Functions in C language (detailed explanation)

Catalog

1. What is a function

2.c Classification of functions in language ：

2.1. Library function

2. Custom function

3. The parameters of the function

3.1 The actual parameter （ Actual parameters ）

3.2 Formal parameters （ Shape parameter ）

4. Function call ：

4.1 Value transfer call

4.2 Address call

5. Nested calls and chained access to functions  

5.1 Nested calls  

5.2 Chained access

6. Function declaration and definition

6.1 Function declaration ：

6.2 Function definition ：

7. Function recursion

7.1 What is recursion ？ 

7.2 Two necessary conditions for recursion  

---

1. What is a function

In Wikipedia , The definition of function is Subroutines . Subroutines It's a piece of code in a large program , Consists of one or more statement blocks , He is responsible for completing a specific task , And compared to other code , With relative independence .

Generally, there will be input Parameters And it has a return value , Provide encapsulation of the process and hiding of details . These codes are usually integrated into software
Parts library .

2.c Classification of functions in language ：

2.1. Library function

Why are there library functions ？

In the early c Language has no function , He only stipulates your grammar , such as for How to write loops and so on, and then the rules of various grammatical details are very clear , Is that the code you write can be edited . But one day ,A When you want to print a message on the screen A It implements a similar printf1 Functions of the function . At that time ,B say , I also want a print function , So he wrote a printf2, later c say , He also wants to print . What about him , Also wrote a printf3 function .

These people don't know each other , So let's take a look at . When each of us implements the function of printing , This code , It becomes redundant .

What is the second ？ Low development efficiency , Each of us is repeating the creation theory , You write one , He also wrote a , Others write another . It is written with similar functions , Of course, the development efficiency is low .

The third is not standard , What you write is the same function , But the implementation method may be different . The parameters may also be different , The return value type may also be different .

So for the above reasons . Can you implement some commonly used functions into functions ？ At that time , There is the concept of library function , As long as the parameters of this function are specified . The return type specification is dead . The function name is dead , Then his use method must be exactly the same , The emergence of library functions makes our code development efficiency higher . The code is more standardized .

notes ： However, the use of library functions must include the corresponding header file

Here are two websites that recommend learning ：cppreference.com

cplusplus.com - The C++ Resources Network

So how to learn library functions ？
Here we have a simple look ：http://www.cplusplus.com

This is a c The library of , We can quickly find the functions we used in the left part , Then I will take you to learn with an example Library function . 

strcpy This function is contained in string.h In this header file , This function requires two arguments The pointer , The return value is a character pointer , The pointer is the address ,char * strcpy ( char * destination, const char * source );

Copies the C string pointed by source into the array pointed by destination, including the terminating null character (and stopping at that point).

Translate it into source Point to the C Copy string to destination In the array that points to , Include terminated empty characters ( And stop at that point ). Include '\0' character .

Pointer to the destination array where the content is to be copied.

Pointer to the target array , Where the contents of the target array are copied .

C string to be copied.

To be copied c character string

  The return value is destiination,destiination It's a character pointer .

To sum up , Namely strcpy The function copies one string to another .

2. Custom function

If library functions can do everything , What do programmers do ？ Therefore, it is more important to customize functions . Custom functions are the same as library functions , There's a function name , Return value types and function parameters . But the difference is that these are all designed by ourselves . This gives programmers a lot of room to play .

Composition of functions ：

ret_type fun_name(para1, * )
{
statement;// Statement item
}
ret_type Return type
fun_name Function name
para1    Function parameter

Give me an example , At a glance .

3. The parameters of the function

3.1 The actual parameter （ Actual parameters ）

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 .

3.2 Formal parameters （ Shape parameter ）

Formal parameters refer to the variables in brackets after the function name , Because formal parameters are only instantiated when the function is called （ Within distribution
Storage unit ）, 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 .

The names of formal parameters and arguments can be the same , It doesn't matter .

4. Function call ：

4.1 Value transfer call

The formal and actual parameters of a function occupy different memory blocks , Modification of a parameter does not affect the argument .

Write a function to exchange the contents of two shaping variables  

void swap(int p1, int p2)
{
	int tmp = 0;
	tmp = p1;
	p1 = p2;
	p2 = tmp;
}
int main()
{
	int a = 0;
	int b = 0;
	scanf("%d%d", &a, &b);
	printf(" Exchange before ,a = %d b = %d\n", a, b);
	swap(a, b);
	/*int p1 = &a;
	int p2 = &b;
	swap(p1, p2);*/
	printf(" After exchanging ,a = %d b = %d\n", a, b);
	return 0;
}

It can be seen that , I clearly passed in the parameters , Why haven't the parameters been exchanged yet ？ We are going to use important address calls .

4.2 Address call

Address call is a way to call a function by passing the memory address of the created variable outside the function to the function parameter . This parameter transfer method can establish a real relationship between the function and the variables outside the function , In other words, the variables outside the function can be directly manipulated inside the function .

void swap(int* p1 , int* p2)
{
	int tmp = 0;
	tmp = *p1;
	*p1 = *p2;
	*p2 = tmp;
}
int main()
{
	int a = 0;
	int b = 0;
	scanf("%d%d", &a, &b);
	printf(" Exchange before ,a = %d b = %d\n", a, b);
	swap(&a, &b);
	/*int p1 = &a;
	int p2 = &b;
	swap(p1, p2);*/
	printf(" After exchanging ,a = %d b = %d\n", a, b);
	return 0;
}

In argument , I will a,b The address of was passed in , Formal parameters are used again p1 and p2 Two pointers store a,b The address of , Then use * Operator found a,b And modify the contents .

5. Nested calls and chained access to functions  

5.1 Nested calls  

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

Functions can be called nested , But you can't nest definitions .

5.2 Chained access

Take the return value of one function as the parameter of another function .

Why is this printed 4321 Well ？ We need to check printf The return value of . So the result is 4321 No wonder .

6. Function declaration and definition

6.1 Function declaration ：

  The program is executed step by step , The following picture is the writing method in the textbook , When we call a function , There should be a function declaration , But if the function is written in main Function above , There is no need to declare .

 1. Tell the compiler that there is a function called , What are the parameters , What is the return type . But does it exist , function
    The statement does not determine .
2. The declaration of a function usually precedes the use of the function . To satisfy the requirement of declaration before use .
3. The declaration of the function is usually placed in the header file .

6.2 Function definition ：

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

7. Function recursion

7.1 What is recursion ？ 

The programming skill of program calling itself is called recursion （ recursion）. Recursion as an algorithm is widely used in programming languages . A procedure or function has a method that directly or indirectly calls itself in its definition or description , It usually transforms a large and complex problem into a smaller problem similar to the original problem to solve , The recursion strategy only needs a few programs to describe the repeated calculation needed in the process of solving problems , Greatly reduces the amount of code in the program . The main way to think about recursion is ： Turn the big thing into a small one

7.2 Two necessary conditions for recursion  

There are restrictions , When this constraint is met , Recursion doesn't continue .
After each recursive call, it gets closer and closer to this constraint

I will explain some topics of recursion in detail in the next article , I hope you will like it a little  ！