Unit 3: Arrays, structures and unions

An array is a collection of elements of the same type stored in consecutive memory locations under one name. Elements are accessed by an index starting from 0.

• One-dimensional: int marks[5] = {70, 82, 65, 90, 77}; — marks[0] is 70.
• Two-dimensional (matrix): int a[3][3]; — a[i][j] is row i, column j.
• Passing an array to a function passes the address of its first element, so the function works on the original array: void display(int a[], int n);

c/* Largest element of an array */
int max = a[0];
for (int i = 1; i < n; i++)
    if (a[i] > max) max = a[i];

c/* Matrix addition and multiplication */
int a[2][2] = {{1, 2}, {3, 4}}, b[2][2] = {{5, 6}, {7, 8}}, c[2][2] = {0};
for (int i = 0; i < 2; i++)
    for (int j = 0; j < 2; j++)
        for (int k = 0; k < 2; k++)
            c[i][j] += a[i][k] * b[k][j];   /* c = {{19, 22}, {43, 50}} */

• Row-major storage: element a[i][j] of an m × n array is at base + (i × n + j) × size.
• Common programs: addition, multiplication (columns of A = rows of B), transpose, sum of diagonals, checking symmetry.

A string is an array of characters ending with the null character '\0'. char name[10] = "SBS"; uses 4 bytes ('S', 'B', 'S', '\0').

A structure groups variables of different data types under one name. Each variable inside it is a member.

cstruct student {
    int roll;
    char name[20];
    float marks;
};
struct student s1 = {1, "Ana", 88.5};
printf("%s scored %.1f", s1.name, s1.marks);

• Members are accessed with the dot operator (s1.roll).
• An array of structures stores many records: struct student cls[60];
• typedef creates a user-defined type name: typedef struct student Student;
• Structures can be passed to functions by value (a copy) or by address (a pointer).

A union is like a structure, but all members share the same memory, so only one member holds a value at a time. Its size equals the size of its largest member.