C Input/Output (I/O)

Input/Output in C is performed using functions from the <stdio.h> (Standard Input/Output) library.

Common I/O Functions

Function	Purpose
printf()	Prints output to the console
scanf()	Reads input from the keyboard
fprintf()	Writes output to a file
fscanf()	Reads input from a file
sprintf()	Writes formatted output to a string
sscanf()	Reads formatted input from a string

Format Specifiers

A format specifier begins with % and tells the compiler the data type of the value to print or read.

Specifier	Data Type
%d	int
%u	unsigned int
%c	char
%s	String (char[])
%f	float (scanf), float/double (printf)
%lf	double (scanf)
%x	Hexadecimal
%o	Octal
%p	Pointer (memory address)
%%	Prints %

printf()

printf() is used to display formatted output on the console.

Syntax

printf("format_string", arguments);

Example

int age = 20;
float cgpa = 8.7;
char grade = 'A';

printf("Age = %d\n", age);
printf("CGPA = %f\n", cgpa);
printf("Grade = %c\n", grade);

Important Notes

• Pass values, not addresses.
• Uses format specifiers to determine how data is printed.
• float is automatically promoted to double, so both float and double use %f.

float f = 3.5;
double d = 3.5;

printf("%f\n", f);
printf("%f\n", d);

scanf()

scanf() is used to read formatted input from the keyboard.

Syntax

scanf("format_string", &variable);

Example

int age;
float cgpa;

scanf("%d", &age);
scanf("%f", &cgpa);

Important Notes

• Pass the address of variables using &.
• scanf() stores the user’s input in the given memory location.
• float uses %f, while double uses %lf.

float f;
double d;

scanf("%f", &f);
scanf("%lf", &d);

Why printf() uses x but scanf() uses &x

printf() requires the value

printf() only reads the value and prints it.

int x = 10;
printf("%d", x);

Here, x (value 10) is passed.

scanf() requires the address

scanf() needs to store the input into the variable, so it requires the variable’s memory address.

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

Here, &x (address of x) is passed.

Difference between x and &x

x   → Value stored in the variable
&x  → Memory address of the variable

Example:

int x = 25;

x    = 25
&x   = 0x61FF08   // Example address

Why Arrays (Strings) Don’t Use &

For arrays, the array name itself represents the address of its first element.

char str[20];

scanf("%s", str);
printf("%s", str);

Here,

str == &str[0]

Therefore,

scanf("%s", str);     // Correct
scanf("%s", &str);    // Incorrect

Commonly Used Format Specifiers

%d   // int
%u   // unsigned int
%c   // char
%s   // string
%f   // float (scanf), float/double (printf)
%lf  // double (scanf)
%x   // hexadecimal
%o   // octal
%p   // pointer (address)
%%   // prints %

Summary

• printf() → Displays output → Pass values.
• scanf() → Reads input → Pass addresses using &.
• Exception: Arrays (e.g., strings) do not use & because the array name is already an address.
• In printf(), both float and double use %f.
• In scanf(), float uses %f and double uses %lf.
• Always use the correct format specifier to match the variable’s data type.