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: <br>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.