6 function

6.1 general

Function: encapsulate a piece of frequently used code to reduce repeated code

A large program is generally divided into several program blocks, and each module realizes specific functions.

6.2 definition of function

The definition of a function generally consists of five steps:

1. Return value type

2. Function name

3. Parameter table column

4. Function body statement

5. return expression

Syntax:

Return value type function name (parameter list)
{

       Function body statement

       return expression

}

• Return value type: a function can return a value. In function definition
• Function name: give the function a name
• Parameter list: the data passed in when using this function
• Function body statement: the code in curly braces and the statement to be executed in the function
• Return expression: linked to the return value type. After the function is executed, the corresponding data is returned

Example: define an addition function to add two numbers

//Function definition
int add(int num1, int num2)
{
	int sum = num1 + num2;
	return sum;
}

6.3 function call

Function: use defined functions

Syntax: function name (parameter)

Example:

//Function definition
int add(int num1, int num2) //Num1 and num2 in the definition are called formal parameters, or formal parameters for short
{
	int sum = num1 + num2;
	return sum;
}

int main() {

	int a = 10;
	int b = 10;
	//Call the add function
	int sum = add(a, b);//a and b at the time of calling are called actual parameters, which are referred to as actual parameters for short
	cout << "sum = " << sum << endl;

	a = 100;
	b = 100;

	sum = add(a, b);
	cout << "sum = " << sum << endl;

	system("pause");

	return 0;
}

Summary: the parentheses in the function definition are called formal parameters, and the parameters passed in during function call are called arguments

6.4 value transfer

• The so-called value passing means that the real parameter passes the value to the formal parameter when the function is called
• When a value is passed, if a formal parameter occurs, the argument is not affected

Example:

void swap(int num1, int num2)
{
	cout << "Before exchange:" << endl;
	cout << "num1 = " << num1 << endl;
	cout << "num2 = " << num2 << endl;

	int temp = num1;
	num1 = num2;
	num2 = temp;

	cout << "After exchange:" << endl;
	cout << "num1 = " << num1 << endl;
	cout << "num2 = " << num2 << endl;

	//return ;  When a function is declared, there is no need to return a value. You can not write return
}

int main() {

	int a = 10;
	int b = 20;

	swap(a, b);

	cout << "mian Medium a = " << a << endl;
	cout << "mian Medium b = " << b << endl;

	system("pause");

	return 0;
}

Summary: formal parameters cannot modify arguments when passing values

6.5 common styles of functions

There are four common function styles

1. No reference, no return
2. There is no return
3. Return without participation
4. Participation and return

Example:

//Common styles for functions
//1. No reference, no return
void test01()
{
	//void a = 10; // No type. Cannot create variable because memory cannot be allocated
	cout << "this is test01" << endl;
	//test01();  function call
}

//2. There is no return
void test02(int a)
{
	cout << "this is test02" << endl;
	cout << "a = " << a << endl;
}

//3. Return without participation
int test03()
{
	cout << "this is test03 " << endl;
	return 10;
}

//4. Participation and return
int test04(int a, int b)
{
	cout << "this is test04 " << endl;
	int sum = a + b;
	return sum;
}

6.6 function declaration

Function: tell the compiler the function name and how to call the function. The actual body of a function can be defined separately.

• A function can be declared multiple times, but a function can only be defined once

Example:

//You can declare multiple times and define only once
//statement
int max(int a, int b);
int max(int a, int b);
//definition
int max(int a, int b)
{
	return a > b ? a : b;
}

int main() {

	int a = 100;
	int b = 200;

	cout << max(a, b) << endl;

	system("pause");

	return 0;
}

6.7 function sub file preparation

Function: make the code structure clearer

Function sub file writing generally has four steps

1. Create a header file with the suffix. h
2. Create a source file with the suffix. cpp
3. Write the declaration of the function in the header file
4. Write the definition of the function in the source file

Example:

//swap.h file
#include<iostream>
using namespace std;

//A function declaration that implements the exchange of two numbers
void swap(int a, int b);

//swap.cpp file
#include "swap.h"

void swap(int a, int b)
{
	int temp = a;
	a = b;
	b = temp;

	cout << "a = " << a << endl;
	cout << "b = " << b << endl;
}

//main function file
#include "swap.h"
int main() {

	int a = 100;
	int b = 200;
	swap(a, b);

	system("pause");

	return 0;
}

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