The C programming language (2nd) -- Notes -- 1.7

 1.7 function （Functions）

        C The function in is equivalent to Fortran The subroutine of （subroutine） Or functions , It's also equivalent to Pascal The process of （procedure） Or functions . Function provides a simple method for computing encapsulation , Later, when using functions, you don't need to consider how to implement them . Use the function designed correctly , Programmers don't have to think about how the function is implemented , Just know what functions it has .C Medium simple 、 convenient 、 Use functions efficiently . We often see short functions that are called only once after definition , This makes the code snippet more legible .

        So far, , Functions used （printf、getchar、putchar） Are functions provided by the function library . Now I have compiled some functions . C Didn't like Fortran Provide similar to ** Exponentiation operator of , Now write a power function power(m, n) To illustrate the method of function definition .power(m, n) Used to calculate integers m Of n The next power ,n It's a positive integer. . Function call power(2, 3) As the result of the 32. This function is not a practical power function , Can only handle A positive integer of a smaller integer The next power , But this is enough to illustrate the problem .（ A calculation is provided in the standard library x Of y Power function pow(x, y).）

         Here are the functions power(m, n) The definition of and the main program that calls it , You can see the complete program structure .

#include <stdio.h>

int power(int m, int n);

/* test power function */

main()
{
    int i;

    for (i = 0; i<10; ++i) 
        printf("%d %d %d\n", i, power(2,i),power(-3,i));
    return 0;
}

/* power: raise base to n-th power; n >= 0 */
int power(int base, int n)
{
    int i, p;

	p = 1;
    for(i = 1; i <= n; ++i)
        p = p * base;
    return p;
}

The general form of function definition :

 return type   Function name (0 Or multiple parameter declarations )
{
     Declaration part 
     Statement sequence 
}

         Function definition We can use In any order Appear in the One or more source files in , But the same function cannot be stored in multiple files . If the source program is scattered in multiple files , Then you need to do more work when compiling and loading , But this is the operating system , It's not the attribute of language . Tentative assumption main and power The two functions are placed in the same file , What I learned earlier is related to function C Language program Your knowledge is still valid .

         main The function was called twice in the following statement power function ：

printf("%d %d %d\n", i, power(2, i), power(-i, 3));

On every call ,main Functional direction power Function passes two arguments ; When the call execution is completed ,power Functional direction main Function returns a formatted integer and prints . In the expression ,power(2, i) Same as 2 and i The same is an integer （ Not all functions result in integer values , See the first 4 Chapter ）.

         power Function number 1 That's ok

int power(int base, int n)

Statement Type of parameter 、 name as well as The type of the result returned by this function .power The arguments to the function use names only in power Function internal valid , Not visible to any other function ： Other functions can use the same parameter name without conflict . Variable i And p Again ：power Function i And main Function i irrelevant .

        Usually put Function definition Appear in the list in parentheses Variable be called Formal parameters , But the Function call China and The value corresponding to the formal parameter be called The actual parameter .

         power The result of the function passes return Statement returned to main function . keyword return Can be followed by any expression , form ：

return  expression ; 

Functions do not always have return values ; Without expression return Statement will return control to caller , But no useful value is returned . This is equivalent to drawing curly braces at the right end of the function , The function is “ At the end ”. The main function (the calling fuction) You can also ignore the value returned by the function .

        Be careful ,main There is a clause at the end of the function return sentence .main It's also a function , It can also caller Returns a value , The caller is actually The execution environment of the program . In a general way ,‘ The return value is 0’ Indicates normal termination ,‘ The return value is not 0’ Indicates an abnormal condition or an error ending condition . For simplicity , Ahead main Functions are omitted return sentence , But in the future main Function contains return Sentence to remind everyone of ： The program also returns status to its execution environment .

        Appear in the main Declaration statement before function

int power(int m, int n) ;

indicate power There are two functions int Parameters of type , And return a int Type value . This declaration is called a function prototype , It has to work with power The definition and usage of the function are consistent . If the definition of function 、 Usage is inconsistent with the prototype of the function , Will make a mistake .

        The function prototype And Statement in Parameter name The same is not required . in fact , The parameter name in the function prototype is optional , The above function prototype can also be written as ：

 int power (int, int);

        but , Appropriate Parameter name can rise To very good Illustrative effect , Therefore, the parameter name is always indicated in the function prototype .

        review ,ANSI C Same as earlier versions C The biggest difference between languages is Function declaration and definition . Press C The original definition of ,power The function should be written as ：

/* power: raise base to n-th power;n>=0 */
/*         (old-style version) */
power(base, n)
int base, n;
{
    int i, p;

    p = 1;
    for (i = 1; i <= n; ++i)
        p = p * base;
    return p; 
}

among , Parameter name Specify , Parameter type Declare before the left curly bracket . If the type of a parameter is not declared , The default is int type . Function body and ANSI C In the same form .

        stay C In the original definition of language , It can be declared at the beginning of the program in the following form power function ：

int power();

Parameter lists are not allowed in function declarations , So the compiler cannot check at this time power Legitimacy of function calls . in fact , power By default, the function will be assumed to return int Type value , Therefore, the declaration of the whole function can be ignored .

        stay ANSI C In the function prototype syntax defined in , The compiler can easily detect errors in the number and type of parameters in function calls . ANSI  C Old style function declarations and definitions are still supported , This can at least have a transitional stage . But it is strongly recommended that ： When using the new compiler , It's best to use the new function prototype declaration .

        practice 1-15         Rearrange 1.2 Temperature conversion procedure in section , Use function to realize temperature conversion calculation .