C constant

Integer constants

Integer constants can be decimal, octal or hexadecimal constants. The prefix specifies the base: 0x or 0X for hexadecimal, 0 for octal, and without a prefix the default is decimal.

Integer constants can also have a suffix. The suffix is ​​a combination of U and L. U represents an unsigned integer (unsigned) and L represents a long integer (long). The suffix can be uppercase or lowercase, and U and L can be in any order.

The following are examples of several integer constants:

212         /* 合法的 */215u        /* 合法的 */0xFeeL      /* 合法的 */078         /* 非法的：8 不是八进制的数字 */032UU       /* 非法的：不能重复后缀 */

The following are examples of various types of integer constants:

85         /* 十进制 */0213       /* 八进制 */0x4b       /* 十六进制 */30         /* 整数 */30u        /* 无符号整数 */30l        /* 长整数 */30ul       /* 无符号长整数 */

Floating point constants

Floating Point constants consist of an integer part, a decimal point, a fractional part, and an exponent part. You can represent floating-point constants in decimal or exponential form.

When expressed in decimal form, the decimal point, exponent, or both must be included. When expressed in exponential form, an integer part, a decimal part, or both must be included. Signed exponents are introduced with e or E.

The following are examples of floating point constants:

3.14159       /* 合法的 */314159E-5L    /* 合法的 */510E          /* 非法的：不完整的指数 */210f          /* 非法的：没有小数或指数 */.e55          /* 非法的：缺少整数或分数 */

Character constants

Character constants are enclosed in single quotes, for example, 'x' can be stored in ## In a simple variable of type #char.

Character constant can be an ordinary character (such as 'x'), an escape sequence (such as '\t'), or a universal character (such as '\u02C0').

In C, there are some specific characters. When they are preceded by a backslash, they have special meanings and are used to represent newlines (\n) or tabs (\t )wait. The following table lists some such escape sequence codes:

#Escape sequenceMeaning \\\ characters\'' characters\"" Characters\?? Characters\aAlarm ringtone\bBackspace key\fForm feed\nLine break\rEnter\tHorizontal tab##\v\ooo##\xhh . . .One or more digits in hexadecimal number

The following example shows some escape sequence characters:

#include <stdio.h>int main(){
   printf("Hello\tWorld\n\n");   return 0;}

When the above code is compiled and executed, it produces the following results:

Hello   World

String constants

String literals or constants are enclosed in double quotes "". A string contains characters similar to character constants: ordinary characters, escape sequences, and universal characters.

You can use spaces as delimiters to separate a long string constant into lines.

The following example shows some string constants. The strings displayed in the following three forms are the same.

"hello, dear""hello, \

dear""hello, " "d" "ear"

Define constants

In C, there are two simple ways to define constants:

1. Use #define preprocessor.

2. Use the const keyword.

#define preprocessor

The following is the form of using the #define preprocessor to define constants:

#define identifier value

Please see the following example for details:

#include <stdio.h>#define LENGTH 10   #define WIDTH  5#define NEWLINE '\n'int main(){   int area;  
  
   area = LENGTH * WIDTH;
   printf("value of area : %d", area);
   printf("%c", NEWLINE);   return 0;}

When the above code is compiled and executed, it produces the following results:

value of area : 50

const Keywords

You can use the const prefix Declare a constant of the specified type as follows:

const type variable = value;

Please see the following example for details:

#include <stdio.h>int main(){   const int  LENGTH = 10;   const int  WIDTH  = 5;   const char NEWLINE = '\n';   int area;  
   
   area = LENGTH * WIDTH;
   printf("value of area : %d", area);
   printf("%c", NEWLINE);   return 0;}

When the above code is compiled and executed, it will produce the following results:

value of area : 50

Please note that it is a good programming practice to define constants in uppercase letters.

Vertical tab
One to three digits in octal number