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 flip side of printf

If printf sends text to the screen, scanf reads input from the keyboard. They're mirror images of each other.

📤 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 three-step pattern is the bread and butter of console I/O.

⚠️ Why `&` (ampersand) is required

The number-one rule of scanf: put & in front of every variable. Forgetting it usually crashes the program.

🔴 What happens if you forget &

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

What happens:
• The compiler emits a warning (take warnings seriously!)
• Running the program almost always results in a segmentation fault (crash)
• Worst case: it writes into some unrelated memory region, causing a subtle bug that's hard to track down

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

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

      a = the contents of the box (the value)

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

      scanf needs to know where to put the new value,

      so we hand it the address (&a).

💡 Rule of thumb: always prefix scanf variables with &, and never prefix printf's. (printf just reads the value — it doesn't need an address.) The full mechanics are 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 (that's a special C rule). For now, just remember: "arrays are the exception."

🧩 The %lf puzzle — scanning a double

In scanf, double uses %lf. That's different from printf, and it's 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 something huge). It's undefined behavior. Compile with -Wall to catch these.

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

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);
Leaving off & will usually crash the program

Pitfall ② %f for double

scanf("%lf", &b);
In scanf, double needs %lf

Pitfall ③ extra chars

scanf("%d\n", &a); ← don't add \n
scanf("Enter %d", &a); ← don't embed prose

Pitfall ④ consecutive scanfs

Leftover newlines can bite you 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 the number of items it successfully read, which is handy for catching bad input.

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

For real-world input handling, prefer fgets + sscanf. See Safe String Handling.

Try it yourself — scanf

The browser simulator swaps in a prompt dialog wherever scanf would normally read from the keyboard.

scanf_demo.c

Output

Press "Run" to execute...

💡 A few things to try

Drop the & on purpose to see the crash (it's a useful lesson)
Read a double with %f and watch the value get corrupted
Use scanf("%d %d", &a, &b); to read two ints separated by a space
Read a char with %c
Read a string into char s[50] with %s (it stops at whitespace)

Review Quiz

Check your understanding of this lesson!

Q1. Which is the correct way to read an integer into `int x;`?

scanf("%d", &x);

scanf("%d", x);

scanf(x, "%d");

scanf needs the address where it should write the value, so prefix the variable name with &.

Q2. Why is `&` needed on `scanf` arguments?

To pass the variable's address so scanf can write into it

So the function can return a pointer

To treat the value as a string

scanf updates the caller's variable, so it needs a pointer (an address) rather than the value itself.

Q3. Which is correct for reading a string into `char name[50];`?

scanf("%s", name);

scanf("%s", &name);

scanf("%c", name);

Array names decay to the address of their first element, so & isn't needed. %s reads a single word up to whitespace.

Lesson 5: scanf — Read keyboard input

scanf basics — the flip side of printf

The standard pattern

⚠️ Why `&` (ampersand) is required

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

Exception: strings (arrays)

🧩 The %lf puzzle — scanning a double

scanf simulator

Common pitfalls

✅ Safe template

💡 Bonus: check the return value

Try it yourself — scanf

Related Lessons

Review Quiz

Q1. Which is the correct way to read an integer into `int x;`?

Q2. Why is `&` needed on `scanf` arguments?

Q3. Which is correct for reading a string into `char name[50];`?

scanf basics — the flip side of printf

The standard pattern

⚠️ Why & (ampersand) is required

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

Exception: strings (arrays)

🧩 The %lf puzzle — scanning a double

scanf simulator

Common pitfalls

✅ Safe template

💡 Bonus: check the return value

Try it yourself — scanf

Related Lessons

Review Quiz

Q1. Which is the correct way to read an integer into int x;?

Q2. Why is & needed on scanf arguments?

Q3. Which is correct for reading a string into char name[50];?

⚠️ Why `&` (ampersand) is required

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

Q1. Which is the correct way to read an integer into `int x;`?

Q2. Why is `&` needed on `scanf` arguments?

Q3. Which is correct for reading a string into `char name[50];`?