46. Array Manipulation-i

Question.4

A developer reverses an ADC sample buffer of size 5. The buffer contains {10, 20, 30, 40, 50}. He writes:

int i = 0, j = n - 1;
while (i < j) {
   int temp = arr[i];
   arr[i] = arr[j];
   arr[j] = temp;
   i++; j--;
}

After the loop, what does the array look like?

Need Help? Refer to the Quick Guide below

What is an Array?

An array in C is a contiguous block of memory that stores multiple elements of the same data type. It allows you to:

Group variables together under a single name
Access each item using an index
Efficiently perform operations like iteration, sum, reverse, etc.

Core Array Syntax and Usage in C

Array Declaration

int numbers[10];        // Array of 10 integers
uint8_t buffer[32];     // 32-byte buffer (commonly used in firmware)
Declares a fixed-size array — memory is allocated statically.

Array Initialization

int values[4] = {10, 20, 30, 40};     // full initialization
int empty[4] = {0};                   // all elements = 0
char message[] = "Hi";                // string-style init

If you skip the size (like message[ ]) , the compiler counts the size automatically.

Accessing Array Elements

int arr[3] = {5, 10, 15};
printf("%d", arr[1]);     // prints 10
arr[2] = 20;               // modify element at index 2

The index starts from 0. Always ensure you stay within 0 to n-1.

Array Size with sizeof()

int arr[5];
int total_bytes = sizeof(arr);        // e.g., 20 if int = 4 bytes
int element_bytes = sizeof(arr[0]);  // gives size of int i.e. 4 bytes 
int element_count = sizeof(arr) / sizeof(arr[0]);  // gives 5

This only works within the same scope where the array is declared (not when passed to a function).

Memory Layout (Example)

int arr[4] = {10, 20, 30, 40};

Assuming the starting address is 0x2000, the memory looks like:

Address	Value
0x2000	10
0x2004	20
0x2008	30
0x200C	40

Each int = 4 bytes (on most systems).

Where Arrays Are Used in Firmware

In embedded systems, arrays are used for:

Data buffers (UART, SPI, ADC)
Sensor samples
Circular queues
Protocol packets/frames
Bitmaps/lookup tables

Relevance in Embedded

Arrays = lightweight, deterministic memory structures
You work without dynamic memory, so statically allocated arrays are essential
Used in interrupt routines, communication, and data processing

Common Pitfalls (Practical Tips)

Off-by-one errors: If n = 5, then valid indices are 0 to 4 (not 5).
Wrong type for storage: If you’re reading bytes, use uint8_t[] not int[].
Forgetting to initialize: Especially in statically defined buffers.
Overwriting data in fixed-size buffers — always check bounds.
Mixing pointers and arrays incorrectly — be clear whether you’re passing array base or pointer offset.

Select Answer

{50, 40, 30, 20, 10}
{50, 20, 30, 40, 10}
{10, 20, 30, 20, 10}
{50, 40, 30, 40, 50}