C Formatted I/O

Formatted input and output functions in C allow you to control exactly how data is displayed or read. The printf() and scanf() functions are the most commonly used formatted I/O functions in C programming.

The printf() Function

The printf() function is used to display formatted output to the standard output (usually the console/terminal).

Basic Syntax

c

int printf(const char *format, ...);

• Returns the number of characters printed, or a negative value if an error occurs
• format is a string that may contain text and format specifiers
• Format specifiers start with % and are replaced by the values of the additional arguments

Common Format Specifiers

Specifier	Description	Example
%d	Signed decimal integer	printf("%d", 42); → 42
%u	Unsigned decimal integer	printf("%u", 42); → 42
%f	Decimal floating point	printf("%f", 3.14159); → 3.141590
%e	Scientific notation	printf("%e", 3.14159); → 3.141590e+00
%c	Character	printf("%c", 'A'); → A
%s	String of characters	printf("%s", "Hello"); → Hello
%x	Hexadecimal (lowercase)	printf("%x", 255); → ff
%X	Hexadecimal (uppercase)	printf("%X", 255); → FF
%o	Octal	printf("%o", 8); → 10
%p	Pointer address	printf("%p", &num); → 0x7ffee13be8dc
%%	Percent sign	printf("%%"); → %

Format Modifiers

You can use modifiers to control width, precision, alignment, and more:

c

printf("%10d", 42);      // Right-aligned, width 10 →  "        42"
printf("%-10d", 42);     // Left-aligned, width 10 → "42        "
printf("%.3f", 3.14159); // 3 decimal places → "3.142"
printf("%+d", 42);       // Show sign → "+42"
printf("%04d", 42);      // Zero padding → "0042"

The scanf() Function

The scanf() function is used to read formatted input from the standard input.

Basic Syntax

c

int scanf(const char *format, ...);

• Returns the number of input items successfully matched and assigned
• format specifies the expected format of the input
• Subsequent arguments must be pointers to variables where the input will be stored

Important Note

When using scanf(), you need to pass the address of the variable (using &), except for arrays (like strings) which are already pointers.

Examples

c

#include <stdio.h>

int main() {
    int num;
    float decimal;
    char letter;
    char name[50];
    
    printf("Enter an integer: ");
    scanf("%d", &num);  // Note the & operator
    
    printf("Enter a decimal number: ");
    scanf("%f", &decimal);  // Note the & operator
    
    printf("Enter a single character: ");
    scanf(" %c", &letter);  // Note the space before %c to consume any whitespace
    
    printf("Enter your name: ");
    scanf("%s", name);  // No & needed for arrays
    
    printf("\nYou entered:\n");
    printf("Integer: %d\n", num);
    printf("Decimal: %.2f\n", decimal);
    printf("Character: %c\n", letter);
    printf("Name: %s\n", name);
    
    return 0;
}

Advanced Format Specifications

Width and Precision

Both printf() and scanf() support width and precision specifications:

c

printf("%*d", width, number);       // Width specified as a variable
printf("%.*f", precision, number);  // Precision specified as a variable

// Limit input to specific width
scanf("%5s", shortString);  // Read max 5 characters into shortString

Scanning with Field Width

c

int a, b;
scanf("%2d%3d", &a, &b);  // If input is "12345", a=12, b=345

Common Mistakes and Tips

scanf() Limitations

1. scanf() with %s stops reading at whitespace:

c

scanf("%s", name);  // Input "John Doe" will only store "John"

2. To read a line including spaces, use fgets() instead:

c

fgets(name, sizeof(name), stdin);
// Remove trailing newline if necessary
if (name[strlen(name)-1] == '\n')
    name[strlen(name)-1] = '\0';

3. Always check the return value of scanf():

c

if (scanf("%d", &num) != 1) {
    printf("Invalid input!\n");
    // Handle error
}

4. Always include a trailing space when using scanf() repeatedly:

c

scanf("%d ", &num);  // Note the space after %d

Buffer Issues

After calling scanf() to read numeric data, if you want to read character data, it's often necessary to clear the input buffer:

c

int age;
char name[50];

scanf("%d", &age);

// Clear input buffer (consume newline)
while (getchar() != '\n');

printf("Enter name: ");
fgets(name, sizeof(name), stdin);

Summary

• printf() formats and displays data to the standard output
• scanf() reads and parses data from the standard input
• Format specifiers (starting with %) control how data is displayed or interpreted
• Format modifiers allow control of width, precision, alignment, etc.
• Always use the & operator with scanf() except for arrays (strings)
• Be aware of buffer issues and limitations when using these functions

With mastery of formatted I/O, you'll have precise control over how your C programs interact with users.