C - Format Specifiers

A format specifiers is a sub-sequences beginning with % in the format string. It helps the compiler to understand the data types and formats in the input or output stream. It is used with scanf() and printf() family of functions while taking input and printing the output.

• Format specifier for printf() function
• Format specifier for scanf() function

Format specifier for printf() function

A standard format specifier string takes the following form when using with printf() family of functions:

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

Where:

• % - starting character of any format specifier
• [flags][width][.precision][length] - sub-specifiers. Additional format values which can be placed between the % and the specifier.

Lets learn about specifier and sub-specifiers in detail:

Specifier

The possible values of %specifier are:

Sub-specifiers

Additional format values can be placed between the % and the specifier (e.g. %.3f) also known as sub-specifier. These sub-specifier are:

Flags

Width

An integer or * that specifies minimum field width. In the case when * is used, the width is not specified in the format string, but as an additional integer value argument preceding the argument that has to be formatted. If the value to be printed is shorter than the width, the result is padded with blank spaces. (Note: This is the minimum width: The value is never truncated.)

Precision

Period . followed by an integer or *. In the case when * is used, the precision is not specified in the format string, but as an additional integer value argument preceding the argument that has to be converted. If the value of this argument is negative, it is ignored. If neither a number nor * is used, the precision is taken as zero.

• For d, i, o, u, x and X specifiers: It signifies the minimum number of digits.
• For a, A, e, E, f and F specifiers: It signifies the number of decimal digits.
• For g and G specifiers: It signifies the maximum number of significant digits.
• For s specifier: It specifies a cutoff point, setting maximum number of characters.

Length

Modifies the length of the data type. For example - %i is used for signed decimal integer, with ll length sub-specifier it becomes %lli which can be used for long long int data type. Here is the complete list of length sub-specifier.

Length sub-specifier for printf() family of functions

See <inttypes.h> for the specifiers for extended types.

Note: If multiple sub-specifiers are provided, they must be in %[flags][width][.precision][length]specifier order.

Example: Different specifiers

In the example below, the printf() function is used to print formatted data.

#include <stdio.h>
 
int main (){
  int x = 10;
  int * ptr;
  ptr = &x;

  printf("Characters: %c %c \n", 'b', 66);
  printf("Decimals: %d %i \n", 200, 300);
  printf("More Decimals: %ld %li \n", 20000L, 30000L);
  printf("Octals: %o %#o \n", 100, 100);
  printf("Hexadecimals: %x %#x %X %#X \n", 100, 100, 100, 100);
  printf("Strings: %s \n", "Hello");
  printf("Scientific notation: %e %E \n", 123.45, 123.45);
  printf("Floats: %2.0f %2.2f %2.4f \n", 3.1416, 3.1416, 3.1416);
  printf("Positive signed number = %+.2f \n", 3.1416); 
  printf("Padded number = %05d \n", 89);
  printf("Number with Width = %*d \n", 5, 89);
  printf("Pointer address = %p \n\n", ptr);

  int y;
  printf("Number of character printed so far in this line: %n", &y);
  printf("%d", y);
  return 0;
}

The output of the above code will be:

Characters: b B 
Decimals: 200 300 
More Decimals: 20000 30000 
Octals: 144 0144 
Hexadecimals: 64 0x64 64 0X64 
Strings: Hello 
Scientific notation: 1.234500e+02 1.234500E+02 
Floats:  3 3.14 3.1416 
Positive signed number = +3.14 
Padded number = 00089 
Number with Width =    89 
Pointer address = 0x7fff07a75f68 

Number of character printed so far in this line: 49

Example: Format date string

In the example below, this function is used to print formatted date string .

#include <stdio.h>
 
int main (){
  int year =  2015;
  int month = 5;
  int day = 1;
  
  //each format specifier specifies padding 
  //with 0 and width of the field
  printf("%04d-%02d-%02d", year, month, day);
  return 0;
}

The output of the above code will be:

2015-05-01

Example: String specifiers

Consider one more example to see how to use string specifiers with a given character string.

#include <stdio.h>
 
int main (){
  char x[] = "catfish";
  const char* y = "many catfishes";

  printf("[%s]\n", x);            //standard string
  printf("[%10s]\n", x);          //right-justified with spaces
  printf("[%*s]\n", 10, x);       //right-justified with spaces
  printf("[%-10s]\n", x);         //left-justified with spaces
  printf("[%-*s]\n", 10, x);      //left-justified with spaces
  printf("[%10.8s]\n",  y);       //right-justified (8 characters cutoff)
  printf("[%-10.8s]\n", y);       //left-justified (8 characters cutoff)
  printf("[%-*.*s]\n", 10, 8, y); //left-justified (8 characters cutoff)
  return 0;
}

The output of the above code will be:

[catfish]
[   catfish]
[   catfish]
[catfish   ]
[catfish   ]
[  many cat]
[many cat  ]
[many cat  ]

Format specifier for scanf() function

The format string contains a sequence of characters that control how characters extracted from the stream are treated:

• Whitespace characters: any single whitespace character in the format string consumes all available consecutive whitespace characters from the input (determined as if by calling isspace() in a loop). Note that there is no difference between "\n", " ", "\t\t", or other whitespace in the format string.
• Non-whitespace character, except format specifier (%): each such character in the format string consumes exactly one identical character from the input stream, or causes the function to fail if the next character on the stream does not compare equal.
• Format specifiers: A sub-sequence beginning with % is used to specify the type and format of the data to be retrieved from the stream and stored into the locations pointed by the additional arguments.

A format specifier for scanf() family of functions follows this prototype:

%[*][width][length]specifier

Where:

• % - starting character of any format specifier
• [*][width][length] - sub-specifiers. Additional format values which can be placed between the % and the specifier.

Lets learn about specifier and sub-specifiers in detail:

Specifier

The following format specifiers are available:

Except for n, at least one character shall be consumed by any specifier. Otherwise the match fails, and the scan ends there.

Sub-specifiers

Additional format values can be placed between the % and the specifier (also known as sub-specifier), which are as follows:

Length sub-specifier for scanf() family of functions

See <inttypes.h> for the specifiers for extended types.

Example: scanf() example

The example below shows the usage of scanf() function.

#include <stdio.h>
 
int main (){
  char fname [31], lname [31];
  int age;
  int i;

  printf("Enter first name: ");
  scanf("%30s", fname);  
  printf("Enter last name: ");
  scanf("%30s", lname); 
  printf("Enter age: ");
  scanf("%d", &age);
  printf("%s %s is %d years old.\n\n", fname, lname, age);

  printf("Enter a hexadecimal number: ");
  scanf("%x", &i);
  printf("Entered value: %#x (%d)\n", i, i);

  return 0;
}

The output of the above code will be:

Enter first name: John
Enter last name: Smith
Enter age: 25
John Smith is 25 years old.

Enter a hexadecimal number: ff
Entered value: 0xff (255)

Example: sscanf() example

The example below shows the usage of sscanf() function.

#include <stdio.h>
 
int main (){
  char mystr [] = "John  25  3000";
  char name [20];
  int age, salary;

  sscanf(mystr, "%s %i %i", name, &age, &salary);
  printf("%s is %i years old and earns %i dollars.\n", name, age, salary);

  return 0;
}

The output of the above code will be:

John is 25 years old and earns 3000 dollars.