) [OK]

Common Mistakes:

• Confusing p[2] with arr[2]
• Forgetting pointer offset starts at arr + 1
• Assuming p[2] is out of bounds