What is the define command for?

define translates to definition in English;

define x

That is to say, x is not defined as a program, and if x appears, then its value is;

double a=;

Then a=;

It is to define a number into another character, if you define this data first, you can replace it with the defined character in the subsequent programming, simplifying the program and making the program easy to understand, and then if the number still changes, you can directly change the number after defining. There is no need to change them one by one in the program, which is very troublesome.

It's just my personal understanding. For example, in the y=f(x) equation, define is the x, and if you enter a value of x, you can get the corresponding y

There are three uses, such as substitution of multi-line **, conditional compilation, and a typical macro definition, which are as follows:

1. The most important use of define is conditional compilation.

#ifdefwindows

#endif

#ifdeflinux

#endif

You can set the compilation environment with define at compile time.

2. Typical usage methods.

With macro definitions, we can define and even change the grammatical conventions of the C language ourselves according to our own habits, such as:

#definebegin

intmain()begin

printf("define---n");

end defines a loop.

#defineloopfor(;;

Redefine data types.

#defineitint

3. Define can replace the multi-line **, add one at each line break"\"

#definemax(x,y)dowhile(0)/*notrailing;)*

define uchar unsigned char" means that all the "uchar" in ** after that represents "unsigned char", and the actual replacement operation is performed when the compilation preprocessing is performed.

The define function defines a constant macro. During compilation preprocessing, all "macro names" that appear in the program are replaced with strings in the returner macro definition, which is called "macro substitution" or "macro". Macro definition is done by macro definition commands in the source program.

Correct the upstairs:

define is a macro definition that defines a global variable that can be used throughout the file.

For example, define a 3

i.e. a is considered to be equivalent to 3

int d=10%a;Equivalent to int d=10%3;

The result is d = 1

Macro definitions, e.g. define a b;

This means that when a is encountered in the program, a is replaced with b

#define a b

means. In the program below.

All A is replaced with B

It's all character substitution.

So be careful when using it.

If you define a x+y

When you use a*a in the back.

It will be replaced with x+y*x+y

instead of (x+y)*(x+y).

define is the meaning of macro definition.

A macro definition is to use the macro name to represent a string, and then replace the macro name with that string in the macro case, which is just a simple substitution, the string can contain any character, it can be a constant, or it can be an expression, and the preprocessor does not do any checking on it. If there are errors, they can only be found when compiling the source program that has been macromed.

A macro definition is not a description or statement, and it does not need to be a semicolon at the end of the line, and if a semicolon is added, the semicolon is replaced with it.

A macro definition is scoped from the macro definition command to the end of the source program. To terminate its scope, you can use the undef command.

Other notes: ifdef "Logo".

**Segment. If you define the logo, it ......

ifndef "Logo".

**Segment. If you don't have a define, you're ......

#define

Needless to say, this one.

#endif

Use the condition definition to end with this.

That is, all the places in the program where x occurs are replaced with (y=5).

For example: int main

Replace with Yes.

int main

Question added: define x (y=5).

define is an expression that defines a global constant. Global variables are valid in the global.

It's to replace the individual x with (y=5) in all the ** you write, just like the substitution of text operations.

To see x is to understand x as (y=5).

The macro definition is simply a substitution of x with (y=5)!