Array Insertion at Middle Index in DSA C - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the first step when inserting an element in the middle of an array?

Find the middle index where the new element will be inserted.

beginner

Why do we need to shift elements when inserting in the middle of an array?

To make space for the new element, all elements from the middle index to the end must be moved one position to the right.

intermediate

In C, what must you be careful about when inserting into an array?

Ensure the array has enough space to hold the new element to avoid overwriting memory.

intermediate

What happens if you insert an element at the middle index of an array with an even number of elements?

The new element is inserted at the index equal to the size divided by 2, shifting elements from that index onwards.

beginner

How do you calculate the middle index for insertion in an array of size n?

Middle index = n / 2 (integer division).

What is the middle index of an array with 7 elements for insertion?

A 2

B 3

C 4

D 5

Middle index = 7 / 2 = 3 (integer division).

What must you do before inserting an element in the middle of an array?

A Replace the middle element directly

B Shift elements to the left from the middle index

C Shift elements to the right from the middle index

D Delete the last element

Elements from the middle index to the end must be shifted right to make space.

If an array has size 6, at which index will the new element be inserted in the middle?

A 2

B 5

C 4

D 3

Middle index = 6 / 2 = 3.

What is a risk when inserting into a fixed-size array in C without checking capacity?

A Memory overflow or corruption

B Faster insertion

C Automatic resizing

D No risk

Inserting without space can overwrite memory causing errors.

Which operation is NOT needed when inserting an element in the middle of an array?

A Sorting the array

B Shifting elements to the right

C Finding the middle index

D Assigning the new element

Sorting is not required for insertion at middle index.

Explain the steps to insert an element at the middle index of an array in C.

Think about making space and placing the new element.

What challenges might you face when inserting in the middle of a fixed-size array and how can you handle them?

Consider memory and size limits.

Practice

1. What is the correct middle index to insert an element in an array of size 6?

easy

A. 5

B. 2

C. 4

D. 3

Solution

1. Step 1: Calculate middle index for even-sized array

   For an array of size 6, middle index is size/2 = 6/2 = 3.

2. Step 2: Confirm insertion position

   Inserting at index 3 places the element exactly in the middle.

3. Final Answer:

   3 -> Option D

4. Quick Check:

   Middle index = size/2 = 3 [OK]

Hint: Middle index = array size divided by 2 [OK]

Common Mistakes:

• Using size-1 as middle index
• Using size/2 - 1 for even size
• Confusing zero-based indexing

2. Which of the following C code snippets correctly inserts an element at the middle index of an array arr of size n?

easy

A. int mid = n / 2; for(int i = 0; i < mid; i++) arr[i] = arr[i+1]; arr[mid] = x;

B. int mid = n / 2; for(int i = n; i > mid; i--) arr[i] = arr[i-1]; arr[mid] = x;

C. int mid = n / 2; arr[mid] = x;

D. int mid = n / 2; for(int i = mid; i < n; i++) arr[i] = arr[i+1]; arr[mid] = x;

Solution

1. Step 1: Identify correct shifting direction

   To insert at middle, elements from middle to end must shift right to make space.

2. Step 2: Check code correctness

   int mid = n / 2; for(int i = n; i > mid; i--) arr[i] = arr[i-1]; arr[mid] = x; shifts elements right from end to middle, then inserts x at mid. Others shift incorrectly or overwrite.

3. Final Answer:

   int mid = n / 2; for(int i = n; i > mid; i--) arr[i] = arr[i-1]; arr[mid] = x; -> Option B

4. Quick Check:

   Shift right from end to mid, then insert [OK]

Hint: Shift elements right from end to middle before insertion [OK]

Common Mistakes:

• Shifting elements left instead of right
• Not shifting elements before insertion
• Overwriting elements without shifting

3. What will be the content of the array after inserting 99 at the middle index of arr = {10, 20, 30, 40, 50} using the correct insertion method?

medium

A. [10, 20, 30, 40, 99, 50]

B. [10, 20, 99, 30, 40, 50]

C. [10, 20, 30, 99, 40, 50]

D. [10, 20, 30, 40, 50, 99]

Solution

1. Step 1: Calculate middle index

   Array size is 5, middle index = 5/2 = 2 (integer division).

2. Step 2: Shift elements right and insert 99

   Shift elements from index 4 to 2 right by one position. Insert 99 at index 2.

3. Final Answer:

   [10, 20, 99, 30, 40, 50] -> Option C

4. Quick Check:

   Insert at index 2 shifts elements right [OK]

Hint: Middle index = size/2; shift right, then insert [OK]

Common Mistakes:

• Inserting at wrong index
• Not shifting elements before insertion
• Overwriting elements

4. Identify the error in the following code snippet that inserts an element x at the middle of array arr of size n:

int mid = n / 2;
for(int i = mid; i < n; i++) {
    arr[i+1] = arr[i];
}
arr[mid] = x;

medium

A. Loop should run backwards from n to mid

B. Index mid is incorrect for insertion

C. No need to shift elements before insertion

D. Array size n should be incremented before insertion

Solution

1. Step 1: Analyze loop direction

   The loop shifts elements from mid to n forward, but does so from low to high index, causing overwrite.

2. Step 2: Correct loop direction

   Loop must run backwards from n to mid to shift elements right safely without overwriting.

3. Final Answer:

   Loop should run backwards from n to mid -> Option A

4. Quick Check:

   Shift right must be done backwards [OK]

Hint: Shift elements right by looping backwards from end [OK]

Common Mistakes:

• Looping forward causing data overwrite
• Not updating array size after insertion
• Incorrect middle index calculation

5. Given an array arr = {5, 10, 15, 20, 25, 30} of size 6, write the final array after inserting 99 at the middle index and then inserting 77 at the new middle index of the updated array.

hard

A. [5, 10, 15, 77, 99, 20, 25, 30]

B. [5, 10, 77, 99, 15, 20, 25, 30]

C. [5, 10, 99, 15, 77, 20, 25, 30]

D. [5, 10, 15, 99, 77, 20, 25, 30]

Solution

1. Step 1: Insert 99 at middle of original array

   Original size = 6, middle index = 3. Insert 99 at index 3 shifting elements right: [5, 10, 15, 99, 20, 25, 30]

2. Step 2: Insert 77 at middle of updated array

   New size = 7, middle index = 7/2 = 3. Insert 77 at index 3 shifting elements right: [5, 10, 15, 77, 99, 20, 25, 30]

3. Final Answer:

   [5, 10, 15, 77, 99, 20, 25, 30] -> Option A

4. Quick Check:

   Insert twice at middle recalculating index each time [OK]

Hint: Recalculate middle after each insertion before next insert [OK]

Common Mistakes:

• Not recalculating middle index after first insertion
• Overwriting elements without shifting
• Inserting at wrong index second time