C scanf Tutorial | Reading Keyboard Input Safely

📖 What to learn on this page

✅ Must-know essentials

scanf("%d", &x); reads into a variable
Always put & before the variable
double uses %lf (not %f!)
Don't put extra characters in the format string

⭐ Read if you have time

Check scanf's return value for errors
Whitespace skipping rules
Leftover-input problems and fixes
For strings, prefer fgets (safer)

scanf basics — the opposite of printf

Where printf "sends text to the screen", scanf "reads input from the keyboard". It's the mirror image.

📤 printf (output)

printf("%d\n", a);
// print variable a to screen

📥 scanf (input)

scanf("%d", &a);
// read from keyboard into variable a

The standard pattern

int a;
printf("Enter a: ");   // prompt
scanf("%d", &a);       // read input
printf("a = %d\n", a);

Flow: ① prompt with printf → ② read with scanf → ③ use the value. This 3-step pattern is the bread and butter.

⚠️ Why `&` (ampersand) is required

The #1 rule of scanf: put & before every variable. Forgetting it typically crashes the program.

🔴 What happens if you forget &

int a;
scanf("%d", a);    // ❌ forgot &

Result:
• The compiler emits a warning (take warnings seriously)
• Running it almost always produces a segmentation fault (crash)
• Worst case: writes into another memory region and causes a hard-to-find bug elsewhere

What `&` really is: the variable's "address"

The variable a holds a value. &a is "the memory address where a lives".

      a = "contents of the box" (value)

      &a = "location of the box" (address)

      scanf needs to know where to write the value.

      So we pass the address (&a).

💡 Rule: always prefix scanf variables with &; never prefix printf's. (printf only reads values; it doesn't need an address.) The detailed mechanism is covered in Pointer Basics.

Exception: strings (arrays)

char name[50];
scanf("%s", name);   // ← no & for array name

Why: an array name is already an address (special rule). For now, just remember: "arrays are the exception".

🧩 The %lf mystery — scanning a double

In scanf, double uses %lf. Different from printf, and a classic source of confusion.

Type	scanf specifier	printf specifier
`int`	`%d`	`%d`
`float`	`%f`	`%f`
`double`	`%lf` ⚠️	`%f` (`%lf` also OK)
`char`	`%c`	`%c`
string	`%s`	`%s`

🔴 Common mistake: %f for a double

double b;
scanf("%f", &b);    // ❌ double needs %lf

Result: b ends up with a completely wrong value (often 0.0 or huge). It's undefined behaviour. Compile with -Wall to get warnings.

Mnemonic: scanf + double → use %lf (the l is for "long"). Only scanf is picky here.

scanf simulator

Type a value and press Enter to see how scanf would store it.

Enter a:

variable a

(empty)

printf("%d", a)

—

Common pitfalls

Pitfall ① missing &

scanf("%d", &a);
Omitting & usually crashes

Pitfall ② %f for double

scanf("%lf", &b);
double uses %lf in scanf

Pitfall ③ extra chars

scanf("%d\n", &a); ← no \n
scanf("Enter %d", &a); ← no prose

Pitfall ④ consecutive scanfs

Leftover newlines can bite when mixing %c with other specifiers.

✅ Safe template

int a;
double b;

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

printf("Enter double: ");
scanf("%lf", &b);  // lf!

printf("a=%d, b=%f\n", a, b);

💡 Bonus: check the return value

scanf returns "how many items it read successfully". Useful for detecting bad input.

int a;
if (scanf("%d", &a) != 1) {
    printf("Please enter an integer\n");
    return 1;
}

For production input, prefer fgets + sscanf. See Safe String Handling.

Try it yourself — scanf

The browser simulator substitutes a prompt for scanf input.

scanf_demo.c

Output

Press "Run" to execute...

💡 Try these ideas

Deliberately drop & to see the crash (educational)
Read a double with %f and watch the value corrupt
scanf("%d %d", &a, &b); to read two ints separated by space
Read a char with %c
Read a string into char s[50] with %s (stops at space)

Lesson 5: scanf — Read keyboard input

scanf basics — the opposite of printf

The standard pattern

⚠️ Why `&` (ampersand) is required

What `&` really is: the variable's "address"

Exception: strings (arrays)

🧩 The %lf mystery — scanning a double

scanf simulator

Common pitfalls

✅ Safe template

💡 Bonus: check the return value

Try it yourself — scanf

Related Lessons

scanf basics — the opposite of printf

The standard pattern

⚠️ Why & (ampersand) is required

What & really is: the variable's "address"

Exception: strings (arrays)

🧩 The %lf mystery — scanning a double

scanf simulator

Common pitfalls

✅ Safe template

💡 Bonus: check the return value

Try it yourself — scanf

Related Lessons

⚠️ Why `&` (ampersand) is required

What `&` really is: the variable's "address"