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46. Array Manipulation-i

Question.5

A developer left-rotates an array by k positions using the brute-force approach. The array has 5 elements. He calls rotate_left(arr, 5, 7):

void rotate_left(int arr[], int n, int k) {
   for (int r = 0; r < k; r++) {
       int temp = arr[0];
       for (int i = 0; i < n - 1; i++)
            arr[i] = arr[i + 1];
       arr[n - 1] = temp;
   }
}

The code works, but what's the performance problem?

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:

AddressValue
0x200010
0x200420
0x200830
0x200C40

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.
     

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