C language preprocessing two (macro definition--#define)

//#define macro definition (macro definition general capitalization)// Point -and-#define的作用域: Starting with # define, from top to bottom, if you encounter #undef, it ends at #undef, if not the current entire file#include<stdio.h>#include<stdlib.h>// Knowledge Point two ---Macro definition function//macro Definition Action 1: can be used to define constants #defineX 10//macro Definition Action 2: Replace any part of the code (usually for easy language, or encrypted code)#defineWater is the source of all things, main.//Note: The macro definition is not a normal C statement, there is no need to add a semicolon at the end of the line, such as a semicolon is also substituted#defineY 100;// Knowledge Point three --and macro definition with parameters (can partially override function calls)//① a macro with a parameter is simply a substitution, does not find the value of the actual parameter, the value of the parameter is computed when the function is called//when the ② function is called, the parameters are allocated memory, and the macros are simply replacements, not memory//The ③ function has the concept of a return value, the macro has no concept of return value, an expression is returned, and the result of the expression is equivalent to the return value of the function.//The ④ function is called when there is a type, and the macro substitution does not require a type//the code is longer when the ⑤ macro replaces the expansion, and the function call does not//Jump when function is called, return time consuming, resource occupied//multiple function calls increase program run time without affecting preprocessing time//multiple macro calls, which can increase preprocessing time and run short#defineFL (x) x*x//find the minimum number of two numbers#defineMymin (x, y) x>y?y:x#defineMytex (x, y) x*y#defineMYTEX2 (x, Y) (×) * (y)//#define MYTEX2 ((x), (y)) (x) * (y) Such a macro definition is wrong, the parameters of the macro with parameters can not have ()// Knowledge Point four ----the role of # in a macro definition statement is to change a parameter to a string#defineMYTEF (x) printf ("%s\n", #x)#defineMYTEF2 (x) printf ("%s---%d\n", #x, X)// Knowledge Point Five --in the C language macro, "# #" is called a connector//It is a preprocessing operator that uses two language symbols to form a single language symbol//the language symbol here is not necessarily a macro variable, and the "# #" operator can "glue" two tokens (for example, identifiers) together to become a marker//Example:#defineL (x) s# #x//for example L (1) is equivalent to S1#defineL1 (x) person # #x//for example L (1) is equivalent to people 1#definePN (x) etes# #x//for example L (1) is equivalent to people 1//Note: The "# #" operator cannot be present as the first or last element//#define PN2 (x) # #x Error//#define PN2 (x) x## error (direct execution is not wrong, should # #会把 "error" two characters as a symbol)//if the macro definition with parameters is longer, you can use the ' \ ' connection#defineMytre (x) if (x>1) {\printf ("this number is greater than 1, AH \ n");}Else{printf ("that's not really a big number."); }voidgo1 () {//NOTE: #define is not restricted by block statements    #defineZ 10.0//the following main function can still use the z}voidEtes1 () {printf ("I'm a function etes1\n .");}voidEtes2 () {printf ("I'm a function etes1\n .");}voidWater is the source of all things () {intA[x] = {0 }; //X = 5; BUG: Error 1 C2106: "=": the left operand must be an lvalue, indicating that x is a constant//printf ("%d", Y); Error: 2 err C2059: syntax error: ")"//because y is here;printf ("%d\n", X); printf ("%d\n", Z);//print 0, because here Z is a double type of//Note: Macro expansion is simply a substitution that can contain any character in a string,//It can be a constant, or it can be an expression, and the preprocessor does not perform a syntax check for macro expansion.     #undefX//macro definition for end x    //printf ("%d\n", X); Error//macro definition with Parametersprintf"%d\n", FL (3)); printf ("%d\n", Mymin (3,8));//Printing 3printf ("%d\n", Mytex (1+7,2+7));//Print//Note: Define is only replaced, so Mytex (1+7, 2+7)) is equivalent to 1+7*2+7printf"%d\n", MYTEX2 (1+7,2+7));//Print//application of # in macro definitionMytef (hello);//print "Hello"//in the macro definition # also applies to the name of the print variable    intA1 =9;    MYTEF2 (A1); Mytre (2)        //use of the "# #" operator        intL1), L1 (1); //the L (1) here is S1S1 =Ten; Person 1=3; printf ("the value of L (1) is%d\n"L1)); MYTEF2 (L1 (1)); //the "# #" operator is used to invoke various functionsPN (1)(); //PN (1) is equivalent to Etes1System ("Pause");}

C language preprocessing two (macro definition--#define)