Array Insertion at End in DSA C++ - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does 'Array Insertion at End' mean?

It means adding a new element to the last position of an array, increasing its size by one.

beginner

In C++, what must you check before inserting an element at the end of a fixed-size array?

You must check if the array has space left; otherwise, insertion will cause overflow or errors.

beginner

What is the time complexity of inserting an element at the end of an array if space is available?

The time complexity is O(1) because you directly place the element at the next free index.

intermediate

Why might inserting at the end of an array sometimes require creating a new array?

Because fixed-size arrays cannot grow, you create a bigger array and copy old elements to it before adding the new element.

beginner

Show the C++ code snippet to insert an element 'x' at the end of an array with current size 'n' and capacity 'capacity'.

if (n < capacity) { arr[n] = x; n++; } else { // Handle overflow or resize }

What happens if you try to insert an element at the end of a full fixed-size array?

A It causes overflow or error

B The element replaces the first element

C The array automatically resizes

D Nothing happens

Fixed-size arrays cannot grow, so inserting beyond capacity causes overflow or error.

What is the index of the new element when inserting at the end of an array with current size n?

A n - 1

B 0

C n

D n + 1

The new element is placed at index n, which is the next free position.

Which of these is the best description of array insertion at end when space is available?

A O(n^2) time due to copying

B O(n) time because all elements shift

C O(log n) time due to searching

D O(1) time because direct placement

Insertion at the end with space is O(1) because no shifting is needed.

If an array is full, what is a common way to allow insertion at the end?

A Create a new larger array and copy elements

B Delete the first element

C Overwrite the last element

D Do nothing

Creating a larger array and copying elements allows more space for insertion.

In C++, which variable usually tracks the current number of elements in an array for insertion?

A capacity

B size or n

C index

D length

A variable like 'size' or 'n' tracks how many elements are currently stored.

Explain how to insert an element at the end of an array and what checks are necessary.

Think about array size and available space.

Describe the difference between inserting at the end of a fixed-size array and a dynamic array.

Consider what happens when the array is full.

Practice

1. What happens when you insert an element at the end of a fixed-size array in C++ without resizing?

easy

A. The element replaces the last element if the array is full

B. The array automatically resizes to fit the new element

C. The element is added at the next free position if available

D. The program throws a runtime error

Solution

1. Step 1: Understand fixed-size array behavior

   Fixed-size arrays have a set size and cannot grow automatically.

2. Step 2: Inserting at end when full

   If the array is full, you cannot add a new element without overwriting or causing undefined behavior. If there is space available, the element is added at the next free position.

3. Final Answer:

   The element is added at the next free position if available -> Option C

4. Quick Check:

   Fixed-size array adds element only if space available [OK]

Hint: Fixed arrays don't resize; add element only if space available [OK]

Common Mistakes:

• Assuming array resizes automatically
• Thinking insertion always adds new space
• Expecting runtime error on insertion

2. Which of the following is the correct syntax to insert an element `x` at the end of a vector `v` in C++?

easy

A. v.append(x);

B. v.push_back(x);

C. v.add(x);

D. v.insert_end(x);

Solution

1. Step 1: Recall vector insertion method

   The standard method to add an element at the end of a vector is push_back.

2. Step 2: Check syntax correctness

   Only v.push_back(x); is valid C++ syntax for vector insertion at end.

3. Final Answer:

   v.push_back(x); -> Option B

4. Quick Check:

   Use push_back to insert at vector end [OK]

Hint: Use push_back() to add at vector end [OK]

Common Mistakes:

• Using non-existent methods like insert_end or add
• Confusing vector with other container methods
• Using append which is not a C++ vector method

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

int arr[5] = {1, 2, 3, 4, 5};
int n = 5;
int x = 10;
if (n < 5) {
  arr[n] = x;
  n++;
}
for (int i = 0; i < n; i++) {
  std::cout << arr[i] << " ";
}

medium

A. 1 2 3 4 10

B. 1 2 3 4 5 10

C. Compilation error

D. 1 2 3 4 5

Solution

1. Step 1: Check condition for insertion

   The condition if (n < 5) is false because n equals 5, so insertion does not happen.

2. Step 2: Print array elements

   The loop prints all elements from index 0 to 4, which are 1 2 3 4 5.

3. Final Answer:

   1 2 3 4 5 -> Option D

4. Quick Check:

   Insertion skipped, original array printed [OK]

Hint: Check insertion condition before adding element [OK]

Common Mistakes:

• Assuming insertion happens even if condition false
• Printing beyond array size
• Confusing array size with variable n

4. Identify the error in the following code that tries to insert an element at the end of an array:

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

medium

A. Array index out of bounds when inserting element

B. Variable n is not incremented correctly

C. Missing semicolon after arr[n] = x

D. Loop condition should be i <= n

Solution

1. Step 1: Check array size and insertion index

   Array size is 3, valid indices are 0,1,2. Inserting at arr[3] is out of bounds.

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

   Accessing arr[3] causes undefined behavior or runtime error.

3. Final Answer:

   Array index out of bounds when inserting element -> Option A

4. Quick Check:

   Insertion index must be within array size [OK]

Hint: Array indices go from 0 to size-1 only [OK]

Common Mistakes:

• Ignoring array bounds when inserting
• Assuming array resizes automatically
• Incorrect loop boundary causing extra prints

5. You have an array `int arr[6] = {1, 2, 3, 4, 5};` and a variable `int n = 5;` representing the current number of elements. Write the correct code snippet to insert `10` at the end safely, considering array size limits.

hard

A. if (n < 6) { arr[n] = 10; n++; }

B. arr[n] = 10; n++;

C. if (n <= 6) { arr[n] = 10; n++; }

D. if (n < 5) { arr[n] = 10; n++; }

Solution

1. Step 1: Check array capacity before insertion

   Array size is 6, so insertion is safe only if n < 6.

2. Step 2: Insert element and update count

   If condition is true, assign arr[n] = 10 and increment n to reflect new size.

3. Final Answer:

   if (n < 6) { arr[n] = 10; n++; } -> Option A

4. Quick Check:

   Check capacity before insert, then add element [OK]

Hint: Always check n < array size before inserting [OK]

Common Mistakes:

• Not checking array size before insertion
• Using <= instead of < causing overflow
• Wrong size limit in condition