C printf and scanf Tutorial | Format Specifiers & I/O Basics

printf — Display a String

printf displays a string (a sequence of characters) on the screen.
\n is a special escape sequence that means "newline."

printf("Hello World!\n");       // Print a string
printf("5 x 6 = %d\n", 30);    // Embed a number inside the string
printf("result = %d\n", value); // Display a variable's value

Format Specifiers — %d and %f

When printing a variable with printf, you must use the format specifier that matches the variable's type.

Integer → %d

int a = 5;
printf("%d", a); → 5

Width: %2d → " 5", %4d → " 5"

Floating point → %f

double b = 5.23;
printf("%f", b); → 5.230000

Precision: %5.2f → " 1.52" (width 5, 2 decimals)

Mismatched type and format produce garbage output! Using %f for int or %d for double prints 0 or undefined values.

Format Specifier Tester

Type: Value: Format:

Result

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scanf — Read Input from the Keyboard

scanf is a function that reads input from the keyboard. It essentially does the opposite of printf.

int a;
printf("Enter a value for a: ");
scanf("%d", &a);           // Don't forget the &!

double b;
printf("Enter a value for b: ");
scanf("%lf", &b);          // double uses %lf (NOT %f)

scanf Simulator

Type a value below and press "Enter" to see how the value gets stored in the variable.

Enter a value for a:

Value of variable a

(no input)

Result of printf("%d", a)

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Common scanf Pitfalls

Pitfall 1: Don't forget &

scanf("%d", &a);
Without &, the program crashes.

Pitfall 2: double uses %lf

scanf("%lf", &b);
Use %lf, not %f.

Pitfall 3: Don't add extra text

Do not include prompt text or \n in the format string.

Pitfall 4: Beware consecutive scanf

Avoid multiple scanf calls in a row; put a printf between them.

Summary:
int → scanf: %d / printf: %d
double → scanf: %lf / printf: %f

Try It Yourself — printf & scanf

In this simulator, scanf is replaced by an input dialog.

io_demo.c

Output

Click "Run" to execute...

Review Quiz

Check your understanding of this lesson!

Q1. What is the output of printf("%d", 10);?

10.000000

%d is the format specifier for integers. The value 10 is printed as-is.

Q2. Which scanf call correctly reads an integer?

scanf("%d", x);

scanf("%d", &x);

scanf("%d", *x);

scanf requires the address of the variable, so you must prefix it with & (the address-of operator).

Q3. What is the output of printf("%.2f", 3.14159);?

3.14

3.14159

3.1

%.2f prints a floating-point number with two digits after the decimal point. 3.14159 becomes 3.14.

Lesson 4: printf & scanf

printf — Display a String

Format Specifiers — %d and %f

Format Specifier Tester

scanf — Read Input from the Keyboard

scanf Simulator

Common scanf Pitfalls

Try It Yourself — printf & scanf

Related Lessons

Review Quiz

Q1. What is the output of printf("%d", 10);?

Q2. Which scanf call correctly reads an integer?

Q3. What is the output of printf("%.2f", 3.14159);?