Array size and bounds in C++

Introduction

Arrays hold many items in order. Knowing their size and limits helps avoid errors when using them.

When you want to store a fixed number of items like scores or names.

When you need to loop through all items safely without going outside the array.

When you want to check if an index is valid before accessing an array element.

Syntax

C++

const int ARRAY_SIZE = 5;
int numbers[ARRAY_SIZE];

// Access elements from 0 to ARRAY_SIZE - 1
for (int index = 0; index < ARRAY_SIZE; ++index) {
    // Use numbers[index]
}

Array indexes start at 0 and go up to size - 1.

Accessing outside these bounds causes undefined behavior (errors).

Examples

This array has no elements, so you cannot access any index.

C++

int emptyArray[0]; // Array with zero size (rarely used)

Array with one element. Only index 0 is valid.

C++

int singleElementArray[1] = {10};
// Valid index: 0
// Invalid index: 1 or more

Accessing index equal to size or more is out of bounds.

C++

int colors[3] = {1, 2, 3};
// Valid indexes: 0, 1, 2
// Accessing colors[3] is out of bounds and unsafe

Sample Program

This program creates an array of 4 scores, prints the size, and prints all scores safely by looping from 0 to size - 1.

C++

#include <iostream>

int main() {
    const int ARRAY_SIZE = 4;
    int scores[ARRAY_SIZE] = {85, 90, 78, 92};

    std::cout << "Array size: " << ARRAY_SIZE << "\n";
    std::cout << "Scores: ";
    for (int index = 0; index < ARRAY_SIZE; ++index) {
        std::cout << scores[index] << " ";
    }
    std::cout << "\n";

    // Trying to access out of bounds (commented out to avoid error)
    // std::cout << scores[ARRAY_SIZE] << "\n"; // Unsafe!

    return 0;
}

Important Notes

Time complexity to access an element by index is O(1).

Always check that index is between 0 and size - 1 before accessing.

Common mistake: Accessing index equal to or greater than size causes bugs or crashes.

Use loops with conditions based on array size to avoid out-of-bounds errors.

Summary

Arrays have a fixed size known at creation.

Valid indexes go from 0 up to size minus 1.

Accessing outside these bounds causes errors and must be avoided.

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