Array of structures

Introduction

An array of structures lets you store many related groups of data together. It helps organize information like a list of people, each with their own details.

When you want to keep a list of students, each with a name, age, and grade.

When you need to store multiple books, each with a title, author, and year.

When you want to manage a collection of products, each with an ID, price, and quantity.

When you want to group related data items together and access them by index.

Syntax

C

struct Person {
    char name[50];
    int age;
    float height;
};

struct Person people[10];

You first define the structure type with struct.

Then you create an array of that structure type to hold many items.

Examples

This creates an array of 5 books, each with title, author, and year.

C

struct Book {
    char title[100];
    char author[50];
    int year;
};

struct Book library[5];

An empty array of structures (size 0) means no elements are stored yet.

C

struct Point {
    int x;
    int y;
};

struct Point points[0];

An array with one element can store exactly one item.

C

struct Item {
    int id;
    float price;
};

struct Item singleItem[1];

Sample Program

This program creates an array of 3 students, fills their details, and prints them one by one.

C

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

struct Student {
    char name[50];
    int age;
    float grade;
};

int main() {
    struct Student students[3];

    // Fill data for each student
    strcpy(students[0].name, "Alice");
    students[0].age = 20;
    students[0].grade = 88.5f;

    strcpy(students[1].name, "Bob");
    students[1].age = 22;
    students[1].grade = 91.0f;

    strcpy(students[2].name, "Charlie");
    students[2].age = 19;
    students[2].grade = 85.0f;

    // Print all students
    printf("List of students:\n");
    for (int i = 0; i < 3; i++) {
        printf("Name: %s, Age: %d, Grade: %.1f\n", students[i].name, students[i].age, students[i].grade);
    }

    return 0;
}

Important Notes

Access elements by index: array[index].

Use strcpy to copy strings into char arrays inside structures.

Time complexity to access an element is O(1) because arrays allow direct access.

Remember to not exceed the array size to avoid memory errors.

Summary

An array of structures stores many grouped data items together.

Define the structure first, then create an array of that structure type.

Access each element by its index to read or change data.

Practice

1. What is the correct way to declare an array of 5 structures named Student in C?

easy

A. Student arr[5];

B. struct Student arr;

C. Student arr;

D. struct arr[5];

Solution

1. Step 1: Understand structure array declaration

   To declare an array of structures, use the structure type followed by the array name and size in square brackets.

2. Step 2: Apply correct syntax for array of structures

   Since Student is the structure type, Student arr[5]; declares an array of 5 Student structures.

3. Final Answer:

   Student arr[5]; -> Option A

4. Quick Check:

   Array of structures = type name[size] [OK]

Hint: Use type name[size] to declare structure arrays [OK]

Common Mistakes:

• Forgetting 'struct' keyword if not typedefed
• Declaring single structure instead of array
• Using wrong syntax like struct arr[5];

2. Which of the following is the correct syntax to access the age member of the 3rd element in an array students of structures?

easy

A. students->age[3];

B. students.age[3];

C. students[3].age;

D. students[2].age;

Solution

1. Step 1: Understand zero-based indexing in arrays

   Arrays in C start at index 0, so the 3rd element is at index 2.

2. Step 2: Access structure member using dot operator

   Use students[2].age to access the age member of the 3rd element.

3. Final Answer:

   students[2].age; -> Option D

4. Quick Check:

   3rd element = index 2, use dot for member [OK]

Hint: Remember arrays start at 0; 3rd element is index 2 [OK]

Common Mistakes:

• Using index 3 instead of 2 for 3rd element
• Using arrow operator on array elements
• Accessing member as students.age[3]

3. Consider the code:

typedef struct {
  int id;
  char name[10];
} Item;

Item items[2] = {{1, "Pen"}, {2, "Book"}};

printf("%s", items[1].name);

What will be the output?

medium

A. Pen

B. Book

C. items[1].name

D. Compilation error

Solution

1. Step 1: Identify the array initialization

   The array items has two elements: first with id=1, name="Pen"; second with id=2, name="Book".

2. Step 2: Understand printf access

   items[1].name accesses the second element's name, which is "Book".

3. Final Answer:

   Book -> Option B

4. Quick Check:

   items[1].name = "Book" [OK]

Hint: Index 1 accesses second element's name [OK]

Common Mistakes:

• Confusing index 0 and 1
• Expecting id to print instead of name
• Syntax errors in initialization

4. What is wrong with this code snippet?

struct Point {
  int x, y;
};

struct Point points[3];
points[0].x = 10;
points[0].y = 20;
points[1].x = 30;
points[1].y = 40;
points[2].x = 50;
points[2].y = 60;

printf("%d", points[3].x);

medium

A. Array size should be 4 instead of 3

B. Missing semicolon after struct definition

C. Accessing points[3] is out of bounds

D. Incorrect struct member access syntax

Solution

1. Step 1: Check array size and indexing

   The array points has size 3, valid indices are 0, 1, 2.

2. Step 2: Identify invalid access

   Accessing points[3] is out of bounds and causes undefined behavior.

3. Final Answer:

   Accessing points[3] is out of bounds -> Option C

4. Quick Check:

   Array index must be less than size [OK]

Hint: Array indices go from 0 to size-1 only [OK]

Common Mistakes:

• Using index equal to array size
• Ignoring array bounds errors
• Assuming array auto-expands

5. You have a structure Employee with members int id; and float salary;. How do you write a function in C that takes an array of Employee and its size, then returns the average salary?

hard

A. float averageSalary(Employee emp[], int n) { float sum=0; for(int i=0;i

B. float averageSalary(Employee *emp, int n) { float sum=0; for(int i=0;isalary; return sum/n; }

C. float averageSalary(Employee emp[], int n) { float sum=0; for(int i=0;i

D. float averageSalary(Employee emp[], int n) { float sum=0; for(int i=0;i

Solution

1. Step 1: Loop through array elements correctly

   Use a for loop from 0 to n-1 and access each element with emp[i].

2. Step 2: Sum the salary member of each element

   Access salary as emp[i].salary and add to sum, then divide by n.

3. Final Answer:

   float averageSalary(Employee emp[], int n) { float sum=0; for(int i=0;i -> Option A

4. Quick Check:

   Loop + emp[i].salary sum / n = average [OK]

Hint: Use emp[i].member to access array struct members [OK]

Common Mistakes:

• Using emp->salary inside loop instead of emp[i].salary
• Accessing emp.salary instead of emp[i].salary
• Adding whole struct instead of member