Data Input and Output (I/O) in C refers to the process of receiving data from the user, reading data from files, and displaying data on the screen. The C language provides several built-in functions through the stdio.h library to handle standard input and output operations.

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1. Standard Input and Output in C

C uses stdin (standard input) and stdout (standard output) to communicate with the user.

• Input: Getting data from the keyboard
• Output: Displaying data on the screen

The main library for this is:

#include <stdio.h>

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2. Output Functions

a) printf()

Used to display formatted output.

Syntax

printf("format specifier", variables);

Example

int age = 20;
printf("Age = %d", age);

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b) puts()

Used to display strings only.

puts("Hello World");

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c) putchar()

Prints a single character.

putchar('A');

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3. Input Functions

a) scanf()

Reads formatted input from the user.

Syntax

scanf("format specifier", &variables);

Example

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

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b) gets() (Deprecated)

Reads an entire line including spaces.
Note: Not recommended due to security issues.

gets(name);

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c) fgets()

A safe alternative to gets().

fgets(name, sizeof(name), stdin);

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d) getchar()

Reads a single character from user input.

char c = getchar();

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4. Format Specifiers

Data Type	Format Specifier
int	%d
char	%c
float	%f
double	%lf
string	%s

Example:

int roll;
float marks;
scanf("%d %f", &roll, &marks);

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5. Example Program for Input and Output

#include <stdio.h>

int main() {
    int age;
    float salary;

    printf("Enter your age: ");
    scanf("%d", &age);

    printf("Enter your salary: ");
    scanf("%f", &salary);

    printf("\n--- Output ---\n");
    printf("Age: %d\n", age);
    printf("Salary: %.2f\n", salary);

    return 0;
}

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6. Difference Between Input and Output Functions

Function Type	Common Functions	Purpose
Input	scanf(), getchar(), fgets()	Read data from user
Output	printf(), puts(), putchar()	Display data to user

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7. Key Points

• All I/O functions belong to stdio.h
• scanf() requires address operator (&) for variables
• printf() is used for formatted output
• Prefer fgets() over gets() for safety
• C uses buffering, meaning input is stored temporarily before processing

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🔗 External C Documentation:
https://www.w3schools.com/c/