Array of Structure

Programming Language & Its Applications — Pointers, Structure and Data Files in C, NEC licence examination syllabus (Nepal Engineering Council).

Array of Structure

One record is a struct; a table of records is an array of structs — and that is a database in miniature.

Almost every real program manages a collection of records: students in a class, items in an inventory, employees on a payroll. In C that is an array of structs, laid out contiguously with each element the full struct size. Understanding the memory arithmetic lets you compute addresses, and understanding the alternative layout (arrays of separate fields) explains a performance idea that turns up in graphics and data engineering.

Declaration and layout

#include <stdio.h>
#include <string.h>

typedef struct {
    int   roll;
    char  name[20];
    float marks;
} Student;

int main(void) {
    Student cls[5] = {
        {101, "Ram Bahadur",  87.5f},
        {102, "Sita Devi",    91.0f},
        {103, "Hari Prasad",  76.5f},
        {104, "Gita Kumari",  68.0f},
        {105, "Bikash Thapa", 94.5f}
    };
    int n = sizeof cls / sizeof cls[0];

    printf("sizeof(Student)=%zu  array=%zu  n=%d\n",
           sizeof(Student), sizeof cls, n);

    printf("\n%-6s %-16s %7s\n", "Roll", "Name", "Marks");
    printf("-------------------------------\n");
    float total = 0;
    for (int i = 0; i < n; i++) {
        printf("%-6d %-16s %7.1f\n",
               cls[i].roll, cls[i].name, cls[i].marks);
        total += cls[i].marks;
    }
    printf("-------------------------------\n");
    printf("%-23s %7.2f\n", "Class average", total/n);

    /* addresses show the stride */
    printf("\n&cls[0]=%p\n&cls[1]=%p  (stride %ld bytes)\n",
        (void*)&cls[0], (void*)&cls[1],
        (char*)&cls[1] - (char*)&cls[0]);
    return 0;
}
Output:
sizeof(Student)=28  array=140  n=5

Roll   Name               Marks
-------------------------------
101    Ram Bahadur         87.5
102    Sita Devi           91.0
103    Hari Prasad         76.5
104    Gita Kumari         68.0
105    Bikash Thapa        94.5
-------------------------------
Class average             83.50

&cls[0]=0x7ffd1a2b3400
&cls[1]=0x7ffd1a2b341c  (stride 28 bytes)

Worked example 1 — searching and sorting records

#include <stdio.h>
#include <string.h>
#include <stdlib.h>

typedef struct { int roll; char name[20]; float marks; } Student;

/* linear search by roll: O(n) */
int find_by_roll(const Student *a, int n, int roll) {
    for (int i = 0; i < n; i++) if (a[i].roll == roll) return i;
    return -1;
}

/* binary search needs the array SORTED by roll: O(log n) */
int bsearch_roll(const Student *a, int n, int roll) {
    int lo = 0, hi = n - 1;
    while (lo <= hi) {
        int mid = lo + (hi - lo)/2;      /* no overflow */
        if (a[mid].roll == roll) return mid;
        if (a[mid].roll <  roll) lo = mid + 1;
        else                     hi = mid - 1;
    }
    return -1;
}

/* comparators for qsort */
int by_marks_desc(const void *x, const void *y) {
    float d = ((const Student*)y)->marks
            - ((const Student*)x)->marks;
    return (d > 0) - (d < 0);       /* safe float compare */
}
int by_name(const void *x, const void *y) {
    return strcmp(((const Student*)x)->name,
                  ((const Student*)y)->name);
}

int main(void) {
    Student cls[5] = {
        {103,"Hari",76.5f}, {101,"Ram",87.5f},
        {105,"Bikash",94.5f},{102,"Sita",91.0f},
        {104,"Gita",68.0f}
    };
    int n = 5;

    int i = find_by_roll(cls, n, 104);
    printf("linear: roll 104 -> %s\n",
           i >= 0 ? cls[i].name : "not found");

    qsort(cls, n, sizeof cls[0], by_marks_desc);
    printf("\nRank by marks:\n");
    for (int k = 0; k < n; k++)
        printf("  %d. %-8s %.1f\n", k+1, cls[k].name,
               cls[k].marks);

    qsort(cls, n, sizeof cls[0], by_name);
    printf("\nAlphabetical: ");
    for (int k = 0; k < n; k++) printf("%s ", cls[k].name);
    printf("\n");
    return 0;
}
Output:
linear: roll 104 -> Gita

Rank by marks:
  1. Bikash   94.5
  2. Sita     91.0
  3. Ram      87.5
  4. Hari     76.5
  5. Gita     68.0

Alphabetical: Bikash Gita Hari Ram Sita

Worked example 2 — nested structures

#include <stdio.h>

typedef struct { int dd, mm, yyyy; } Date;

typedef struct {
    int   id;
    char  name[24];
    Date  joined;          /* a struct INSIDE a struct */
    float salary;
} Employee;

void print_emp(const Employee *e) {
    printf("%-4d %-14s %02d/%02d/%4d  Rs %9.2f\n",
        e->id, e->name,
        e->joined.dd, e->joined.mm, e->joined.yyyy,
        e->salary);
}

int main(void) {
    Employee staff[3] = {
        {1, "Ram Shrestha",  {15, 3, 2019}, 68000.0f},
        {2, "Sita Adhikari", { 1, 7, 2021}, 54500.0f},
        {3, "Hari Magar",    {22,11, 2017}, 81250.0f}
    };

    printf("%-4s %-14s %-12s %12s\n",
           "ID","Name","Joined","Salary");
    float payroll = 0;
    for (int i = 0; i < 3; i++) {
        print_emp(&staff[i]);
        payroll += staff[i].salary;
    }
    printf("\nMonthly payroll: Rs %.2f\n", payroll);
    printf("Annual payroll : Rs %.2f\n", payroll * 12);

    /* nested access chains the dot operator */
    printf("staff[2] joined in year %d\n",
           staff[2].joined.yyyy);
    return 0;
}
Output:
ID   Name           Joined             Salary
1    Ram Shrestha   15/03/2019  Rs  68000.00
2    Sita Adhikari  01/07/2021  Rs  54500.00
3    Hari Magar     22/11/2017  Rs  81250.00

Monthly payroll: Rs 203750.00
Annual payroll : Rs 2445000.00
staff[2] joined in year 2017

Array of structs vs struct of arrays

Syllabus points

• Declaring & using array of structures

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Related topics in Pointers, Structure and Data Files in C

• Pointer Arithmetic
• Pointer and Array
• Passing Pointer to Function
• Structure vs Union
• Passing Structure to Function
• Structure and Pointer
• File I/O Operations
• Sequential and Random Access to File