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

📖 What to learn on this page

✅ Must-know essentials

printf("...%d...\n", x); prints a value
%d=int, %f=double, %c=char, %s=string
scanf("%d", &x); reads input — don't forget &
\n is newline

⭐ Read if you have time

Width/precision (%5d, %.2f)
%x (hex), %o (octal)
scanf return value and error handling

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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Try It Yourself — printf & scanf

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

io_demo.c

Output

Click "Run" to execute...

💡 Try these ideas too

Use one printf with %d %d %d for three variables
Try width/precision specifiers like %5d, %.3f, %-10s
Use \n (newline) and \t (tab) to align text
Print with %c (char), %x (hex), %o (octal)

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