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This article introduces the relevant knowledge of "JavaScript conditional compilation application case analysis". In the operation of actual cases, many people will encounter such a dilemma, so let the editor lead you to learn how to deal with these situations. I hope you can read it carefully and be able to achieve something!

Conditional compilation instructions are mainly used to determine which code in the program can be compiled and which is not compiled. The commands commonly used in conditional compilation are: 1, # if # else and # endif compilation command groups; 2, # ifdef # undef and # endif compilation command groups; 3, # ifndef # define and # endif compilation command groups.

Specifically, it is:

(1)

# if expression

Code segment

# endif

If the expression is true, the following code is compiled, otherwise it is not compiled.

# endif is the end of the # if preprocessing instruction and cannot be omitted.

Setting the expression to 0 can serve as a comment on the code.

(2)

# if expression

Code snippet 1

# else

Code snippet 2

# endif

If the expression is true, snippet 1 is compiled and executed, otherwise snippet 2 is executed.

Program case:

/ *

Objective: to design a compiler switch, which controls the execution of the program by compiling the switch. If the switch is turned on, it will

Lowercase letters in known strings are converted to uppercase letters, otherwise uppercase letters are converted to lowercase letters.

, /

# include

# define LEN 100

# define LOWERCASE 1

Int main ()

{

Char str [LEN]

Char c

Int I = 0

Printf ("Enter a string:\ n")

Gets (str)

While ((c = str [I])! ='\ 0') / / traversal string

{

I + +

# if LOWERCASE

If (c > ='A'& & c ='a'& & c ='a'& & telegram [I] ='Z'& & telegram [I] <'Z'))

{

Telegram [I] + = 1; / / becomes the next letter

}

Else if (('z' = = telegram [I]) | ('Z' = = telegram [I]))

{

Telegram [I]-= 25

}

}

Printf ("Telegraph password message:")

Puts (telegram)

Printf ("\ n")

# else

Printf ("Telegraph message:")

Puts (telegram)

Printf ("\ n")

# endif

Return 0

}

/ *

The output in Code::Blocks is as follows:

Please enter a telegram message:

Abcdefghijklmnopqrstuvwxyz

The telegram password message is: bcdefghijklmnopqrstuvwxyza

, /

(4)

# ifndef Macro name

Code snippet 1

# else

Code snippet 2

# endif

The macro name can be any defined or undefined macro. If the program has never defined a macro before, compile and execute snippet 1, otherwise compile and execute snippet 2.

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