🟢 Beginner · Lesson 09

Input & Output – printf, scanf

Q: How do printf and scanf work in C?

printf displays text and values on the screen, using format specifiers like %d and %f to show numbers. scanf reads input typed by the user into variables, using the same kind of specifiers and the address of each variable with &.

Q: What are format specifiers in C?

Format specifiers are placeholders in printf and scanf that tell C the type of data being printed or read. Common ones are %d for integers, %f for floats, %c for characters and %s for strings.

Q: Why do you use & in scanf but not in printf?

scanf needs the memory address of a variable so it can store the input there, which is why you write &x. printf only needs the value to display, so it takes the variable directly without &.

Q: What is the difference between getchar and scanf?

getchar reads a single character from input, while scanf can read formatted data of various types such as numbers and strings. getchar is simpler and useful for reading one character at a time, often in a loop.

Output with printf()

In C, printf() is the standard function to display output on the screen. It is defined in <stdio.h>.

Syntax: printf("format string", variables);

C Language

#include <stdio.h>
int main() {
    printf("Hello, World!\n");              // Simple string
    printf("My age is %d\n", 20);           // Integer
    printf("Price: %.2f\n", 99.99f);         // Float, 2 decimals
    printf("Grade: %c\n", 'A');              // Character
    printf("%d + %d = %d\n", 5, 3, 5+3);    // Multiple values
    return 0;
}

Hello, World! My age is 20 Price: 99.99 Grade: A 5 + 3 = 8

Format Specifiers

Format specifiers tell printf/scanf what type of data to expect or print.

Specifier	Data Type	Example	Output
`%d`	int	`printf("%d", 42)`	42
`%i`	int (same as %d)	`printf("%i", 42)`	42
`%f`	float	`printf("%f", 3.14f)`	3.140000
`%.2f`	float, 2 decimals	`printf("%.2f", 3.14f)`	3.14
`%lf`	double	`printf("%lf", 3.14)`	3.140000
`%c`	char	`printf("%c", 'A')`	A
`%s`	string	`printf("%s", "Hello")`	Hello
`%ld`	long int	`printf("%ld", 100L)`	100
`%lld`	long long int	`printf("%lld", 99999LL)`	99999
`%u`	unsigned int	`printf("%u", 200)`	200
`%o`	Octal	`printf("%o", 8)`	10
`%x`	Hexadecimal	`printf("%x", 255)`	ff
`%p`	Pointer address	`printf("%p", ptr)`	0x7fff...
`%%`	Literal %	`printf("100%%")`	100%

Width and Precision Formatting

C Language

printf("%5d\n", 42);     // "   42"  — right-aligned, width 5
printf("%-5d|\n", 42);    // "42   |" — left-aligned, width 5
printf("%05d\n", 42);    // "00042" — zero-padded, width 5
printf("%.4f\n", 3.14159f); // "3.1416" — 4 decimal places
printf("%8.2f\n", 3.14f); // "    3.14" — width 8, 2 decimals

Escape Sequences

Special characters that can't be typed directly.

Escape Seq.	Meaning	Effect
`\n`	Newline	Moves to next line
`\t`	Tab	Inserts horizontal tab (8 spaces)
`\\`	Backslash	Prints a \
`\"`	Double quote	Prints a "
`\'`	Single quote	Prints a '
`\r`	Carriage return	Moves to beginning of line
`\0`	Null character	String terminator
`\a`	Bell	Audio beep (system)
`\b`	Backspace	Moves cursor back one position

Input with scanf()

scanf() reads input from the user. The & (address-of) operator is required before variable names (except strings).

Syntax: scanf("format", &variable);

C Language

#include <stdio.h>
int main() {
    int age;
    printf("Enter your age: ");
    scanf("%d", &age);  // & is required — gives address of 'age'
    printf("You are %d years old.\n", age);
    return 0;
}

Enter your age: 20 You are 20 years old.

⚠️ Don't forget &

Forgetting & in scanf causes undefined behavior (program crash or wrong output). Always write scanf("%d", &variable) not scanf("%d", variable). Exception: char arrays/strings don't need &.

Reading Multiple Inputs

C Language

#include <stdio.h>
int main() {
    int a, b;
    printf("Enter two numbers: ");
    scanf("%d %d", &a, &b);   // Two integers, space-separated
    printf("Sum = %d\n", a + b);

    float price;
    char  grade;
    scanf("%f %c", &price, &grade);

    return 0;
}

Complete Programs

Program: Simple Calculator

C Language – Calculator

#include <stdio.h>
int main() {
    float a, b;
    printf("Enter two numbers: ");
    scanf("%f %f", &a, &b);

    printf("Sum        = %.2f\n", a + b);
    printf("Difference = %.2f\n", a - b);
    printf("Product    = %.2f\n", a * b);
    if (b != 0)
        printf("Division   = %.2f\n", a / b);
    else
        printf("Division by zero!\n");
    return 0;
}

Enter two numbers: 10 4 Sum = 14.00 Difference = 6.00 Product = 40.00 Division = 2.50

getchar() and putchar()

getchar() reads a single character. putchar() prints a single character.

C Language

#include <stdio.h>
int main() {
    char ch;
    printf("Enter a character: ");
    ch = getchar();
    printf("You entered: ");
    putchar(ch);
    putchar('\n');
    return 0;
}

Summary

printf() is used to display output — requires format specifiers
scanf() reads user input — requires & before variable names
Format specifiers: %d (int), %f (float), %lf (double), %c (char), %s (string)
Escape sequences: \n (newline), \t (tab), \\ (backslash)
Width/precision: %5d, %.2f, %8.3lf
Never forget & in scanf — causes undefined behavior!

Frequently Asked Questions

How do printf and scanf work in C?

printf displays text and values on the screen, using format specifiers like %d and %f to show numbers. scanf reads input typed by the user into variables, using the same kind of specifiers and the address of each variable with &.

What are format specifiers in C?

Format specifiers are placeholders in printf and scanf that tell C the type of data being printed or read. Common ones are %d for integers, %f for floats, %c for characters and %s for strings.

Why do you use & in scanf but not in printf?

scanf needs the memory address of a variable so it can store the input there, which is why you write &x. printf only needs the value to display, so it takes the variable directly without &.

What are escape sequences in C?

Escape sequences are special character combinations starting with a backslash that represent things you cannot type directly, such as \n for a new line and \t for a tab. They are used inside strings to control formatting.

What is the difference between getchar and scanf?

getchar reads a single character from input, while scanf can read formatted data of various types such as numbers and strings. getchar is simpler and useful for reading one character at a time, often in a loop.

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