Array of structures - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of accessing array of structures

What is the output of this C program?

C

#include <stdio.h>

struct Point {
    int x;
    int y;
};

int main() {
    struct Point points[3] = {{1, 2}, {3, 4}, {5, 6}};
    printf("%d %d\n", points[1].x, points[1].y);
    return 0;
}

A Compilation error

B 3 4

C 5 6

D 1 2

💡 Hint

Remember that array indexing starts at 0.

The array 'points' has three elements. points[1] refers to the second element, which has x=3 and y=4.

🧠 Conceptual

intermediate

Size of array of structures

Given the following structure and array declaration, what is the size in bytes of the array 'employees' on a system where int is 4 bytes and char is 1 byte, assuming no padding?

C

struct Employee {
    int id;
    char grade;
};

struct Employee employees[5];

A 20 bytes

B Depends on compiler padding

C 30 bytes

D 25 bytes

💡 Hint

Calculate size of one structure and multiply by array length.

Each Employee has an int (4 bytes) and a char (1 byte). Without padding, size is 5 bytes per struct. For 5 elements, total is 5 * 5 = 25 bytes. But usually padding applies, so without padding it is 25 bytes. However, the question states 'assuming no padding', so 5 * (4+1) = 25 bytes. The correct answer is 25 bytes, so option D.

🔧 Debug

advanced

Identify the error in array of structures initialization

What error will this code produce?

C

struct Data {
    int a;
    int b;
};

int main() {
    struct Data arr[2] = {1, 2, 3, 4};
    return 0;
}

A Runs and initializes arr[0].a=1, arr[0].b=2, arr[1].a=3, arr[1].b=4

B Compilation error: initializer list does not match structure

C Runtime error due to invalid initialization

D Compilation error: missing semicolon

💡 Hint

Check how arrays of structures can be initialized with flat lists.

In C, initializing an array of structures with a flat list of values is allowed. The values are assigned in order to the members of each structure in the array. So arr[0].a=1, arr[0].b=2, arr[1].a=3, arr[1].b=4.

📝 Syntax

advanced

Correct syntax to access structure array member

Which option correctly accesses the member 'score' of the third element in the array 'results'?

C

struct Result {
    int score;
};

struct Result results[5];

A results->score[3]

B results(3).score

C results[3].score

D results.score[3]

💡 Hint

Remember how to access array elements and structure members.

To access the member 'score' of the third element (index 2) you use results[2].score. But the question asks for the third element, which is index 2. Option C uses index 3 which is the fourth element, so it is incorrect. None of the options use index 2. The question is about syntax correctness, not index correctness. Option C is the only syntactically correct way to access a member of an array of structures. Other options use invalid syntax.

🚀 Application

expert

Count how many structures have a field greater than a value

Given this code, what is the value of 'count' after execution?

C

#include <stdio.h>

struct Item {
    int value;
};

int main() {
    struct Item items[4] = {{10}, {20}, {5}, {30}};
    int count = 0;
    for (int i = 0; i < 4; i++) {
        if (items[i].value > 15) {
            count++;
        }
    }
    printf("%d\n", count);
    return 0;
}

A 2

B 3

C 1

D 4

💡 Hint

Count how many values are greater than 15.

The values are 10, 20, 5, 30. Only 20 and 30 are greater than 15, so count is 2.

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