Programming learning records - Lesson 6 [functions]

function

In computer science , A function is a piece of code in a large program , Consists of one or more statement blocks . It's responsible for a particular task , And 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 libraries .

Library function

In order to support portability and improve program efficiency , therefore C A series of similar library functions are provided in the basic library of the language , It is convenient for programmers to develop software .

There's a function name , Return value types and function parameters .

example memset Use of functions , Is the string ptr Of the 1 to num Use... Characters value Instead of .

void * memset ( void * ptr, int value, size_t num );

Library function query tool ：

MSDN(Microsoft Developer Network)

www.cplusplus.com

http://en.cppreference.com（ English version ）

http://zh.cppreference.com（ Chinese version ）

When using library functions, you should pay attention to , You need a header file containing this function ; The library function can master its usage .

Custom function

Like library functions ; Custom functions include There's a function name , Return value types and function parameters .

Unlike library functions , User defined functions are designed by programmers , Give programmers a lot of room to play .

Custom functions are defined by Return type , Function parameter , Statement item composition

Arguments and formal parameters

Shape parameter ： Formal parameters are formal parameters , 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 ）, Only valid in functions , After the function call is completed, it is destroyed .

Actual parameters ： Arguments are actual parameters , That is, the value actually passed to the function , 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 , So that we can send these values to the shape ginseng .

Function call

1. Value transfer call

A formal parameter is essentially a temporary copy of an argument , Just pass the value to the function , The modification of formal parameters in the function will not affect the actual parameters .

2. Address call

Address calling is a form of creating variables outside the function and passing the memory address to the function parameters , Make the variables inside and outside the function establish a real connection , Thus, the variables outside the function can be modified directly in the function .

You can modify variables inside functions , example ：

// Exchange integer 
int exchange(int* x, int* y)
{
	int tmp = *x;
	*x = *y;
	*y = tmp;
	return 0;
}
int main()
{
	int i = 0;int  j=0;
	scanf("%d", &i, &j);
	exchange(&i, &j);
	printf("%d %d", i, j);
	return 0;
}

It realizes the exchange of the values of two variables inside the function .

Nested calls and chained access to functions

Nested calls

Combine functions according to their own needs , A function can be called inside another function , It is called nested call , Functions can be called nested , But you can't nest definitions , That is, you cannot define another function within a function , in addition , Call another function within one function , This function to be called is required to be defined before .

Chained access

Pass the value returned by one function to another function as a parameter . It is called chain access .

example ：

int main()
{
    printf("%d", printf("%d", printf("%d", 43)));
    return 0; }

The result of printing is 4321, because printf The value returned by the function is the number of characters printed .
Practice code today ：

// Prime judgment 
int  sushu(int i)
{
    	int j = 0;
		for (j = 2;j < i;j++)
		{
			if (i % j == 0) { return 0; }
			else if (j == i - 1) { return 1; }
		}
}
int main()
{
	for (int i = 100;i <= 200;i++)
	{
		if (sushu(i) == 1) { printf("%d ", i); }
	}
	return 0;
}
// Leap year judgment 
int run(int year)
{
	if (year % 4 == 0 && year % 100 != 0) return 1;
	else if (year % 400 == 0) return 1;
	else return 0;
}
int main()
{
	int year = 0;
	scanf("%d", &year);
	if (run(year) == 1) printf(" It's a leap year ");
	else printf(" It's not a leap year ");
	return 0;
}
// Exchange integer 
int exchange(int* x, int* y)
{
	int tmp = *x;
	*x = *y;
	*y = tmp;
	return 0;
}
int main()
{
	int i = 0;int  j=0;
	scanf("%d", &i, &j);
	exchange(&i, &j);
	printf("%d %d", i, j);
	return 0;
}
// Multiplication table 
int biao(int i)
{
	int m = 0;int n = 0;
	for (m = 1;m <= i;m++)
	{
		for (n = 1;n <= m;n++)
		{
			printf("%d * %d=%2d ", n, m, m * n);
		}
		printf("\n");
	}
	return 0;
}
int main()
{
	int i = 0;
	scanf("%d", &i);
	biao(i);
	return 0;
}