[C language - zero foundation lesson 9] love and hate in functions

Preface

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This issue is about love and hate in functions , If there is any mistake, please point out thank you ！

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Catalog

What is a function

Definition of function

Declaration of functions

The return value of the function

The parameters of the function

Function call

Parameter call

Recursively call

The value transfer of a function

---

What is a function

Let's take an example , You are the boss now. You ask your secretary to complete a task. You can tell him to do a task , Or don't tell him to do a task （ Hint him ） Of course you have to be the boss . As long as you are the boss, you can ask the Secretary to do a task unlimited times , And he didn't complain .

Let's see The literal definition of the function ：

A function is a piece of code that can be reused , Used to perform a function independently , It can receive data transmitted by users , You can also not accept . The function receiving user data shall specify parameters when defining , If user data is not received, there is no need to indicate , According to this point, functions can be divided into parametric functions and nonparametric functions .

Definition of function

The format of the function definition ：

return type Function name （ Parameters , Parameters ,……）

{

Execute statement

}

#include<stdio.h>
void fun (){
		printf("HelloWold");
}
int main(){
	fun();
	return 0;
} 

Be careful ：

1. Functions should be defined before use

2.C The source program starts with the main function , End with the main function .（ The main function is main function ）

3. Arguments are responsible for passing , Formal parameters are responsible for accepting , Assign the argument to the corresponding formal parameter when calling the function

Arguments are actual parameters , Formal parameters are formal parameters .

The actual parameters are in the main function , The function that enters the call cannot be used . You need to pass parameters , The scope of the shape parameter is within the function being called , This function doesn't work . The formal parameter cannot change the value of the actual parameter .

Be careful : Functions can be called nested , However, nested calls are not allowed

Declaration of functions

Function should be defined first and then called. , If the call to the function occurs before the definition , The function is declared .

#include<stdio.h>
void fun();// Declaration of functions  
int main(){
	fun();
	return 0;
}
void fun (){
		printf("HelloWold");
} 

The return value of the function

There is a return value

form 1 : return（ expression ）

#include<stdio.h>
int fun();// Declaration of functions  
int main(){
 printf("%d",fun())	;
	return 0;
}
int fun (){
		return (1+2);
} 

form 2 :return expression ;

#include<stdio.h>
int fun();// Declaration of functions  
int main(){
 printf("%d",fun())	;
	return 0;
}
int fun (){
		return 3+2;
} 

form 3 : adopt return sentence , Function can only return one value

#include<stdio.h>
int fun();// Declaration of functions  
int main(){
 printf("%d",fun())	;
	return 0;
}
int fun (){
		return 10;
} 

The default return value type is int type

No return value void

#include<stdio.h>
void fun();// Declaration of functions  
int main(){
	fun();
	return 0;
}
void fun (){
		printf("HelloWold");
} 

  Be careful

void A defined function body cannot have return sentence

void A defined function cannot be used to assign a value to a variable

Return value refers to return The value behind

---

The parameters of the function

Classification of functions ：

Shape parameter ： Formal parameters when defining a function

Actual parameters ： Parameters when calling the function

Parameter passing

1. Arguments are passed unidirectionally to shape parameters
2. Actual parameters , The order and number of formal parameter types correspond to each other one by one
3. No space is occupied when defining formal parameters , System allocated space at call time

Function call

Function call format

1. The statement calls

Fun(x,y);

2. Expression call

A=fun（x,y）;

Parameter call

Call the return value of the function as an argument , Parameter calls called functions

#include<stdio.h>
int fun (int x,int y){
		return y-x;
} 
int main(){
	int a=10,b=5;
 printf("%d",fun(a,fun(a,b)))	;
	return 0;
}

Recursively call

Recursive conditions ：

1. The main functions of the function

2. Clear end conditions

3. Find out the equivalent relation of function

#include<stdio.h>
int fun (int x){
	if(x==1){// The end condition 
		return 1;
	}
	return  x+fun(x-1); // Function relation 
} 
int main(){

 printf("%d",fun(10));

}

The value transfer of a function

The parameter transfer of function is divided into address transfer （ The pointer will say ） And value passing

Be careful ：

The type of the argument can be different from the argument type , The formal parameter type shall prevail

Storage units occupied by arguments and formal parameters

Formal parameters need to be defined separately , It's a temporary variable , Allocate a storage unit to it when called , Release its storage unit after the function call .

The default type of function is int type , Omission .

The type of the return value of a function is determined by the return value type when the function is defined .

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