How does the C language see whether the sign constants are legitimate?

Integers are divided into prefixes x) and suffixes (u, l), and the numbers used in each base system are different;

Floating-point number suffixes f, l, exponential indicates whether the attention is within the range;

Characters and strings should be followed by the escaping characters to see if they are legal.

a: commas are not allowed in integers;

B: Scientific notation is expressed to the power of multiplying by 10, since the exponential part of scientific notation can only be an integer.

c: The sign is an escape symbol, followed by other symbols to represent specific characters, and the backticks are written without anything here;

d: The string constant is 2 consecutive characters, the ASCII code of the first character is 7, the ASCII code of the second character is 0 as the string ending symbol, and the first character is the bell symbol, which will make the default speaker sound when output.

a", double quotation marks indicate that it is a string, not a character.

12', not a character. , is an escape character flag, and there is no escape description after it, and it is not a character at all.

x101', escaped followed by x, indicating that it is a hexadecimal representation, with a maximum of 2 hexadecimal representations, and there are 3 bits here.

128', the escape is directly followed by 3 digits, indicating that it should be a number in the base of eight, and there can be no 8 in the base of eight

2a', ditto, not a character.

Legal analysis: It can only be composed of a-z, a-z, 0-9 and underscores (i.e. no spaces or other characters). You can't start with numbers.

Legal basis: "C Language Programming" Chapter 2 Constant Measure Ruler -E-5"/045" 'basic'0fc 0x4d00 e5 ' variable int auto auto 2-or turbo c 169 inside the constant I know that the numbers 1,4,5,8 are not valid. Constant.

e-5 "/045" 'basic'

0fc 0x4d00 e5 ‘\

Variable. int _auto_ auto 2-orturbo_c _169

Legal Analysis: Constants in the C language: direct constants and symbolic constants.

Direct constants refer to a kind of garbled formula directly expressed by specific data, and direct constants are divided into integer constants, real constants, character constants and modulus string constants. Symbolic constants refer to constants defined by C language identifiers.

Legal basis: Article 35 of the Constitution of the People's Republic of China Citizens of the People's Republic of China have freedom of speech, publication, assembly, association, procession and demonstration.

12. 0xa23 ，-0xf .5e-3 .177 legal.

Real constants: There must be a decimal point, for example, it is wrong to define double a=1, it must be written as double a=, and it must be written as a=1, and 0 is a comma in the middle, which is even more wrong.

e and e represent the power of 10 as the base, and e and e must be followed by an integer, if the decimal is also wrong, for example, here is a decimal, so it is wrong.

Items A and B are both legitimate. 0abc and 10,000 in item c are not legal: because numbers starting with 0 are octal, and octal will not have symbols such as abc; A comma in 10,000 is not allowed.

A second, incorrect.

C low four is incorrect.

1. Imitation First, define a Boolean variable and save the result of the judgment.

2. Next, define a character variable and save the entered identifier.

3. Enter an identifier and save it in the variable ID.

4. Determine the first letter of the identifier to see if it meets the identifier requirements.

5. Use the for loop to determine each character of the identifier to see if it meets the requirements of the identifier.

6. Use the if statement to determine whether each character of the identifier meets the requirements of the identifier.

7. If a character does not meet the requirements, the output identifier is invalid, and the judgment result will be marked as false and the loop will be exited.

8. If the first character does not meet the requirements, the output identifier is invalid.

9. Finally, if the judgment result is true, the output identifier is a legal identifier.