Input and output (I/O) operations allow a C program to interact with the user or other systems.
The standard library provides functions for reading input and displaying output.

• Used to display text and variable values on the screen.
• Supports format specifiers for different data types.

Example:

#include <stdio.h>

int main() {
    int age = 20;
    float height = 5.9;
    printf("Age: %d\n", age);         // %d for int
    printf("Height: %.1f\n", height); // %f for float
    printf("Hello, World!\n");        // prints a string
    return 0;
}

• %d : integer
• %f : float/double (in printf)
• %c : character
• %s : string
• %lf : double (in scanf)

• Used to read input from the user.
• Requires the address of the variable (except for strings).

Example:

#include <stdio.h>

int main() {
    int num;
    float fnum;
    char ch;
    char str[20];

    printf("Enter an integer: ");
    scanf("%d", &num);

    printf("Enter a float: ");
    scanf("%f", &fnum);

    printf("Enter a character: ");
    scanf(" %c", &ch); // space before %c to consume newline

    printf("Enter a string: ");
    scanf("%s", str); // no & needed for arrays (strings)

    printf("\nYou entered: %d, %.2f, %c, %s\n", num, fnum, ch, str);
    return 0;
}

• For strings, do not use & before the variable name.
• For characters, use " %c" to avoid reading leftover newline characters.
• For double, use %lf in scanf() and %f in printf().

• getchar() : Reads a single character from input.
• putchar() : Writes a single character to output.
• gets() and puts() : Used for string input/output (not recommended, use fgets() instead for safety).

Example:

char c = getchar();   // reads a character
putchar(c);           // prints a character

char line[50];
fgets(line, sizeof(line), stdin); // safe way to read a string
puts(line);                      // prints the string

• Use printf() for output and scanf() for input.
• Always match format specifiers with variable types.
• For string input, use arrays and avoid using & in scanf.
• For safe string input, prefer fgets() over gets().

Input and output are essential for interactive C programs and for communicating with users.