Format Specifiers in C

Format specifiers are used in C to specify the type of data that should be displayed or read during input and output operations. They begin with the % symbol and are commonly used with functions like printf() and scanf().

• Format specifiers tell the compiler how to interpret and process different data types.
• They are used to print or read values such as integers, characters, floats, and strings.

#include <stdio.h>

int main(){
    
    int age = 25;
    float salary = 45000.50;
    char grade = 'A';

    printf("Age: %d\n", age);
    printf("Salary: %.2f\n", salary);
    printf("Grade: %c\n", grade);

    return 0;
}

Age: 25
Salary: 45000.50
Grade: A

Explanation: %d is used to print the integer age, %.2f prints the floating-point value salary with two decimal places, and %c prints the character grade.

Uses of Format Specifiers

Format specifiers are used to tell the compiler how a value should be interpreted during input and output operations. They help to:

• Print values of different data types correctly.
• Read user input according to the required data type.
• Control the formatting of output, such as width, precision, and alignment.
• Improve the readability of program output.

Commonly Used Format Specifiers in C

The following are some of the commonly used format specifiers in C for handling different data types:

1. Character Format Specifier - %c

The %c is the format specifier for the char data type in C language. It can be used for both formatted input and formatted output in C language.

#include <stdio.h>

int main(){
    char c;
    
    // Using %c for character input
    scanf("%c", &c);

    // Using %c for character output
    printf("The entered character: %c", c);
    return 0;
}

Input

R

Output

The entered character: R

Explanation: %c reads a single character using scanf() and prints it using printf().

2. Signed Integer Format Specifier - %d and %i

The %d and %i format specifiers are used with signed integer values.

#include <stdio.h>

int main() {

    int x;

    scanf("%d", &x);

    printf("Using %%d: %d\n", x);
    printf("Using %%i: %i", x);

    return 0;
}

Input

3

Output

Using %d: 3
Using %i: 3

Explanation: Both %d and %i can be used with int values in printf(). In scanf(), %i additionally interprets integer input according to its prefix, allowing decimal, octal, or hexadecimal input.

3. Unsigned Integer Format Specifier - %u

The %u is the format specifier for the unsigned integer data type. If we specify a negative integer value to the %u, it converts the integer to its 2's complement.

#include <stdio.h>

int main(){
    unsigned int var;
    scanf("%u", &var);
    printf("Unsigned Integer: %u\n", var);

    // trying to print negative value using %u
    printf("Printing -10 using %%u: %u\n", -10);
    return 0;
}

Output

3 (Enter by user)
Unsigned Integer: 3
Printing -10 using %u: 4294967286

Explanation: %u formats an unsigned int as a decimal number.

4. Floating-point format specifier - %f

The %f is the floating-point format specifier in C language that can be used inside the formatted input and output of float data type. Apart from %f, we can use %e or %E format specifiers to print the floating-point value in the exponential form.

#include <stdio.h>

int main(){
    float a = 12.67;
    printf("Using %%f: %f\n", a);
    printf("Using %%e: %e\n", a);
    printf("Using %%E: %E", a);
    return 0;
}

Using %f: 12.670000
Using %e: 1.267000e+01
Using %E: 1.267000E+01

Explanation: %f prints the value in decimal notation, while %e and %E print it in scientific notation. The e and E versions differ in the case of the exponent character.

5. Unsigned Octal number for integer - %o

We can use the %o format specifier in the C program to print or take input for the unsigned octal integer number.

#include <stdio.h>

int main() {
    int a = 67;
    printf("%o\n", a);
    return 0;
}

103

Explanation: The decimal value 67 is represented as 103 in octal notation.

6. Unsigned Hexadecimal for integer - %x

The %x format specifier is used in the formatted string for hexadecimal integers. In this case, the alphabets in the hexadecimal numbers will be in lowercase. For uppercase alphabet digits, we use %X instead.

#include <stdio.h>
int main() {
    int a = 15454;
    printf("%x\n", a);
    printf("%X", a);
    return 0;
}

3c5e
3C5E

Explanation: %x prints hexadecimal digits using lowercase letters, while %X uses uppercase letters.

7. String Format Specifier - %s

The %s in C is used to print strings or take strings as input.

#include <stdio.h>

int main(){
    char a[] = "Hi Geeks";
    printf("%s", a);
    return 0;
}

Hi Geeks

Explanation: %s prints characters from the string until the terminating null character ('\0') is encountered.

Using %s with scanf()

#include <stdio.h>

int main() {

    char str[50];

    scanf("%49s", str);

    printf("Entered String: %s", str);

    return 0;
} 

Input

Hi Geeks

Output

Entered String: Hi

Explanation: %s in scanf() reads characters until whitespace is encountered. The field width 49 prevents more than 49 characters from being stored, leaving space for the terminating '\0'.

To read a string containing spaces, functions such as fgets() or an appropriate scansets can be used.

8. Address Format Specifier - %p

The %p format specifier is used to print a pointer value.

#include <stdio.h>

int main() {

    int a = 10;

    printf("Address of a: %p", (void *)&a);

    return 0;
}

Address of a: 0x7fffffffe9cc

Explanation: %p prints the value of a pointer, which is the address stored in the pointer. The argument is converted to void * for portable use with %p.

Input and Output Formatting

Format specifiers can be combined with formatting options to control how output is displayed. Common formatting options include:

• - flag: Left-aligns the output within the specified field width.
• Width: Specifies the minimum number of characters used for the output.
• Precision: Controls the number of digits for floating-point values, the minimum number of digits for integers, or the maximum number of characters printed for strings.
• 0 flag: Pads numeric output with leading zeros when applicable.
• + flag: Displays a plus sign for positive signed numbers.

Example of I/O Formatting

#include <stdio.h>
int main()
{
    char str[] = "geeksforgeeks";
    printf("%20s\n", str);
    printf("%-20s\n", str);
    printf("%20.5s\n", str);
    printf("%-20.5s\n", str);
    return 0;
}

       geeksforgeeks
geeksforgeeks       
               geeks
geeks               

Explanation: %20s right-aligns the string within a minimum width of 20 characters. %-20s left-aligns it. The .5 precision limits the string to at most five characters.

List of C Format Specifiers

The below table contains the most commonly used C format specifiers: