Programming Essentials in C - Assignment Two

Task 1: Store and Display Total Marks Using a 2D Array

• Objective: Write a C program to store marks of 5 students in 3 subjects using a 2D array, then calculate and display total marks for each student.
• Code:

#include <stdio.h>

int main() {
  // 2D array for 5 students, 3 subjects
  int marks[5][3];
  int total; // Variable for total marks per student

  // Input marks
  for (int i = 0; i < 5; i++) { // Loop through students
    printf("\nEnter marks for student %d:\n", i + 1);

    for (int j = 0; j < 3; j++) { // Loop through subjects
      printf("Subject %d: ", j + 1);
      scanf("%d", &marks[i][j]); // Read marks
    }
  }

  printf("\n"); // Newline for output formatting

  // Calculate and display total marks
  for (int i = 0; i < 5; i++) { // Loop through students
    total = 0; // Reset total for each student

    for (int j = 0; j < 3; j++) { // Loop through subjects to sum marks
      total += marks[i][j]; // Add subject mark to total
    }

    // Display total marks for the current student
    printf("Total marks for student %d: %d\n", i + 1, total);
  }

  return 0; // Indicate success
}

Task 2: Create and Use a Student Structure

• Objective: Define a student structure with ID, name, and three subject marks, then input and display data for 3 students.
• Code:

#include <stdio.h>

// Define a structure named Student
struct Student {
  int id; // Student ID
  char name[50]; // Student name (character array to hold the string)
  int marks[3]; // Array to hold marks for 3 subjects
};

int main() {
  // Declare an array of 3 Student structures
  struct Student students[3];

  // Input data for 3 students
  for (int i = 0; i < 3; i++) { // Loop through each student
    printf("Enter details for student %d:\n", i + 1); // Prompt for student details
    printf("ID: ");
    scanf("%d", &students[i].id); // Read student ID

    printf("Name: ");
    // Read student name. scanf("%s", ...) reads a single word and stops at whitespace.
    // Be cautious with scanf("%s") for names with spaces; fgets is generally safer.
    scanf("%s", students[i].name);

    // Input marks for the 3 subjects for the current student
    for (int j = 0; j < 3; j++) { // Loop through each subject
      printf("Subject %d marks: ", j + 1);
      scanf("%d", &students[i].marks[j]); // Read subject marks
    }
  }

  printf("---------------------\n"); // Separator line for output

  // Display data for the 3 students
  for (int i = 0; i < 3; i++) { // Loop through each student
    printf("\nStudent %d:\n", i + 1); // Display student number
    printf("ID: %d\n", students[i].id); // Display student ID
    printf("Name: %s\n", students[i].name); // Display student name

    // Display marks for the 3 subjects for the current student
    for (int j = 0; j < 3; j++) { // Loop through each subject
      printf("Subject %d marks: %d\n", j + 1, students[i].marks[j]); // Display subject marks
    }
  }

  return 0;
}

Task 3: Calculate Average with a Function

• Objective: Write a calculateAverage() function that takes a student structure, calculates the average of marks, and updates the structure.
• Code:

#include <stdio.h>

// Define a structure named Student
struct Student {
  int id; // Student ID
  char name[50]; // Student name
  int marks[3]; // Array to hold marks for 3 subjects
  float average; // Variable to store the calculated average
};

// Function to calculate the average marks for a student
// Takes a pointer to a Student structure as input
void calculateAverage(struct Student *s) {
  float sum = 0; // Initialize sum of marks

  // Loop through the marks array to calculate the sum
  for (int i = 0; i < 3; i++) {
    sum += s->marks[i]; // Add current subject's mark to sum (using pointer dereference ->)
  }

  // Calculate the average and store it in the average field of the structure
  s->average = sum / 3.0; // Use 3.0 for floating-point division
}

int main() {
  // Declare and initialize a Student structure variable 's1'
  struct Student s1 = {
    .id = 1, // Initialize ID
    .name = "John", // Initialize name
    .marks = { 80, 85, 90 }, // Initialize marks for 3 subjects
    .average = 0.0 // Initialize average (will be calculated later)
  };

  // Display student details before calculating the average
  printf("Before\n");
  printf("Student ID: %d\nName: %s\nAverage: %.2f\n", s1.id, s1.name, s1.average);

  // Call the calculateAverage function, passing the address of s1
  // This allows the function to modify the s1 structure directly
  calculateAverage(&s1);

  // Display student details after calculating the average
  printf("\nAfter\n");
  printf("Student ID: %d\nName: %s\nAverage: %.2f\n", s1.id, s1.name, s1.average);

  return 0;
}

Task 4: Use Pointers to Modify Student ID

• Objective: Declare a student ID variable and use pointers to modify and display it.
• Code:

#include <stdio.h>

int main() {
  // Declare an integer variable and initialize it
  int studentId = 100;

  // Declare an integer pointer and initialize it to point to the memory address of studentId
  int* idPtr = &studentId;

  // Print the initial value of studentId using the variable name
  printf("student ID before edit: %d\n", studentId);

  // Change the value of the variable studentId using the pointer
  // The '*' operator dereferences the pointer, accessing the value at the memory address it points to
  *idPtr = 101;

  // Print the updated value of studentId using the variable name
  printf("student ID after edit (var): %d\n", studentId);

  // Print the updated value of studentId using the pointer dereference
  // This shows that the value pointed to by idPtr has also changed
  printf("student ID after edit (pointer): %d\n", *idPtr);

  return 0;
}

Task 5: Compare Arrays and Structures

• Arrays

  • Hold multiple elements of the same type (e.g. all int or all double).
  • Laid out contiguously in memory—you can index into them (arr[5]) and perform pointer‑arithmetic.
  • Good when you need a simple list or table of values, all governed by the same operations.

• Structures

  • Bundle together one or more variables, possibly of different types, under a single name.
  • Each member can be accessed by name (person.age, person.name), improving clarity when you have logically related but type‑varying data.
  • Memory layout may include padding to satisfy alignment, but you don't lose the benefit of grouping.

When to prefer a struct over separate arrays?
Use a struct when you're modelling an entity that has multiple attributes—especially if those attributes are not all the same data type. With arrays you'd need parallel arrays (e.g. ages[i], names[i], grades[i]), which is error‑prone and scatters related data. A struct keeps each "record" intact, makes function interfaces cleaner, and aligns with best practices for data encapsulation.

#include <stdio.h>

/* Define a student record with mixed fields */
struct Student {
  char name[32];
  int age;
  float gpa;
} ;

int main() {
  /* Array of structures: each element is a complete student record */
  struct Student classroom[3] = {
    { "Alice", 20, 3.8f },
    { "Bob", 22, 3.2f },
    { "Cara", 19, 3.9f }
  };

  /* Print each student's data */
  for (int i = 0; i < 3; i++) {
    printf(
      "Student %d: %s, age %d, GPA %.2f\n",
      i + 1,
      classroom[i].name,
      classroom[i].age,
      classroom[i].gpa
    );
  }

  return 0;
}

• Why this is better than parallel arrays
  • You avoid mistakes like shifting one array and not the others.
  • Passing one Student to a function is simpler than passing three separate arrays plus an index.
  • The code is self‑documenting: student.age vs. ages[i].

Task 6: Dynamic Access with Pointers

• Objective: Store marks of 5 students in a 1D array and use pointers to display them.
• Code:

#include <stdio.h>

int main() {
  // Declare a one-dimensional array to store marks for 5 students in 3 subjects.
  // Total elements = 5 students * 3 subjects = 15.
  int marks[15];

  // Input marks for 5 students (3 subjects each) into the 1D array.
  // The index calculation i * 3 + j maps the 2D logic (student i, subject j)
  // to the 1D array index.
  for (int i = 0; i < 5; i++) { // Loop through each student
    printf("Enter marks for student %d:\n", i + 1);

    for (int j = 0; j < 3; j++) { // Loop through each subject
      printf("Subject %d: ", j + 1);
      scanf("%d", &marks[i * 3 + j]); // Read marks into the calculated 1D index
    }
  }

  // Declare an integer pointer and initialize it to point to the beginning of the 'marks' array.
  // The array name 'marks' itself acts as a pointer to its first element.
  int* ptr = marks;

  printf("---------------------\n"); // Separator line for output


  // Loop to display marks using the pointer to traverse the array.
  for (int i = 0; i < 5; i++) { // Loop through each student
    printf("Student %d marks: ", i + 1);

    for (int j = 0; j < 3; j++) { // Loop through each subject for the current student
      printf("%d ", *ptr); // Dereference the pointer to access the value at the current memory location
      ptr++; // Increment the pointer to move to the next integer element in the array
    }

    printf("\n"); // Newline after displaying marks for each student
  }


  return 0;
}

Task 7: Complete Student Grading System

• Objective: Build a system using arrays, structures, functions, and pointers to input, calculate averages, and display a report.
• Code:

#include <stdio.h>
#include <string.h> // Include string library for string manipulation functions

// Define a structure named Student
struct Student {
  int id; // Student ID
  char name[50]; // Student name
  int marks[3]; // Array to hold marks for 3 subjects
  float average; // Variable to store the calculated average
};

// Function to calculate the average marks for a student
// Takes a pointer to a Student structure as input
void calculateAverage(struct Student *s) {
  float sum = 0; // Initialize sum of marks

  // Loop through the marks array to calculate the sum
  for (int i = 0; i < 3; i++) {
    sum += s->marks[i]; // Add current subject's mark to sum (using pointer dereference ->)
  }

  // Calculate the average and store it in the average field of the structure
  s->average = sum / 3.0; // Use 3.0 for floating-point division
}

// Function to read a line from input, removing the newline character
void readline(char* restrict s, int n, FILE *restrict stream) {
  fgets(s, n, stream);
  s[strcspn(s, "\n")] = '\0'; // Remove the newline '\n' character
}

// Function to clear the input buffer
void clearInputBuffer() {
  int c;

  while ((c = getchar()) != '\n' && c != EOF);
}

int main() {
  // Declare an array of 5 Student structures
  struct Student students[5];

  // Input data for 5 students
  for (int i = 0; i < 5; i++) { // Loop through each student
    printf("Enter details for student %d:\n", i + 1); // Prompt for student details

    printf("ID: ");
    scanf("%d", &students[i].id); // Read student ID
    clearInputBuffer(); // Clear buffer after reading integer

    printf("Name: ");
    readline(students[i].name, 50, stdin); // Read student name (handles spaces)

    // Input marks for the 3 subjects for the current student
    for (int j = 0; j < 3; j++) { // Loop through each subject
      printf("Subject %d marks: ", j + 1);
      scanf("%d", &students[i].marks[j]); // Read subject marks
    }

    calculateAverage(&students[i]); // Calculate average for the student
  }

  printf("\nStudent Grading Report:\n"); // Header for the report

  // Display data for the 5 students
  for (int i = 0; i < 5; i++) { // Loop through each student
    struct Student s = students[i]; // Create a copy for easier access (optional, could use students[i] directly)
    printf(
      "ID: %d, Name: '%s', Marks: %d, %d, %d, Average: %.2f\n",
      s.id, s.name, s.marks[0], s.marks[1], s.marks[2], s.average // Print student details and calculated average
    );
  }

  // Demonstrate updating a student's mark using a pointer
  struct Student *p = &students[0]; // Declare a pointer 'p' and point it to the first student structure

  p->marks[0] = 90; // Update the first subject's mark for the first student using the pointer

  calculateAverage(p); // Recalculate the average for the first student after updating marks

  printf("\nAfter updating student 1's subject 1 mark to 90:\n"); // Message indicating the update
  printf(
    "ID: %d, Name: '%s', Marks: %d, %d, %d, Average: %.2f\n",
    p->id, p->name, p->marks[0], p->marks[1], p->marks[2], p->average // Display updated details
  );

  return 0;
}

Task 8: Save Names and Averages to File

• Objective: Save student names and averages to grades.txt.
• Code:

#include <stdio.h>
#include <string.h> // String manipulation functions

// Define filename as a constant
const char FILENAME[11] = "grades.txt";

// Student structure
struct Student {
  char name[50]; // Student name
  int marks[3]; // Marks for 3 subjects
  float average; // Calculated average
};

// Function to calculate average marks
void calculateAverage(struct Student *s) {
  float sum = 0;

  for (int i = 0; i < 3; i++) {
    sum += s->marks[i];
  }

  s->average = sum / 3.0;
}

// Function to read a line, removing newline
void readline(char* restrict s, int n, FILE *restrict stream) {
  fgets(s, n, stream);
  s[strcspn(s, "\n")] = '\0';
}

// Function to clear input buffer
void clearInputBuffer() {
  int c;

  while ((c = getchar()) != '\n' && c != EOF);
}

int main() {
  // Open file in write mode ("w"). Creates or overwrites.
  FILE *file = fopen(FILENAME, "w");
  int character; // Variable for reading characters

  // Check if file opened successfully for writing
  if (file == NULL) {
    printf("Error opening %s!\n", FILENAME);
    return 1;
  }

  // Array of 3 Student structures
  struct Student students[3];

  // Input data for 3 students
  for (int i = 0; i < 3; i++) {
    printf("Enter details for student %d:\n", i + 1);
    printf("Name: ");
    readline(students[i].name, 50, stdin);

    for (int j = 0; j < 3; j++) {
      printf("Subject %d marks: ", j + 1);
      scanf("%d", &students[i].marks[j]);
    }

    clearInputBuffer(); // Clear buffer after scanf

    calculateAverage(&students[i]); // Calculate average
  }

  // Write student name and average to file
  for (int i = 0; i < 3; i++) {
    struct Student s = students[i];
    fprintf(file,
      "Name: '%s', Average: %.2f\n",
      s.name, s.average
    );
  }

  // Close the file after writing
  fclose(file);

  // Reopen the file in read mode ("r")
  file = fopen(FILENAME, "r");

  // Check if file opened successfully for reading
  if (file == NULL) {
    printf("Error reading from %s!\n", FILENAME);
    return 1;
  }

  printf("Reading content from %s:\n\n", FILENAME);

  // Read and print file content character by character
  while ((character = fgetc(file)) != EOF) {
    putchar(character);
  }

  // Close the file after reading
  fclose(file);

  return 0; // Indicate success
}

Task 9: Explain "w" vs. "a" File Modes

Mode	Behavior	Existing file content
"w"	Open for writing. If the file exists, truncate it to zero length (erase all data). If absent, create new file.	Discarded (file is cleared)
"a"	Open for writing. If the file exists, writing always goes to the end (append). If absent, create new file.	Preserved; new data added at end

Key point:

• Use "w" when you want to start fresh and do not care about any previous content.
• Use "a" when you want to preserve existing content and add more data at the end.

• Code:

#include <stdio.h>

int main() {
  FILE *fp;

  // Write mode
  fp = fopen("test.txt", "w");
  fprintf(fp, "This is write mode.\n");
  fclose(fp);

  // Append mode
  fp = fopen("test.txt", "a");
  fprintf(fp, "This is append mode.\n");
  fclose(fp);

  // Read and display
  fp = fopen("test.txt", "r");
  char line[100];
  while(fgets(line, 100, fp) != NULL) {
    printf("%s", line);
  }
  fclose(fp);
  return 0;
}

• Output Explanation: First write creates/overwrites with "This is write mode." Append adds "This is append mode." Result: both lines in the file.

Task 10: Read and Display Records from File

• Objective: Read student records from grades.txt and display them.
• Code:

#include <stdio.h> // Standard I/O library
#include <string.h> // String manipulation functions

// Define filename
const char FILENAME[11] = "grades.txt";

int main() {
  // Open file in read mode ("r")
  FILE *file = fopen(FILENAME, "r");

  // Check for file open errors
  if (file == NULL) {
    printf("Error opening file.\n");
    return 1; // Indicate error
  }

  char line[100]; // Buffer for reading lines
  printf("Student Grades Report:\n"); // Report header

  // Read file line by line using fgets
  while (fgets(line, 100, file) != NULL) {
    char name[50]; // Variable for name
    float average; // Variable for average

    /*
      Parse line using sscanf:
      - "Name: %[^,]" extracts name up to comma.
      - ", " matches literal comma and space.
      - "Average: %f" extracts float after "Average: ".
    */
    sscanf(line, "Name: %[^,], Average: %f", name, &average);

    // Print extracted data
    printf("Name: %s, Average: %.2f\n", name, average);
  }

  // Close the file
  fclose(file);

  return 0; // Indicate success
}

Task 11: Complete Grading Application with File Handling

• Objective: Design an application to load data, update marks, recalculate averages, and save back to a file.
• Code:

#include <stdio.h> // Standard I/O library for file operations, printf, scanf
#include <stdlib.h> // Standard library for exit()
#include <string.h> // String manipulation functions for strcspn, strcpy

// Student structure definition
struct Student {
  int id; // Student ID
  char name[50]; // Student name
  int marks[3]; // Marks for 3 subjects
  float average; // Calculated average
};

// Function to calculate average marks for a student
void calculateAverage(struct Student *s) {
  float sum = 0;

  for (int i = 0; i < 3; i++) {
    sum += s->marks[i];
  }

  s->average = sum / 3.0;
}

// Function to read a line from input, removing newline
void readline(char* restrict s, int n, FILE *restrict stream) {
  fgets(s, n, stream);
  s[strcspn(s, "\n")] = '\0'; // Remove the newline '\n' character
}

// Function to clear input buffer after scanf
void clearInputBuffer() {
  int c;

  while ((c = getchar()) != '\n' && c != EOF);
}

// Function to read student details from user input
void readStudent(struct Student *s) {
  printf("\nEnter student ID: ");
  scanf("%d", &s->id);
  clearInputBuffer(); // Clear buffer after reading integer

  printf("Name: ");
  readline(s->name, 50, stdin); // Read name (handles spaces)

  for (int j = 0; j < 3; j++) {
    printf("Subject %d marks: ", j + 1);
    scanf("%d", &s->marks[j]); // Read subject marks
  }

  clearInputBuffer(); // Clear buffer after reading marks

  calculateAverage(s); // Calculate average after input
}


// Function to create an empty file
// Returns 0 on success, 1 on failure
int createFile(const char* filename) {
  FILE *file;
  file = fopen(filename, "w"); // Open file in write mode (creates or clears)

  if (file == NULL) {
    printf("Error: Could not create file %s\n", filename);
    return 1; // Indicate error
  }

  fclose(file); // Close immediately to leave it empty
  return 0; // Indicate success
}

// Function to save student data to a file
void saveData(struct Student students[], int n, const char* filename) {
  FILE *file = fopen(filename, "w"); // Open file in write mode

  if (file == NULL) {
    printf("Error opening file for saving.\n");
    exit(1); // Exit on critical error
  }

  // Write student data in a formatted way
  for (int i = 0; i < n; i++) {
    fprintf(
      file, "%d %s %d %d %d\n", // Format: ID Name Mark1 Mark2 Mark3
      students[i].id, students[i].name, students[i].marks[0],
      students[i].marks[1], students[i].marks[2]
    );
  }

  fclose(file); // Close file after saving
}


// Function to load student data from a file
// 'checked' flag prevents infinite recursion on file not found
void loadData(struct Student students[], int n, const char* filename, short checked) {
  FILE *fp = fopen(filename, "r"); // Open file in read mode

  if (fp == NULL) {
    if (checked) {
      printf("Error opening file for loading.\n");
      exit(1); // Exit if file should exist but doesn't
    } else {
      // If file doesn't exist on first try, create it, get input, save, and try loading again
      printf("File not found. Creating new file and getting student data.\n");
      createFile(filename);

      for (int i = 0; i < n; i++) {
        readStudent(&students[i]); // Get student data from user
      }

      saveData(students, n, filename); // Save the newly entered data

      loadData(students, n, filename, 1); // Try loading again (checked is now 1)
      return;
    }
  }

  // Read student data from file using fscanf
  for (int i = 0; i < n; i++) {
    fscanf(
      fp, "%d %s %d %d %d", // Format to match saveData
      &students[i].id, students[i].name, &students[i].marks[0],
      &students[i].marks[1], &students[i].marks[2]
    );
    calculateAverage(&students[i]); // Calculate average after loading marks
  }

  fclose(fp); // Close file after loading
}

int main() {
  struct Student students[5]; // Array to hold 5 students
  const char* filename = "students.txt"; // Data file name

  // Load student data from file (or get input if file doesn't exist)
  loadData(students, 5, filename, 0);

  // Display current student data
  printf("\nCurrent Student Data:\n");

  for (int i = 0; i < 5; i++) {
    printf(
      "ID: %d, Name: %s, Marks: %d, %d, %d, Average: %.2f\n",
      students[i].id, students[i].name, students[i].marks[0],
      students[i].marks[1], students[i].marks[2], students[i].average
    );
  }

  // Update marks section
  int id;
  printf("\nEnter student ID to update: ");
  scanf("%d", &id); // Get ID to update
  int index = -1; // Index of student to update

  // Find the student by ID
  for (int i = 0; i < 5; i++) {
    if (students[i].id == id) {
      index = i; // Found student, store index
      break;
    }
  }

  // Handle student not found
  if (index == -1) {
    printf("Student not found.\n");
    return 1; // Exit or handle error
  }

  // Get new marks for the selected student
  printf("Enter new marks for student %d:\n", id);

  for (int j = 0; j < 3; j++) {
    printf("Subject %d: ", j + 1);
    scanf("%d", &students[index].marks[j]); // Read new marks
  }

  calculateAverage(&students[index]); // Recalculate average after update

  // Display updated student data
  printf("\nUpdated Student Data:\n");

  for (int i = 0; i < 5; i++) {
    printf(
      "ID: %d, Name: %s, Marks: %d, %d, %d, Average: %.2f\n",
      students[i].id, students[i].name, students[i].marks[0],
      students[i].marks[1], students[i].marks[2], students[i].average
    );
  }

  // Save the updated data back to the file
  saveData(students, 5, filename);
  printf("\nData saved to %s\n", filename);

  return 0; // Indicate successful execution
}