[function explanation (Part 1)] | | knowledge sorting + code analysis + graphic interpretation

 “ The boy looked back Laugh, I'm not fast enough to keep up .”

 

Catalog

 

 1. What is a function

 2. Library function

3. Custom function

4. The parameters of the function

5. Function call

6. practice

7. Nested calls and chained access to functions

Nested calls

  Chained access

---

1. What is a function

A program may be used many times , If you write such a repeated piece of code every time , It's not only time-consuming and laborious 、 It's easy to make mistakes , And it's troublesome to give it to others , therefore C Language provides a function , It allows us to encapsulate common code into a separate module in a fixed format , As long as you know the name of this module, you can reuse it , This module is called function （Function）. Function compared to other code , With relative independence .

  Generally, there will be input parameters and return values , Provide encapsulation of the process and hiding of details . These codes are usually integrated into Software library .

 2. Library function

C Function classification in language ：

1. Library function

2. Custom function

Why are there library functions ？

Before we learn C In language programming , It's a little bit like scanf,printf,strcpy,strlen And other basic functions , They are not business code , In the process of development, every programmer will use , in order to Support portability and improve program efficiency , therefore C The language base library provides a series of similar library functions , It is convenient for programmers to develop software .

notes ： Use library functions , must #include The corresponding header file .

Take a chestnut ：

1.

int main()
{
	char arr1[] = "#####################";
	char arr2[] = "hello world";
	strcpy(arr1, arr2);
	printf("%s\n", arr1);
	return 0;
}

 

2. 

  

int main()
{
	char arr2[] = "hello world";
	memset(arr2, 'x', 5);
	printf("%s\n", arr2);
	return 0;
}

  notes ：memset It is initialized in bytes .

3. Custom function

Except for library functions , We can still Write your own functions , Expand the function of the program . The function written by yourself is called user-defined function . Custom functions and library functions are written and used in exactly the same way , It's just written by different institutions .

ret_type  fun_name ( para1 )

{ statement;// Statement item }

ret_type Return type   fun_name Function name           para1     Function parameter

Take a chestnut ：

int Max(int a, int b)
{
	return (a > b ? a : b);
}
int main()
{
	int a = 0;
	int b = 0;
	scanf("%d%d", &a, &b);
	int max=Max(a, b);
	printf("%d", max);
	return 0;
}

void Change(int *a, int *b)
{
	int tmp = 0;
	tmp = *a;
	*a =*b;
	*b = tmp;
}
int main()
{
	int a = 0;
	int b = 90;
	printf("a:%db:%d\n", a, b);
	Change(&a,&b);
	printf("a:%db:%d\n", a, b);
	return 0;
}

4. The parameters of the function

Actual parameters ：

The parameters that are actually passed to the function . It can be ： Constant , Variable , expression , Functions, etc . When you make a function call , They all have to have certain values , In order to pass these values to the parameter .

Shape parameter ：

When you define a function , Variables in parentheses after function names , because Formal parameters are instantiated only when called （ Allocate space ）, So it's called formal parameter . Formal parameters will be destroyed after the function is called , Therefore, formal parameters are only valid in functions .

5. Function call

  Value transfer call ：

Function parameters and arguments occupy different memory blocks , Changes to formal parameters do not affect arguments . After the formal parameter is instantiated, it is actually equivalent to a temporary copy of the argument .

Address call ：

Address calling is a calling method that passes the memory address of the variable created outside the function to the function parameters .

This way of parameter transfer can establish a real connection between the function and the variables outside the function . In other words, the variables outside the function can be directly manipulated inside the function .

---

6. practice

1. Write a function to determine whether a number is a prime .

int is_prime(int i)
{
	for (int j = 2; j <= sqrt(i); j++)
	{
		if (i%j == 0)
		{
			return 0;
		}
	}
	return 1;
}
int main()
{
	for (int i = 100; i <= 200; i++)
	{
		if (is_prime(i))
		{
			printf("%d ", i);
		}
	}
	return 0;
}

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

int is_leap_year(int y)
{
	return((y % 4 == 0) && (y % 100 != 0) || (y % 400 == 0));	
}
int main()
{
	int y = 0;
	int i = 0;
	scanf("%d", &y);
	if (is_leap_year(y))
	{
		printf(" It's a leap year ");
	}else
		printf(" It's not a leap year ");
	return 0;
}

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

int binary_search(int k,int arr[], 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 if (arr[mid] == k)
		{
			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(arr[0]);
	int tmp=binary_search(k,arr, sz);
	if (tmp == -1)
	{
		printf(" Can't find ");
	}
	else
	{
		printf(" eureka , Subscript to be %d\n", tmp);
	}
	return 0;
}

4. Write a function , Every time you call this function , Will be num The value of the increase 1.

int Count(int* pnum)
{
	(*pnum)++;
}
int main()
{
	int num = 0;
	Count(&num);
	Count(&num);
	Count(&num);
	Count(&num);
	printf("%d", num);
	return 0;
}

---

7. Nested calls and chained access to functions

Nested calls

void Print()
{
	printf("hello world\n1");
}

void test()
{
	Print();
}
int main()
{
	test();
	return 0;
}

Functions can be called nested, but not defined nested .

// Error model 
void Print()
{
	printf("hello world\n1");
}
int main()
{
	void test()
	{
		Print();
	}
	return 0;
}

  Chained access

Chained access is a function of Return value As Parameters of another function .

#include <stdio.h>
int main()
{
    printf("%d", printf("%d", printf("%d", 43)));
    // The result is what ？
    return 0;
}

analysis ： 1. Execute first  printf("%d", 43), Its return value is 2

2. Re execution printf("%d",2), Its return value is 1

3. Re execution printf("%d",1)