2. Input/Output Operations

2.1 Output Operations

2.1.1 Basic Output - printf()

Function Signature:

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

Python vs C Comparison:

Python	C
print("Hello")	printf("Hello\n");
print("Value:", x)	printf("Value: %d\n", x);
print(f"x = {x}")	printf("x = %d\n", x);

Keep in mind that print() in python automatically creates a new line by default

2.1.2 Format Specifiers

Data Type	Format Specifier	Example
int	%d or %i	printf("%d", 42);
float	%f	printf("%.2f", 3.14);
double	%lf	printf("%.2lf", 3.14159);
char	%c	printf("%c", 'A');
string	%s	printf("%s", "Hello");
hexadecimal	%x or %X	printf("%x", 255);
unsigned int	%u	printf("%u", 42u);

2.1.3 Advanced printf() Features

Width and Precision:

printf("%5d", 42);        // Right-aligned in 5 characters: "   42"
printf("%-5d", 42);       // Left-aligned in 5 characters: "42   "
printf("%05d", 42);       // Zero-padded: "00042"
printf("%.2f", 3.14159);  // 2 decimal places: "3.14"
printf("%8.2f", 3.14159); // 8 characters, 2 decimals: "    3.14"

2.1.4 Escape Characters

Escape characters are special character sequences that represent characters that are difficult or impossible to type directly. They start with a backslash (\).

Common Escape Characters:

Escape Sequence	Character	Description	Example
\n	Newline	Moves cursor to next line	printf("Line 1\nLine 2");
\t	Tab	Horizontal tab (8 spaces)	printf("Name:\tJohn");
\"	Double Quote	Literal double quote	printf("He said \"Hello\"");
\'	Single Quote	Literal single quote	printf("It\'s working");
\\	Backslash	Literal backslash	printf("Path: C:\\Program Files");
\r	Carriage Return	Return to beginning of line	printf("Loading\rDone");
\b	Backspace	Move cursor back one position	printf("ABC\bD"); → "ABD"
\0	Null Character	String terminator	char str[] = "Hi\0lo";
\a	Alert (Bell)	System beep/alert sound	printf("\aError!");
\f	Form Feed	Page break	printf("Page 1\fPage 2");
\v	Vertical Tab	Vertical tab	printf("Line 1\vLine 2");

Python vs C Escape Characters:

Purpose	Python	C
New line	print("Line 1\nLine 2")	printf("Line 1\nLine 2");
Tab spacing	print("Name:\tAge")	printf("Name:\tAge");
Quote in string	print("She said \"Hi\"")	printf("She said \"Hi\"");
Backslash	print("C:\\folder")	printf("C:\\\\folder");

Practical Examples:

// Creating formatted output with escape characters
printf("Student Information:\n");
printf("Name:\t\tJohn Doe\n");
printf("Age:\t\t20\n");
printf("GPA:\t\t3.75\n");

// Output:
// Student Information:
// Name:		John Doe
// Age:		20
// GPA:		3.75

// Using quotes within strings
printf("The teacher said, \"Programming is fun!\"\n");
// Output: The teacher said, "Programming is fun!"

// File paths (especially important for Windows)
printf("Save file to: C:\\Documents\\Programs\\myfile.txt\n");
// Output: Save file to: C:\Documents\Programs\myfile.txt

Important Notes:

• In C strings, \0 (null character) automatically terminates the string
• When counting string length, \n, \t, etc. each count as ONE character
• Escape characters work in both printf() format strings and character/string literals

2.2 Input Operations

2.2.1 Basic Input - scanf()

Function Signature:

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

Python vs C Comparison:

Python	C
x = int(input())	scanf("%d", &x);
name = input()	scanf("%s", name);
x = float(input())	scanf("%f", &x);

2.2.2 Important scanf() Considerations

Address Operator (&):

• Most variables need & before the variable name
• Exception: strings (character arrays) don't need &

int age;
char name[50];
float height;

scanf("%d", &age);     // & required for int
scanf("%s", name);     // & NOT needed for string
scanf("%f", &height);  // & required for float

Input Buffer Issues and Whitespace Handling:

The Whitespace Problem: When you press Enter after typing input, scanf() reads the data but leaves the newline character (\n) in the input buffer. This can cause problems with subsequent input operations.

// Problematic code:
int num;
char ch;

printf("Enter a number: ");
scanf("%d", &num);              // User types "5" and presses Enter
                                // Buffer now contains: \n (leftover newline)
printf("Enter a character: ");
scanf("%c", &ch);               // This reads the leftover \n, not user input!
printf("Character: %c\n", ch);  
// Outputs: 
// Character: (it shows nothing because it prints a newline)

What happens step by step:

1. User types "5" and presses Enter → Input buffer: 5\n
2. scanf("%d", &num) reads "5" → Buffer remaining: \n
3. scanf("%c", &ch) immediately reads the leftover \n
4. Program doesn't wait for new character input

Solutions:

Solution 1: Space before %c

int num;
char ch;

printf("Enter a number: ");
scanf("%d", &num);
printf("Enter a character: ");
scanf(" %c", &ch);  // Space before %c consumes all whitespace (spaces, tabs, newlines)

Solution 2: Explicit buffer clearing

int num;
char ch;

printf("Enter a number: ");
scanf("%d", &num);

// Clear the input buffer
while (getchar() != '\n');  // Read and discard until newline

printf("Enter a character: ");
scanf("%c", &ch);

Solution 3: Using getchar() to consume newline

int num;
char ch;

printf("Enter a number: ");
scanf("%d", &num);
getchar();  // Consume the leftover newline

printf("Enter a character: ");
scanf("%c", &ch);

Whitespace Characters in C:

• \n (newline) - ASCII 10
• \t (tab) - ASCII 9
• \r (carriage return) - ASCII 13
• (space) - ASCII 32
• \f (form feed) - ASCII 12
• \v (vertical tab) - ASCII 11

Important scanf() Whitespace Rules:

• %d, %f, %s automatically skip leading whitespace
• %c does NOT skip whitespace (reads exactly one character)
• Adding a space in format string ( %c) makes scanf() skip whitespace
• %[^\n] does not skip leading whitespace but stops at newline

Advanced scanf() Format Specifiers:

1. Character Set Specifiers [...]:

char name[50];

// Read only alphabetic characters
scanf("%[a-zA-Z]", name);

// Read everything except newline
scanf("%[^\n]", name);  // Reads entire line including spaces

// Read only digits
scanf("%[0-9]", name);

// Read only vowels
scanf("%[aeiouAEIOU]", name);

2. Excluding Character Sets [^...]:

char input[100];

// Read everything EXCEPT newline (gets full line with spaces)
scanf("%[^\n]", input);

// Read everything EXCEPT spaces and tabs
scanf("%[^ \t]", input);

// Read everything EXCEPT digits
scanf("%[^0-9]", input);

// Read until comma is encountered
scanf("%[^,]", input);

3. Width Specifiers:

char buffer[10];

// Read maximum 9 characters (leaving room for \0)
scanf("%9s", buffer);

// Read exactly 5 characters
scanf("%5c", buffer);

4. Practical Examples:

// Example 1: Reading full name (including spaces)
char full_name[100];
printf("Enter your full name: ");
scanf(" %[^\n]", full_name);  // Space before % consumes previous newline

// Example 2: Reading until specific delimiter
char email[50];
printf("Enter email: ");
scanf("%[^@]", email);  // Read until @ symbol

// Example 3: Input validation
char grade[10];
printf("Enter grade (A, B, C, D, F): ");
scanf("%[ABCDFabcdf]", grade);  // Only accept valid grades

5. Combining Multiple Inputs:

int day, month, year;
char separator;

// Reading date in format: dd/mm/yyyy or dd-mm-yyyy
printf("Enter date (dd/mm/yyyy or dd-mm-yyyy): ");
scanf("%d%c%d%c%d", &day, &separator, &month, &separator, &year);

// Alternative: reading with specific separators
printf("Enter date (dd/mm/yyyy): ");
scanf("%d/%d/%d", &day, &month, &year);

2.2.3 Alternative Input Methods

getchar() and putchar():

char ch;
ch = getchar();  // Read single character
putchar(ch);     // Output single character

fgets() for Safe String Input:

char name[50];
printf("Enter your name: ");
fgets(name, sizeof(name), stdin);