C language functions (2)

Preface

Before writing the official content , First, check the last blog （http://t.csdn.cn/JRw3H） Add some knowledge ：

Function declaration and definition

Declaration of functions

The function declaration text is defined as follows ：
1. Tell the compiler that there is a function called , What are the parameters , What is the return type . But does it exist , Function declarations do not determine .
2. The declaration of a function usually precedes the use of the function . To satisfy the requirement of declaration before use .
The code sample is listed in the last blog , Therefore, I will not repeat .

Definition of function ：

The definition of a function refers to the concrete implementation of a function , Explain the function realization .

The figure above shows the graphic display of function definition .

The parameters of the function

There are two arguments to a function , Formal parameters and actual parameters .

Formal parameters （ Shape parameter ）

Its definition is as follows ：
Formal parameters refer to the variables in brackets after the function name , Because formal parameters are only instantiated when the function is called ( Allocate memory units ), So it's called formal parameter . Formal parameters are automatically destroyed when the function call is completed . So formal parameters are only valid in functions .

Formal parameters are only valid within the scope of the function , Automatically destroy after function call , So the formal parameter names of any function can be the same , It can also be the same as the variable name of the main function . The code example is as follows ：

#include <stdio.h>
int add(int x, int y)
{
    
	int result = x + y;
	return result;
}
int main()
{
    
	int x, y;
	scanf("%d%d", &x, &y);
	int c = add(x, y);
	printf("%d", c);
	return 0;
}

Code above , The variables of the main function are x and y, The formal parameter of a function can also be x,y, The program doesn't report errors .

The actual parameter

Its definition is as follows ：
The parameters that are actually passed to the function , It's called the actual parameter .
The argument can be : Constant 、 Variable 、 expression 、 Functions, etc .
Whatever the type of argument is , When you make a function call , They all have to have certain values , In order to pass these values to the parameter .

Pictured above , Among them add（a,b） It can also become add（3,4）、add（1+2,1+3） or add（add（1,2）,add（1,3））. These replacement programs will not report errors .

Nested calls and chained access to functions

Functions and functions can be combined according to actual needs , That is, calling each other .

Nested calls

Here is a code example ：

#include <stdio.h>
void new_line()
{
    
    printf("hehe\n");
}
void three_1ine()
{
    
    int i = 0;
    for (i = 0; i < 3; i++) 
    {
    
        new_line();
    }
}
int main()
{
    
    three_1ine();
    return 0;
}

As shown above in three_1ine() Called in the function new_line() function , This is the nested call of a function . The following is an example .

In addition, pay attention to , Functions can be called nested , But you can't nest definitions .

Chained access

The definition for ：
Take the return value of one function as the parameter of another function . The following is a code example ：

#include <stdio.h>
#include <string.h>
int main()
{
    
	char arr[20] = "he11o";
	int ret = strlen(strcat(arr, "bit"));// Chained access to functions 
	printf("%d\n",ret);
	return 0;
}

If the reader does not know the above procedure strlen Functions and strcat The purpose of function must be searched （ There are many library functions , Some library functions that are not commonly used and have not been seen must give play to the power of search engines , Don't ignore it because you don't often use it or haven't seen it ）.strcat The string returned by the function will be treated as strlen The parameters of the function , This is the chained access of functions . Execution results ：

Example 2：

#include <stdio.h>
int main()
{
    
printf("%d",printf("%d",printf("%d",43)));
return 0;
}

Let's expand here , In the above code example printf The return value of the function is Number of numbers , Such as printf(“%d”,43), It will print first 43, Then return 2, Then the previous one printf Can print numbers 2 Then return 1, final printf Can print numbers 1, Then return 1.
The results are as follows ：

Function recursion

What is recursion ？
1. The programming skill of program calling itself is called recursion .
2. Recursion as an algorithm is widely used in programming languages . A procedure or function has the function of directly or indirectly calling itself in its definition or description
In popular language , Function recursion is a function that calls itself .
Two necessary conditions for using recursion ：
1. There are restrictions , When this constraint is met , Recursion doesn't continue .
2. After each recursive call, it gets closer and closer to this constraint .
If recursion does not meet these two conditions , Then you will program dead recursion , The program will run all the time , Finally, it was forcibly terminated .
Let's start with an example

#include <stdio.h>
int main()
{
    
	printf("hehe\n");
	main();
	return 0;
}

The above is a dead recursion main（） Functions are always using themselves , But there are no termination conditions .
Then let's look at the principle of recursion ：

#include <stdio.h>
void print(int n)
{
    
	if (n > 9)
	{
    
		print(n / 10);
	}
	printf("%d ", n % 10);
}
int main()
{
    
	unsigned int num = 0;
	scanf("%d", &num);
	print(num);
	return 0;
}

The above code is a simple recursion . Let's explain recursion with pictures and texts ：

Finally, expand

Not void function , If you don't write the return value , The function will return a value by default .
Some compilers return ： The result of the last instruction .
The code is as follows ：

#include <stdio.h>
int test()
{
    
	int a = 10;
	int b = 20;
	int c = a + b;
}
int main()
{
    
	printf("%d\n", test());
	return 0;
}

The final production is not easy , If you feel good, please click one button three times , The author is welcome to point out the mistakes in the comment area or private letter .