The formatted input and output in C language is controlled through a format string that dictates how data is read or displayed. These format strings consist of ordinary characters and conversion specifiers, which begin with a percent sign (%).

Basic Structure of Format Strings

%[flags][width][.precision][length]type

1. Conversion Types (Type Specifiers)

This is the core part of the format specifier, specifying the data type to be processed:

1.1 Integer Types

%d or %i – signed decimal integer
%u – unsigned decimal integer
%o – unsigned octal integer
%x – unsigned hexadecimal integer (lowercase letters)
%X – unsigned hexadecimal integer (uppercase letters)

1.2 Floating Point Types

%f – decimal floating-point number (lowercase)
%F – decimal floating-point number (uppercase)
%e – scientific notation (lowercase e)
%E – scientific notation (uppercase E)
%g – automatically selects %f or %e based on value (lowercase)
%G – automatically selects %f or %E based on value (uppercase)
%a – hexadecimal floating-point number (lowercase)
%A – hexadecimal floating-point number (uppercase)

1.3 Characters and Strings

%c – single character
%s – string

1.4 Pointer Types

%p – pointer address

1.5 Special Types

%n – stores the number of characters written so far into the parameter
%% – outputs a percent sign

2. Length Modifiers

Specifies the size of the parameter to distinguish between different sizes of data types:

hh – represents signed char or unsigned char

%hhd – signed character
%hhu – unsigned character
%hhx – unsigned character hexadecimal

h – represents short int or unsigned short int

%hd – short integer
%hu – unsigned short integer

l – represents long int or unsigned long int

%ld – long integer
%lu – unsigned long integer
%lx – long integer hexadecimal

ll – represents long long int or unsigned long long int

%lld – long long integer
%llu – unsigned long long integer

L – represents long double

%Lf – long double floating-point number

z – represents size_t

%zu – size_t type

t – represents ptrdiff_t

%td – ptrdiff_t type

j – represents intmax_t or uintmax_t

%jd – intmax_t type

3. Flags

Controls the alignment, padding, sign, etc. of the output:

- – left-align (default is right-align)
+ – always show sign (even for positive numbers)
(space) – shows space before positive numbers, negative sign before negative numbers
#
– use alternative form:

for %o, add leading 0
for %x and %X, add leading 0x or 0X
for %f, %e, %E, %g, %G, always show decimal point
for %g and %G, keep trailing zeros

0 – use 0 padding (ignored if – flag is present or precision is specified)

4. Width

Specifies the minimum field width:

number – minimum field width
* – width specified by the next argument

5. Precision

Specifies the precision of the output:

.number – precision value
.* – precision specified by the next argument

The meaning of precision depends on the conversion type:

for integers: minimum number of digits (padded with leading zeros if insufficient)
for floating-point numbers: number of digits after the decimal point
for strings: maximum number of characters

Example Code

#include <stdio.h>#include <limits.h>
int main() {    int i = 42;    double d = 3.1415926535;    char c = 'A';    char s[] = "Hello, World!";    unsigned int u = 123;    void *p = &i;
    // Basic examples    printf("Integer: %d\n", i);    printf("Floating point: %f\n", d);    printf("Character: %c\n", c);    printf("String: %s\n", s);    printf("Pointer: %p\n", p);
    // Width and precision    printf("Width 10: %10d\n", i);    printf("Leading zeros: %05d\n", i);    printf("Precision control: %.2f\n", d);    printf("String truncation: %.5s\n", s);
    // Flags    printf("Left align: %-10d end\n", i);    printf("Show sign: %+d\n", i);    printf("Hexadecimal prefix: %#x\n", u);
    // Length modifiers    short si = 123;    long li = 1234567890L;    long double ld = 3.1415926535L;
    printf("Short int: %hd\n", si);    printf("Long int: %ld\n", li);    printf("Long long int: %lld\n", LLONG_MAX);    printf("Long double: %Lf\n", ld);
    // Special formats    printf("Scientific notation: %e\n", d);    printf("Auto select: %g\n", d);    printf("Percentage: 100%%\n");
    // scanf example    int input_int;    char input_str[100];
    printf("Please enter an integer and a string: ");    scanf("%d %s", &input_int, input_str);    printf("You entered: %d and %s\n", input_int, input_str);
    // Using * to skip input    printf("Please enter a date (YYYY-MM-DD): ");    int year, month, day;    scanf("%d-%d-%d", &year, &month, &day);    printf("Date: %d year %d month %d day\n", year, month, day);
    return 0;}

Formatted Input (scanf Series)

The scanf series functions use similar format specifiers, but there are some important differences:

No precision specification is needed (except for %f, %lf, etc. which can specify maximum field width)
For string input, precision does not need to be specified, but maximum width should be specified to prevent buffer overflow
Whitespace characters in the format string match any number of whitespace characters (including zero)
Non-whitespace characters (except for %) must match the input exactly

scanf Format Specifier Example

#include <stdio.h>
int main() {    int a, b;    char c;    char s[20];    float f;
    // Basic input    printf("Please enter two integers: ");    scanf("%d %d", &a, &b);    printf("You entered: %d and %d\n", a, b);
    // Character input    printf("Please enter a character: ");    scanf(" %c", &c); // Note the leading space to skip whitespace characters    printf("You entered: %c\n", c);
    // String input (limit length to prevent overflow)    printf("Please enter a string (up to 19 characters): ");    scanf("%19s", s);    printf("You entered: %s\n", s);
    // Floating point input    printf("Please enter a floating point number: ");    scanf("%f", &f);    printf("You entered: %f\n", f);
    // Using scan set    printf("Please enter a string containing only letters: ");    scanf("%[a-zA-Z]", s);    printf("You entered: %s\n", s);
    // Skipping specific characters    printf("Please enter a date (YYYY/MM/DD): ");    int year, month, day;    scanf("%d/%d/%d", &year, &month, &day);    printf("Date: %d/%d/%d\n", year, month, day);
    return 0;}

Notes

Always limit the length of string input when using scanf to prevent buffer overflow
Check the return value of scanf to ensure all expected inputs are successfully read
For complex input parsing, consider using a combination of fgets and sscanf
Avoid using deprecated functions like gets()

Advanced Usage

1. Positional Parameters

#include&lt;stdio.h&gt;
int main()
{
    // Using positional parameters (n$)
    printf("%2$d %1$d %3$d\n", 10, 20, 30); // Output: 20 10 30
    return 0;
}

2. Dynamic Width and Precision

#include&lt;stdio.h&gt;
int main()
{
    int value = 42;
    int width = 8;
    int precision = 4;
    double pi = 3.1415926535;

    printf("Dynamic width: %*d\n", width, value);
    printf("Dynamic precision: %.*f\n", precision, pi);
    printf("Dynamic width and precision: %*.*f\n", width, precision, pi);

    return 0;
}

3. Scan Sets

#include&lt;stdio.h&gt;
int main()
{
    char s[100];

    // Read a string containing spaces
    printf("Please enter a string containing spaces: ");
    scanf("%[^
]", s); // Read until newline
    printf("You entered: %s\n", s);

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

    // Read specific character set
    printf("Please enter a string containing only numbers and letters: ");
    scanf("%[a-zA-Z0-9]", s);
    printf("You entered: %s\n", s);

    return 0;
}

References

C Library Function – sscanf()C Library Function – printf()Usage of sscanf function

Introduction to Format Input/Output in C Language

Basic Structure of Format Strings

1. Conversion Types (Type Specifiers)

1.1 Integer Types

1.2 Floating Point Types

1.3 Characters and Strings

1.4 Pointer Types

1.5 Special Types

2. Length Modifiers

3. Flags

4. Width

5. Precision

Example Code

Formatted Input (scanf Series)

scanf Format Specifier Example

Notes

Advanced Usage

1. Positional Parameters

2. Dynamic Width and Precision

3. Scan Sets

References

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Basic Structure of Format Strings

1. Conversion Types (Type Specifiers)

1.1 Integer Types

1.2 Floating Point Types

1.3 Characters and Strings

1.4 Pointer Types

1.5 Special Types

2. Length Modifiers

3. Flags

4. Width

5. Precision

Example Code

Formatted Input (scanf Series)

scanf Format Specifier Example

Notes

Advanced Usage

1. Positional Parameters

2. Dynamic Width and Precision

3. Scan Sets

References

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