Lecture 23 — File Operations

The Real World Scenario 🌎

Processing Big Data

Imagine your professor gives you a CSV File (Comma Separated Values) containing the exam marks of 1,000 students. You need to calculate the average and assign a Grade for each. How do you do it?

💡 Automation with C

Would you open the Excel file and type 1000 formulas manually?

Yes, it builds character.

No! I will write a C Program to Read & Write!

✓ Exactly! File Handling isn't just about saving data; it's about Automation. You can read huge files in milliseconds using fscanf(), process data, and generate a new report using fprintf().

students.csvThe Input File

RollNo,Name,Marks
101,Abhay,85
102,Rahul,45
103,Priya,92
... (997 more rows)

Level 1: Characters

Character I/O and EOF

The most basic way to read a file is character-by-character using fgetc(). We keep reading until we hit a special marker called EOF (End Of File).

read_chars.cHover to explore ✶

FILE *ptr = fopen("story.txt", "r");
char ch;

// Read character by character until EOF
while ((ch = fgetc(ptr)) != EOF) {
    printf("%c", ch);
}

fclose(ptr);

🔎 The Magic of fgetc()

When you call fgetc(), it reads a character and automatically moves the cursor to the next character. You don't need a counter!

Write counterpart: fputc('A', ptr); writes a single character to the file.

Application 1

Building a Text Analyzer (Word Count)

Let's use our fgetc() logic to count total characters, words, and lines in a file. This is exactly how tools like MS Word count your essay!

word_count.cApplication Code

FILE *file = fopen("essay.txt", "r");
int chars = 0, words = 0, lines = 0;
char ch;

while ((ch = fgetc(file)) != EOF) {
    chars++; // Every keystroke is a char
    
    if (ch == ' ' || ch == '\n') {
        words++;
    }
    if (ch == '\n') {
        lines++;
    }
}
fclose(file);
printf("Words: %d, Lines: %d", words, lines);

Input: essay.txt

Hello world! This is C programming. File handling is fun.

Output

Chars: 56 Words: 9 Lines: 3

Application 2

Formatted I/O: CSV Grade Calculator

Character-by-character is too slow for formatted data. We use fscanf() to read formatted variables (Strings, Ints, Floats) directly, and fprintf() to write!

csv_processor.cHover to explore ✶

FILE *in  = fopen("marks.txt", "r");
FILE *out = fopen("grades.txt", "w");
char name[50];
int marks;

// Read Name and Marks. Returns EOF when file ends.
while (fscanf(in, "%s %d", name, &marks) != EOF) {
    char grade = (marks >= 40) ? 'P' : 'F';
    
    // Write Name, Marks, and calculated Grade
    fprintf(out, "%s %d %c\n", name, marks, grade);
}

fclose(in);
fclose(out);

marks.txt (Input)

Abhay 85 Rahul 35 Priya 92

grades.txt (Output generated)

Abhay 85 P Rahul 35 F Priya 92 P

Knowledge Check ✍

Let's test what you've learned!

Question 1 of 3

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