Array size and bounds in C++ - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the size of an array represent in C++?

The size of an array represents the total number of elements it can hold. For example, int arr[5]; means the array can store 5 integers.

beginner

What happens if you try to access an array element outside its bounds in C++?

Accessing an element outside the array bounds leads to undefined behavior. This means the program might crash, show wrong data, or behave unpredictably.

intermediate

How do you find the size of a statically declared array in C++?

You can find the number of elements by dividing the total size of the array by the size of one element: int size = sizeof(arr) / sizeof(arr[0]);

beginner

Why is it important to stay within array bounds when accessing elements?

Staying within bounds prevents errors and crashes. It ensures you only access valid memory locations, keeping your program safe and stable.

intermediate

What is the difference between array size and array bounds?

Array size is how many elements the array can hold. Array bounds are the valid index range, usually from 0 to size-1. Accessing outside bounds is unsafe.

If you declare int arr[10];, what is the highest valid index you can use?

A 10

B 11

C 0

D 9

Array indices start at 0, so the highest valid index is size - 1, which is 9.

What does sizeof(arr) return for int arr[5]; if an int is 4 bytes?

A 5

B 20

C 4

D 1

sizeof(arr) returns total bytes. 5 elements × 4 bytes = 20 bytes.

What is the result of accessing arr[10] if int arr[10];?

A Undefined behavior

B Compile error

C Always zero

D Valid access

Index 10 is outside bounds (0-9), so accessing it causes undefined behavior.

How can you safely find the number of elements in a static array?

A Use <code>sizeof(arr) / sizeof(arr[0])</code>

B Use <code>arr.length()</code>

C Use <code>arr.size()</code>

D Use <code>count(arr)</code>

Only sizeof(arr) / sizeof(arr[0]) works for static arrays in C++.

Why should you avoid accessing array elements beyond their declared size?

A It causes syntax errors

B It wastes memory

C It leads to undefined behavior and possible crashes

D It slows down the program

Accessing out-of-bounds elements can cause unpredictable program behavior or crashes.

Explain what array size and array bounds mean in C++ and why they matter.

Think about how many elements an array can hold and which indexes are safe to use.

Describe how to calculate the number of elements in a static array using sizeof in C++.

Remember sizeof(arr) gives total bytes, and sizeof(arr[0]) gives bytes per element.

Practice

1. What is the valid range of indexes for an array declared as int arr[10]; in C++?

easy

A. 1 to 9

B. 1 to 10

C. 0 to 10

D. 0 to 9

Solution

1. Step 1: Understand array size declaration

   The array arr is declared with size 10, meaning it holds 10 elements.

2. Step 2: Determine valid index range

   In C++, array indexing starts at 0, so valid indexes are from 0 up to size-1, which is 9.

3. Final Answer:

   0 to 9 -> Option D

4. Quick Check:

   Array size 10 means indexes 0 to 9 [OK]

Hint: Array indexes start at 0 and end at size minus 1 [OK]

Common Mistakes:

• Thinking indexes start at 1
• Including size as a valid index
• Confusing size with last index

2. Which of the following is the correct way to declare an array of 5 integers in C++?

easy

A. int arr[5];

B. int arr = 5;

C. int arr(5);

D. int arr{5};

Solution

1. Step 1: Review array declaration syntax

   In C++, arrays are declared using square brackets with the size inside, like int arr[5];.

2. Step 2: Check each option

   int arr(5); uses parentheses which is invalid for arrays. int arr = 5; assigns an int, not an array. int arr{5}; uses brace initialization but without specifying size properly.

3. Final Answer:

   int arr[5]; -> Option A

4. Quick Check:

   Array declaration uses square brackets [OK]

Hint: Use square brackets [] to declare array size [OK]

Common Mistakes:

• Using parentheses instead of brackets
• Assigning a single int instead of array
• Confusing brace initialization with size

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

int arr[3] = {10, 20, 30};
std::cout << arr[3] << std::endl;

medium

A. Undefined behavior (may print garbage or crash)

B. 0

C. 30

D. Compilation error

Solution

1. Step 1: Identify array size and index used

   The array has size 3, valid indexes are 0, 1, 2. The code accesses arr[3], which is out of bounds.

2. Step 2: Understand consequences of out-of-bounds access

   Accessing outside array bounds in C++ causes undefined behavior. It may print garbage, crash, or behave unpredictably.

3. Final Answer:

   Undefined behavior (may print garbage or crash) -> Option A

4. Quick Check:

   Out-of-bounds access causes undefined behavior [OK]

Hint: Access only indexes 0 to size-1 to avoid undefined behavior [OK]

Common Mistakes:

• Assuming arr[3] returns last element
• Expecting zero instead of garbage
• Thinking compiler catches out-of-bounds

4. Identify the error in the following code snippet:

int arr[4] = {1, 2, 3, 4};
for (int i = 0; i <= 4; i++) {
    std::cout << arr[i] << " ";
}

medium

A. Array size should be 5

B. Initialization of array is incorrect

C. Loop condition should be i < 4, not i <= 4

D. No error, code runs fine

Solution

1. Step 1: Analyze loop bounds

   The loop runs from i = 0 to i = 4 inclusive, so it tries to access arr[4].

2. Step 2: Check array size and valid indexes

   Array size is 4, valid indexes are 0 to 3. Accessing arr[4] is out of bounds and causes undefined behavior.

3. Final Answer:

   Loop condition should be i < 4, not i <= 4 -> Option C

4. Quick Check:

   Loop must use i < size to avoid out-of-bounds [OK]

Hint: Use < size, not <= size, in loops over arrays [OK]

Common Mistakes:

• Using <= instead of < in loop condition
• Increasing array size unnecessarily
• Ignoring out-of-bounds access

5. You want to create a function that sums all elements of an integer array passed along with its size. Which of the following function signatures correctly allows safe access to the array elements without causing out-of-bounds errors?

hard

A. int sumArray(int arr[10]);

B. int sumArray(int arr[], int size);

C. int sumArray(int* arr);

D. int sumArray(int arr[]);

Solution

1. Step 1: Understand function parameter requirements

   To safely access array elements, the function must know the array size to avoid out-of-bounds access.

2. Step 2: Evaluate each signature

   int sumArray(int arr[], int size); passes both array and size, allowing safe iteration. int sumArray(int arr[10]); fixes size to 10, which is unsafe if actual size differs. int sumArray(int* arr); passes pointer without size, unsafe. int sumArray(int arr[]); passes array without size, unsafe.

3. Final Answer:

   int sumArray(int arr[], int size); -> Option B

4. Quick Check:

   Pass array and size to avoid out-of-bounds [OK]

Hint: Always pass array size with array to functions [OK]

Common Mistakes:

• Assuming fixed size in function parameter
• Not passing size leads to unsafe access
• Using pointer without size information