function （ The first part ）

One 、 What is a function ？

Wikipedia definition of function ： Subroutines

Part of the code in a large program , Consists of one or more statement blocks . It's responsible for a particular task , And compared to other generations code , Have Relative independence .

Two 、 The function classification

（ One ） Library function

classification ：

. IO function

**. ** String manipulation functions

. Character manipulation functions

**. ** Memory manipulation function

**. ** Time / Date function

**. ** Mathematical functions

**. ** Other library functions

give an example 1：strcpy function ：

// This is a function that can be copied , If you put arr Copy values to arr1 in .
#include<stdio.h>
#include<string.h>
int main()
{
    
	char arr1[] = {
     0 };
	char arr2[] = "ABC";
	strcpy(arr1, arr2);
	printf("%s", arr1);
	return 0;
}

give an example 2： memset function ：

It means , hold “void* ptr” after “size-t num” A number is adapted into “int main” Form or content .

#include <stdio.h>
#include <string.h>
int main()
{
    
	char arr[] = "hello world!";
	memset(arr, '@', 5); // It can also be written here “arr+ A number ” Rewrite characters in different positions at will .
	printf("%s", arr// It means that hello this 5 Change characters to ’@‘.
	return 0;
}

** notes ：** But a secret that library functions must know is ： Use library functions , Must contain #include Corresponding header file .

（ Two ） Custom function

​ 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 ：

// Like an addition function 
int Add(int x,int y)   int Is the return type , Add For function name ,   In parentheses are parameters .
{
    
    int z = x+y;
    return z;    // These are statement items .
}

give an example ： Write a function , Find the maximum of both ：

#include<stdio.h>
int get_max(int x, int y)   // Use whatever type is returned here , When there is no need to return , use void It's OK .
{
    
    return (x > y) ? x : y;
}
int main()
{
    
    int a = 0;
    int b = 0;
    scanf("%d%d", &a, &b);
    int A = get_max(a, b);
    printf("%d", A);
    return 0;
}

give an example 2： Write a function that exchanges two numbers ：

#include <stdio.h>
void exchange(int x, int y)
{
    
    int z = 0;
    z = x;
    x = y;
    y = z;
}
int main()
{
    
    int a = 0;
    int b = 0;
    scanf("%d %d", &a, &b);
    exchange(a, b);
    printf("%d %d", a, b);
    return 0;
}

In this way, the values of two numbers cannot be exchanged .

When changing to a pointer variable ：

#include <stdio.h>
void exchange(int* x, int* y)
{
    
    int z = 0;
    z = *x;
    *x = *y;
    *y = z;
}
int main()
{
    
    int a = 0;
    int b = 0;
    scanf("%d %d", &a, &b);
    exchange(&a, &b);
    printf("%d %d", a, b);
    return 0;
}

The reason why something like this happened , Because the shape in the function participates in the influence of the argument . A formal parameter is just a temporary copy of an argument , The change of the parameter does not affect the parameter , therefore , The first method cannot realize the exchange of two numbers .

3、 ... and 、 The shape of a function takes part in an argument

【1】 The actual parameter （ Actual parameters ）

The parameters that are actually passed to the function , It's called an argument .

The argument can be ： Constant 、 Variable 、 expression 、 function 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 .

【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 （ 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 .

Four 、 Function call ：

【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 .

【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 way of transferring parameters can establish a real connection between the function and the external variables , In other words, the variables outside the function can be directly manipulated inside the function .

therefore , To make the exchange of two numbers successful , You should use addressing to call , Using pointer variables , Let the result of internal exchange in the function be transmitted to the original code to realize the exchange of two values .