(important) first knowledge of C language -- function

1. What is a function

C Classification of functions in language ： Library function 、 Custom function

2. Library function

2.1 strcpy

#include <stdio.h>
#include <string.h>
int main()
{
   char arr1[] = "bit";
   char arr2[] = "***********";
   strcpy(arr2, arr1); // hold arr1 Copy the contents of to arr2 in 
   printf("%s\n", arr2);
   return 0;
}

Output results ：bit

2.2 memset

#include <stdio.h>
#include <string.h>
int main()
{
   char arr[] = "hello world";
   memset(arr, '*', 5); // Put it in the array , front 5 Characters set to *
   printf("%s\n", arr);
   return 0;
}

Output results ：***** world

3. Custom function

（1） Compare the size of two numbers

#include <stdio.h>
int getmax(int x, int y)
{
   if (x > y)
      return x;
   else
      return y;
}

int main()
{
   int a = 10;
   int b = 8;
   int max = getmax(a, b);
   printf("%d\n", max);
   return max;
}

（2） Write a function that can exchange the contents of two integer variables

#include <stdio.h>
#include <string.h>

void swap1(int x, int y)
{
   int temp = 0;
   temp = x;
   x = y;
   y = temp;
}

void swap2(int *pa, int *pb)
{
   int tmp = 0;
   tmp = *pa;
   *pa = *pb;
   *pb = tmp;
}
int main()
{
   int a = 10;
   int b = 20;
   printf("a=%d b=%d\n", a, b);
   // swap1(a, b); // This method cannot 
   // because a,b    and x,y The addresses in memory are different ,x,y The values of will be interchanged ,a,b The values of are not interchangeable 
   swap2(&a, &b); //  Delivery address 
   printf("a=%d b=%d\n", a, b);
}

4. Function parameter

• Actual parameters ： The parameters actually passed to the function are called arguments , But constant 、 Variable 、 Expression etc. . No matter what type of variable the argument is , When you make a function call , They all have to have certain values , With These values are passed to the formal parameters .
• Shape parameter ： Receive arguments , Formal parameters are variables in parentheses after the user-defined 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 formal parameters , There is no actual variable space , It will only be instantiated when called , Destroy after calling .

5. Function call

• Value transfer call ： The formal and actual parameters of the function occupy different memory blocks respectively , Modification of a parameter does not affect the argument .
• Address call ： A method of calling a function by passing the memory address of a variable created outside the function to a function parameter . This way of transferring parameters can make the variables outside the function establish a real connection , In other words, the variables outside the function can be directly manipulated inside the function .
• Writing rules ： First write how to use in the calling function , Then write how to implement the function .

practice 1： Write a function to judge whether a number is a prime number .

#include <stdio.h>

//  Function definition 
int is_prime(int n){
	int j = 0;
	for(j=2;j<n;j++){
	if(n%j == 0)
		return 0;
	}
	return 1;  // j=n
}

int main()
{
	int i = 0;
	for(i = 100;i<=200;i++)
	{
	if(is_prime(i) == 1)
		printf("%d\t",i);
	}
   	return 0;
}

practice 2： Write a function to determine whether a year is a leap year .

#include <stdio.h>

//  Function definition 
int is_leap_year(int y){
if((y%4==0&&y%100!=0)||(y%400==0))
	return 1;
}

int main()
{
	int year = 0;
	for(year = 1000;year<=2000;year++)
	{
	//  Judge year Leap year or not 
		if(1==is_leap_year(year))
			printf("%d\t",year);
	}
   	return 0;
}

practice 3： Write a function , To realize binary search of an ordered array .

// Pay attention to the array parameters , Only the address of the first original element is passed
 // The number of array elements must be passed

#include <stdio.h>

// Two points search 
// Find a specific number in an ordered array 
// If you find the subscript of the return number , No return found -1

              // Essentially arr Received pointer 
int binary_search(int arr[],int k,int sz){
	int left = 0;
	int right = sz - 1;
	
	while(left<=right)
	{
	int mid = (left+right)/2;
	if(arr[mid]<k)
		left = mid + 1;
	else if(arr[mid]>k)
		right = mid - 1;
	else
		return mid;
	}
	return -1;
}
int main()
{
	int arr[] = {1,2,3,4,5,6,7,8,9,10};
	int k = 7;
	int sz = sizeof(arr)/sizeof(int);
	int ret = binary_search(arr,k,sz);
	//  Pay attention to the array parameters , Only the address of the first original element is passed 
	//  The number of array elements must be passed 
	if(ret == -1)
		printf(" The specified array cannot be found \n");
	else
		printf(" eureka , The subscript is ：%d\n",ret);
   	return 0;
}

practice 4： Write a function , Every time you call this function , Will be num The value of the add 1

#include <stdio.h>

void Add(int *p){
	(*p)++;
}

int main()
{
	int num = 0;
	Add(&num);
	printf("num = %d\n",num); //1
	Add(&num);
	printf("num = %d\n",num); //2
	Add(&num);
	printf("num = %d\n",num); //3
   	return 0;
}

6. Nested calls and chained access to functions

6.1 Function nested call

Each defined function can call each other

6.2  Chained access

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

7. Function declaration and definition

• Tell the compiler that there is a function called , What are the parameters , What is the return type . But does it exist , Be of no great importance .
• The declaration of a function usually precedes the use of the function , To satisfy the requirement of declaration before use .
• The declaration of a function is usually placed in the header file .

usage ：

Write... In the source file .c file , Write... In the header file .h file

#ifndef LED_H  // Be careful to capitalize , Quoted ****.h Lowercase 
#define LED_H
 
*
*
*
*
 
#endif

Use the function written by yourself "***.h", Reference downloaded or self-contained library functions to use <***.h>

8. Function recursion

• The programming technique of function calls is called recursion (recursion), Call the program yourself . 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 .
• Two necessary conditions for recursion ：
  • There are restrictions , When this constraint is met , Recursion doesn't continue . Prevent stack overflow （ Common mistakes in recursion ,stack overflow）,
  • After each recursive call, it gets closer and closer to this constraint .

practice 1： Receive an integer value （ Unsigned ）, Print each bit of it in order . For example, the input 1234, Output 1 2 3 4

#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);
   //  recursive 
   print(num);
   return 0;
}

practice 2： Writing functions does not allow the creation of temporary variables , Find the length of the string

#include <stdio.h>
#include <string.h>

int my_strlen(char *str)
{
   int count = 0;
   while (*str != '\0')
   {
      count++;
      str++;
   }
   return count;
}

int main()
{
   char arr[] = "hello";
   int len = my_strlen(arr); // arr It's an array , Array parameters , It's not the entire array , It's the address of the first element 
   printf("%d\n", len);
   return 0;
}