6.1 summary

6.1.1 The function classification

C A program is made up of functions , The code we write is composed of the main function main() Started . The function is C The basic modules of the program , Is a unit of program code used to accomplish a specific task .

From the perspective of function definition , Functions can be divided into system functions and user-defined functions ：

1. System function , Library functions ： This is provided by the compilation system , Users don't have to define these functions themselves , You can use them directly , Such as our commonly used print function printf().
2. User defined function ： To solve the special needs of users .

6.1.2  Function function

1. The use of function can save repeated code , Reduce code duplication

//  Find the maximum of two numbers 
int max(int a, int b)
{
	if (a > b)
    {
		return a;
	}
	else
    {
		return b;
	}
}

int main()
{
	
	int a1 = 10, b1 = 20, c1 = 0;
	c1 = max(a1, b1);                   //  call max()

    printf("%d\n",c1);
	return 0;
}

       2. Functions can make programs more modular , This is conducive to the reading of the program , To modify and perfect

6.1.3  Function call ： Generate random numbers

When a function is called , Need to care 5 elements ：

1. The header file ： Contains the specified header file
2. Function name ： The function name must be the same as that declared in the header file
3. function ： You need to know what this function can do before calling
4. Parameters ： Parameter types should match
5. Return value ： Receive return values as needed

6.2 Definition of function

6.2.1  Function definition format

The general form of function definition ：

Return type Function name ( List of formal parameters )

{

        Data definition part ;

        Execute statement part ;

}

6.2.2  Function name 、 Shape parameter 、 The body of the function 、 Return value

1) Function name

Theoretically, you can choose a name at will , The best name is known by name , Users should know the function of this function when they see its name . Be careful , There is a circle substitution after the function name (), This represents a function , It's not an ordinary variable name .

2) Parameter list

Formal parameters specified when defining functions , When there is no function call , They don't take up storage units in memory , So they are called formal parameters or virtual parameters , It's called formal parameter for short , Indicates that they are not actual data , therefore , Variables in formal parameters cannot be assigned .

void max(int a = 10, int b = 20) // error, Formal parameters cannot be assigned

{

}

3) The body of the function

Curly braces { } The content in is the content of the function body , Here for Function function implementation process , This is not much different from previous code writing , Before, we wrote the code in main() In the function , Now just write these into other functions .

4) Return value

The return value of the function is passed through return The statement gets ,return The following value can also be an expression .

1. Try to make sure that return The value of the expression in the statement and the return type of the function are of the same type .

int max()                 //  The return value of the function is int type 
{
	int a = 10;
	return a;             //  Return value a by int type , Function return type is also int, matching 
}

      2. If the function returns the type and return The value of the expression in the statement is inconsistent , The return type of the function shall prevail , namely Function return type determines the type of return value . For numerical data , Automatic type conversion .

double max()         //  The return value of the function is double type 
{
	int a = 10;
	return a;        //  Return value a by int type , It will turn into double Type and then return 
}

       3.return Another function of the statement is to interrupt return The executing function , Be similar to break Break the loop 、switch The sentence is the same .

int max()
{
	return 1;   //  Execute to , The function has been interrupted , So the following return 2 Can't be executed to 
	return 2;   //  No implementation 
}

        4. If the function has a return value ,return It must be followed by a value , If the function does not return a value , The function name must be preceded by void keyword , Now , We can also write code through return Interrupt function ( It can also be done without ), Just then ,return There is no content behind ( A semicolon “;” With the exception of ).

void max()        //  It's better to have void keyword 
{
	return;       //  Interrupt function , This is optional 
}

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