Mastering I/O Functions in C: `printf`, `scanf`, `fprintf`, and `fscanf`

1. Introduction

Input and Output (I/O) operations in C connect the user to the program. Commonly used functions include printf, scanf, fprintf, and fscanf. These use format specifiers to work with variables and structured data.

2. The `printf` Function

int printf(const char *format, ...);

Used to write formatted output to the screen.

Format Specifiers

Format	Meaning
%d	Signed decimal int
%u	Unsigned decimal
%f	Float/double
%c	Character
%s	String
%x	Hexadecimal
%o	Octal
%%	Literal %

3. The `scanf` Function

int scanf(const char *format, ...);

Used to read formatted input from the user.

Example

int x, y;
scanf("x=%d,y=%d", &x, &y);

Input expected: x=10,y=20. Exact formatting must match.

4. Formatting and Alignment

printf("%10d", 123);   // Right-aligned
printf("%-10d", 123);  // Left-aligned

printf("%.2f", 3.1415); // Output: 3.14

5. File I/O with `fprintf` and `fscanf`

`fprintf`

FILE *fp = fopen("data.txt", "w");
fprintf(fp, "x = %d, y = %.2f\n", 10, 3.14);
fclose(fp);

`fscanf`

int x; float y;
FILE *fp = fopen("data.txt", "r");
fscanf(fp, "x = %d, y = %f", &x, &y);
fclose(fp);

6. Data Types and Format Specifiers

Data Type	Specifier
int	%d
unsigned int	%u
float	%f
double	%lf
char	%c
char[]	%s

7. Using Scanset in `scanf`

scanf("%[^"]", str);      // Read until newline
scanf("%[A-Za-z]", name); // Read only alphabets

8. Reading Email Address

char email[100];
scanf("%[A-Za-z0-9@._]", email);

This only allows valid characters. It does not check structure of email.

9. Unsolved Practice Problems

Read full address including punctuation
Write and read an array to file
Print a table of values using alignment
Login system using file I/O
Manual email structure validation
Date input and validation
Read hexadecimal and octal input
Handle comma-separated input
Store sentences to a file
Print padded numbers

10. Conclusion

Formatted I/O is a fundamental skill in C. By mastering printf, scanf, fprintf, and fscanf, you gain fine control over how data is read and displayed. Pay close attention to format specifiers, alignment, and data types to avoid common errors.