Array size and bounds in C++ - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare an array of 5 integers.

C++

int arr[[1]];

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

A-1

B10

C0

D5

Attempts:

3 left

💡 Hint

Common Mistakes

Using zero or negative numbers as array size causes errors.

Forgetting to specify the size inside the brackets.

The number inside the square brackets specifies the size of the array. Here, 5 means the array can hold 5 integers.

2fill in blank

medium

Complete the code to access the third element of the array.

C++

int value = arr[[1]];

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

A2

B1

C3

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using 3 instead of 2 for the third element index.

Confusing 1-based indexing with 0-based indexing.

Array indexing in C++ starts at 0, so the third element is at index 2.

3fill in blank

hard

Fix the error in the code to prevent out-of-bounds access.

C++

for (int i = 0; i < [1]; i++) {
    cout << arr[i] << " ";
}

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

A5

B6

C10

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using a loop limit greater than the array size causes out-of-bounds errors.

Using <= instead of < in the loop condition.

The array size is 5, so the loop should run while i is less than 5 to avoid accessing invalid elements.

4fill in blank

hard

Fill both blanks to create a dictionary that maps array indices to their values for elements greater than 10.

C++

std::map<int, int> result = {
    { [1], arr[[2]] }
};

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

A2

B3

C1

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using different indices for key and value.

Using an index outside the array bounds.

We use the index 2 for both the key and to access the value in the array.

5fill in blank

hard

Fill all three blanks to create a loop that sums all elements in the array.

C++

int sum = 0;
for (int [1] = 0; [2] < [3]; [1]++) {
    sum += arr[[1]];
}

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

Ai

C5

Dn

Attempts:

3 left

💡 Hint

Common Mistakes

Using different variable names in the loop header.

Using a loop limit greater than the array size.

The variable i is used as the loop counter, and the loop runs while i is less than 5, the array size.

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