Common array operations in C++ - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the syntax to declare an array of 5 integers in C++?

You declare it like this: int arr[5]; This creates an array named arr that can hold 5 integers.

beginner

How do you access the third element of an array named arr?

You use the index 2 because counting starts at 0. So, arr[2] gives you the third element.

intermediate

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

Use sizeof(arr) / sizeof(arr[0]). This divides the total bytes of the array by the bytes of one element to get the number of elements.

intermediate

What is the difference between arr[i] and *(arr + i)?

Both access the element at index i. arr[i] is array indexing syntax, while *(arr + i) uses pointer arithmetic. They are equivalent.

intermediate

How do you copy elements from one array to another in C++?

You can use a loop to copy each element one by one, like:
for(int i = 0; i < size; i++) { dest[i] = src[i]; }
Or use std::copy from the <algorithm> header.

What is the index of the first element in a C++ array?

A 0

B 1

C -1

D Depends on the array size

In C++, array indexing starts at 0, so the first element is at index 0.

Which expression correctly gives the number of elements in a statically declared array arr?

A sizeof(arr)

B sizeof(arr) / sizeof(arr[0])

C sizeof(arr) * sizeof(arr[0])

D sizeof(arr[0]) / sizeof(arr)

Dividing the total size of the array by the size of one element gives the number of elements.

What does arr[3] mean in C++?

A The fourth element of the array

B The third element of the array

C The element at index 2

D An invalid syntax

Indexing starts at 0, so arr[3] is the fourth element.

Which of the following copies elements from one array to another correctly?

A copy(dest, src);

B dest = src;

C dest.copy(src);

D for(int i=0; i<size; i++) dest[i] = src[i];

Arrays cannot be assigned directly; you must copy elements one by one or use std::copy.

What is the result of *(arr + 2)?

A The second element of the array

B The first element of the array

C The third element of the array

D A pointer to the array

Pointer arithmetic moves the pointer by 2 elements, so dereferencing gives the third element.

Explain how to declare, access, and find the size of a static array in C++.

Think about how memory size relates to element count.

Describe two ways to copy elements from one array to another in C++.

Remember arrays cannot be assigned directly.

Practice

1. What does the following C++ code do?
int arr[5] = {1, 2, 3, 4, 5};

easy

A. Creates an array of 5 integers with values 1 to 5

B. Creates a vector of 5 integers with values 1 to 5

C. Creates a pointer to an integer array

D. Declares an uninitialized array of size 5

Solution

1. Step 1: Understand array declaration

   The code declares an integer array named arr with size 5.

2. Step 2: Check initialization

   The array is initialized with values 1, 2, 3, 4, 5 in order.

3. Final Answer:

   Creates an array of 5 integers with values 1 to 5 -> Option A

4. Quick Check:

   Array declaration with initialization = Creates array with values [OK]

Hint: Array size and values in braces means initialized array [OK]

Common Mistakes:

• Confusing array with vector
• Thinking it's a pointer declaration
• Assuming array is uninitialized

2. Which of the following is the correct syntax to access the third element of an array named arr in C++?

easy

A. arr[3];

B. arr(3);

C. arr{3};

D. arr[2];

Solution

1. Step 1: Recall zero-based indexing

   Arrays in C++ start at index 0, so the third element is at index 2.

2. Step 2: Identify correct syntax for access

   Array elements are accessed using square brackets, so arr[2] is correct.

3. Final Answer:

   arr[2]; -> Option D

4. Quick Check:

   Third element index = 2, use square brackets [OK]

Hint: Remember arrays start at 0, so third element is index 2 [OK]

Common Mistakes:

• Using parentheses instead of brackets
• Using 1-based indexing
• Using curly braces for access

3. What is the output of this C++ code?

int arr[4] = {10, 20, 30, 40};
int sum = 0;
for(int i = 0; i < 4; i++) {
    sum += arr[i];
}
std::cout << sum;

medium

A. 100

B. 90

C. 40

D. 10

Solution

1. Step 1: Identify array elements and loop

   The array has elements 10, 20, 30, 40 and the loop runs from i=0 to 3.

2. Step 2: Calculate sum of elements

   sum = 10 + 20 + 30 + 40 = 100

3. Final Answer:

   100 -> Option A

4. Quick Check:

   Sum of all elements = 100 [OK]

Hint: Add all elements in loop to get sum [OK]

Common Mistakes:

• Stopping loop early
• Using wrong indices
• Printing sum before loop

4. Find the error in this C++ code that tries to print all elements of an array:

int arr[3] = {5, 10, 15};
for(int i = 1; i <= 3; i++) {
    std::cout << arr[i] << " ";
}

medium

A. Loop index starts at 1 instead of 0

B. Array size is incorrect

C. Using <= causes out-of-bounds access

D. Missing semicolon after cout statement

Solution

1. Step 1: Analyze loop indices

   The loop runs i from 1 to 3 inclusive, so i=3 is included.

2. Step 2: Check array bounds

   Array indices are 0,1,2. Accessing arr[3] is out-of-bounds and causes error.

3. Final Answer:

   Using <= causes out-of-bounds access -> Option C

4. Quick Check:

   Loop must use i < size, not <= [OK]

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

Common Mistakes:

• Starting loop at 1 instead of 0 (not error but unusual)
• Ignoring out-of-bounds access
• Assuming array size is wrong

5. You have an array int arr[6] = {2, 4, 6, 8, 10, 12};. How do you update the element with value 8 to 9 using a loop?

hard

A. arr[3] = 9;

B. for(int i=0; i<6; i++) { if(arr[i] == 8) arr[i] = 9; }

C. for(int i=1; i<=6; i++) { if(arr[i] == 8) arr[i] = 9; }

D. arr[4] = 9;

Solution

1. Step 1: Understand the problem

   We want to find the element with value 8 and update it to 9 using a loop.

2. Step 2: Check loop and condition

   Loop from i=0 to 5 checks each element; if arr[i] == 8, assign arr[i] = 9.

3. Final Answer:

   for(int i=0; i<6; i++) { if(arr[i] == 8) arr[i] = 9; } -> Option B

4. Quick Check:

   Loop with condition to find and update element [OK]

Hint: Loop through array, check value, then update [OK]

Common Mistakes:

• Using wrong indices in loop
• Assigning wrong array index
• Not using loop to find value