Multiple Variables
Specifiers are matched with variables in exact sequential order.
int h = 10, m = 30;
printf("%d hours, %d mins", h, m);
// Output: 10 hours, 30 mins
Talking to the Program
Programs need data to work with, and a way to show results. C provides standard library functions to communicate with the console.
What happens if you write
scanf("%d", age); and forget the & symbol?io_basics.cstdio.h
#include <stdio.h>
int main(void) {
int age;
// Output to console
printf("Enter your age: ");
// Input from console
scanf("%d", &age);
printf("You are %d years old.", age);
return 0;
}
Formatted Output
printf Format Specifiers
The printf function replaces format specifiers (like %d or %f) with the values of the variables provided after the comma.
📐
Width and Precision
Control alignment and decimal places inside the % symbol.
float pi = 3.14159;
printf("%.2f", pi); // 3.14
printf("%03d", 5); // 005
Taking Input
Why scanf needs the & (Address) Operator
When a delivery person brings a package, they need your home address, not just your name. scanf works the exact same way.
The Memory Mailbox
Try delivering the value 42 to the variable score.
42
?
Address: 0x7FFE
The Breakdown
int score = 10;
// Memory creates a box at 0x7FFE containing 10.
Missing the &
scanf("%d", score);
You are passing the value `10` to scanf. Scanf tries to deliver the input to memory address `0x0000000A` (10), which belongs to the operating system. Crash!
Using the &
scanf("%d", &score);
You are passing the address `0x7FFE`. Scanf goes to that exact address and deposits the user's input safely.
Common Pitfalls
The Invisible Newline (\n) Trap
When you type a number and press Enter, both the number and a newline character \n enter the keyboard buffer. scanf("%d") reads the number but leaves the \n behind!
Keyboard Input Buffer
Scenario: The user typed 42, pressed Enter, then typed Y and pressed Enter.
Step 1: Read the Integer
%d automatically ignores leading spaces, reads the digits, and stops exactly at the first non-digit (the \n).
scanf("%d", &age);
Value stored in `age`:
?
Step 2: Read the Character
Unlike %d, the %c specifier reads the very next character in the buffer, even if it is a newline!
Reads the leftover \n
Space skips the \n first
Value stored in `grade`:
?
Alternative Functions
getchar and putchar
While `printf` and `scanf` are versatile, C provides specialized, faster functions when you only need to read or write a single character.
⌨️
getchar()
Reads a single character directly from the standard input (keyboard buffer).
char ch;
printf("Press any key: ");
ch = getchar();
🖥️
putchar()
Writes a single character directly to the standard output (screen).
char initial = 'A';
putchar(initial);
putchar('\n'); // Prints a newline
Knowledge Check
Test Your Understanding
Question 1 of 5
Final recap
Key Takeaways
%
Match Specifiers
Ensure the number and type of % format specifiers exactly match the variables you provide.
&
Use Addresses for Input
Always use the & operator with scanf for basic data types so it knows where to store the input.
\n
Mind the Buffer
Numbers leave \n behind. Use a space before %c in scanf(" %c") to safely skip leftover whitespace.