Array Access and Update at Index in DSA C - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the time complexity of accessing an element at a specific index in an array?

Accessing an element at a specific index in an array takes constant time, O(1), because arrays allow direct access using the index.

beginner

How do you update the value at index 3 in an integer array named arr in C?

You update the value by assigning a new value using the index:
arr[3] = new_value;

intermediate

What happens if you try to access or update an index outside the array bounds in C?

Accessing or updating an index outside the array bounds causes undefined behavior, which can lead to program crashes or incorrect results.

intermediate

Explain why arrays provide fast access to elements compared to linked lists.

Arrays store elements in contiguous memory locations, so accessing an element by index is direct and fast (O(1)). Linked lists require traversal from the head to reach an element, which takes O(n) time.

advanced

In C, how is the expression arr[i] interpreted internally?

The expression arr[i] is interpreted as *(arr + i), which means accessing the memory location at the base address of arr plus i times the size of the element.

What is the result of accessing arr[5] if arr has size 5?

A Valid access to the last element

B Accesses the first element

C Undefined behavior (out of bounds)

D Compiler error

Array indices in C go from 0 to size-1. Accessing index 5 in a size 5 array is out of bounds and causes undefined behavior.

Which of the following is the correct way to update the 2nd element of an array arr to 10?

A arr[1] = 10;

B arr[2] = 10;

C arr(1) = 10;

D arr->1 = 10;

Array indices start at 0, so the 2nd element is at index 1. The correct syntax is arr[1] = 10;

What is the time complexity to update an element at a given index in an array?

A O(1)

B O(n)

C O(log n)

D O(n^2)

Updating an element at a specific index in an array is a direct operation and takes constant time O(1).

In C, what does the expression *(arr + i) mean?

A Pointer to arr plus i

B Access the element at index i of arr

C Access the first element of arr

D Add i to the value of arr

In C, arr[i] is equivalent to *(arr + i), which accesses the element at index i.

Which of these statements about arrays is FALSE?

A Accessing out-of-bounds index causes undefined behavior

B Accessing an element by index is O(1)

C Arrays store elements in contiguous memory

D Arrays can dynamically resize themselves automatically

Arrays in C have fixed size and do not resize automatically. Dynamic resizing requires other data structures.

Describe how to access and update an element at a specific index in a C array.

Think about how you use arr[i] in C.

Explain what happens if you try to access or update an array element outside its valid index range in C.

Consider memory safety and C language behavior.

Practice

1. What does the expression arr[2] do in an array named arr?

easy

A. Accesses the element at index 0 of the array

B. Accesses the element at index 2 of the array

C. Updates the element at index 2 to zero

D. Deletes the element at index 2

Solution

1. Step 1: Understand array indexing

   Arrays use zero-based indexing, so index 2 means the third element.

2. Step 2: Access element at index 2

   The expression arr[2] retrieves the value stored at the third position in the array.

3. Final Answer:

   Accesses the element at index 2 of the array -> Option B

4. Quick Check:

   arr[2] means third element access [OK]

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

Common Mistakes:

• Thinking index starts at 1 instead of 0
• Confusing access with update or delete
• Assuming arr[2] changes the array

2. Which of the following is the correct syntax to update the 4th element of an integer array arr to 10 in C?

easy

A. arr[3] = 10;

B. arr(4) = 10;

C. arr[4] == 10;

D. arr{3} = 10;

Solution

1. Step 1: Identify correct array indexing syntax

   In C, arrays use square brackets with zero-based indexing, so the 4th element is at index 3.

2. Step 2: Use assignment operator correctly

   To update, use single equals =, so arr[3] = 10; is correct.

3. Final Answer:

   arr[3] = 10; -> Option A

4. Quick Check:

   Use arr[index] = value; with zero-based index [OK]

Hint: Use square brackets and single = for update, zero-based index [OK]

Common Mistakes:

• Using parentheses or braces instead of brackets
• Using double equals (==) instead of assignment (=)
• Using wrong index number for 4th element

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

int arr[5] = {1, 2, 3, 4, 5};
arr[1] = arr[3] + 2;
printf("%d", arr[1]);

medium

A. 6

B. 4

C. 5

D. 7

Solution

1. Step 1: Evaluate the expression arr[3] + 2

   arr[3] is the 4th element, which is 4. Adding 2 gives 6.

2. Step 2: Assign and print arr[1]

   arr[1] is updated to 6, then printed, so output is 6.

3. Final Answer:

   6 -> Option A

4. Quick Check:

   arr[3]=4 + 2 = 6 printed [OK]

Hint: Remember arr[3] is 4, add 2, assign to arr[1], print arr[1] [OK]

Common Mistakes:

• Confusing index 1 with index 3 values
• Adding incorrectly or forgetting zero-based index
• Printing wrong array element

4. Identify the error in the following C code snippet:

int arr[3] = {5, 10, 15};
arr[3] = 20;
printf("%d", arr[3]);

medium

A. Syntax error in array declaration

B. Missing semicolon after assignment

C. Array index out of bounds error

D. Incorrect printf format specifier

Solution

1. Step 1: Check array size and valid indices

   Array size is 3, valid indices are 0, 1, 2. Index 3 is out of bounds.

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

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

3. Final Answer:

   Array index out of bounds error -> Option C

4. Quick Check:

   Index 3 invalid for size 3 array [OK]

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

Common Mistakes:

• Using index equal to array size
• Ignoring zero-based indexing
• Assuming no error for out of bounds

5. Given an integer array arr of size 6 initialized as {2, 4, 6, 8, 10, 12}, which code snippet correctly doubles the value at index 4 and then prints the entire array?

hard

A. arr[4] = arr[4] + 2;
for(int i=0; i<5; i++) printf("%d ", arr[i]);

B. arr[5] = arr[4] * 2;
for(int i=0; i<=6; i++) printf("%d ", arr[i]);

C. arr[4] *= 2;
for(int i=1; i<6; i++) printf("%d ", arr[i]);

D. arr[4] = arr[4] * 2;
for(int i=0; i<6; i++) printf("%d ", arr[i]);

Solution

1. Step 1: Correctly update value at index 4

   Index 4 is the 5th element. Doubling means multiply by 2. Both arr[4] = arr[4] * 2; and arr[4] *= 2; work, but check loop next.

2. Step 2: Print entire array correctly

   Array size is 6, so loop must run from 0 to 5 (i<6). arr[4] = arr[4] * 2; for(int i=0; i<6; i++) printf("%d ", arr[i]); uses correct loop range and prints all elements.

3. Final Answer:

   arr[4] = arr[4] * 2; for(int i=0; i<6; i++) printf("%d ", arr[i]); -> Option D

4. Quick Check:

   Update index 4, loop 0 to 5 to print all [OK]

Hint: Use zero-based index and loop from 0 to size-1 [OK]

Common Mistakes:

• Using wrong index for update
• Looping beyond array size causing errors
• Printing partial array missing elements