arrays in C

Complete

lang
/
c

Succeeds: arrays

Summary

Array

fixed size
occupy contiguous memory locations, each index is adjacent

// correct
int numbers[5];                        // [?, ?, ?, ?, ?], mem space is reserved
int scores[5] = {90, 85, 78, 92, 88};  // [90, 85, 78, 92, 88]

char letters[] = {'a', 'b', 'c', 'd'}; // compiler infers size = 4

int zeros[5] = {0};                    // [0, 0, 0, 0, 0], rest get initialized as 0

// initializing more values than declared
int wrong[2] = {1, 2, 3}; // warning: excess elements in array initializer

// initializing after declaration
int late[5];
late[5] = {8, 23, 12, -3, 6}; // error: expected expression before ‘{’ token

Assignment

#define N 10 // good practice to define constants as macros

int source[N] = { 10, 20, 30, 40, 50 };
int dest[N];

// array names are fixed pointers
dest = source; // error: assignment to expression with array type

int i;
for (i = 0; i < N; i++) {
	dest[i] = source[i]; // proper way, copies each index
}

when an array is declared, the OS finds a sufficient chunk of memory, thats why the pointer to that address cannot be altered

Concept

Array parameters in functions

// function prototypes
int sumArray(int [], int);
int sumArray(int arr[], int size);

// function definition
int sumArray(int arr[], int size) { 
	int i, sum = 0;
	
	for (i = 0; i < size; i++) {
		sum += arr[i];
	}
	
	return sum;
}
int sumArray(int arr[8], int size) { // 8 will be ignored, pass in the size of the array as another parameter
	... 
}

// or
// function prototypes
int sumArray(int *, int);

// function definition
int sumArray(int *arr, int size) { 
	int i, sum = 0;
	
	for (i = 0; i < size; i++) {
		sum += *(arr + i); // have to handle pointers differently
	}
	
	return sum;
}

Arrays with pointer arithmetic

int a[3] = {1, 2, 3};

// pointers
printf("%p\n", a);        // base
printf("%p\n", &a[0]);    // base
printf("%p\n", a + 1);    // base + 1 * sizeof(int)
printf("%p\n", &a[1]);    // base + 1 * sizeof(int)

// dereferencing / indexing
printf("%d\n", *a);       // 1
printf("%d\n", a[0]);     // 1
printf("%d\n", *(a + 1)); // 2
printf("%d\n", a[1]);     // 2

Application

Multi-dimensional array

int matrix[3][3] = {
    {1, 2, 3},
    {4, 5, 6},
    {7, 8, 9}
};
printf("%d\n", matrix[1][2]);  // prints 6

Reversing an array

// iterative solution
void reverseArray(int arr[], int size) {
	int i = 0, j = size - 1;
	while (i < j) { // 2 pointer approach
		int tmp = a[i];
		a[i] = a[j];
		a[j] = tmp;
		
		++i;
		--j;
	}
}

// recursive solution
void reverseArray(int arr[], int size) {
	if (size > 1) {
		int tmp = arr[0];
		arr[1] = arr[size - 1];
		arr[size - 1] =  tmp;
		
		reverseArray(arr + 1, size - 2); // &arr[1] would also work
	}
}

Long from two ints

int llong[2] = { 0x61626364, 0x65666768 };

printf("%lx\n", (long) *llong);     // 0x61 62 63 64
printf("%lx\n", *((long *) llong)); // 0x65 66 67 68 61 62 63 64