chapter 4 function basic 函数基础 introduction functions improve clarity and enable software. a...

CHAPTER 4 Function basic 函数基础

Introduction

Functions improve clarity and enable software. A function performs a p

articular task and the returns with its solution.

In this chapter, we explore the fundamental concepts such as invocation

and parameter passing, the iostream library and the preprocessor com

mands.

函数增加了软件的透明性和重用性。函数用于完成一个特定任务，然后返回执行结果。本章讨论函数的基本概念，如函数调用和参数传递。还讨论 iostream 库的使用；预处理指令。

Function is a module that organize their information and its manipulation.

• All major application developments and even most simple programs use

programming schemes in a modular fashion.

• Functions are a crucial component of object-oriented programming.

4.1 FUNCTION BASICS

The act of using a function is referred to as an invocating or call （调用） .

•A function can call others. But a programmer-defined function can not cal

l main function.

• A function even call itself, except main function. This process is known a

s recursion （递归） .

4.1 FUNCTION BASICS

A function can can be passed information to perform its task.

• The information is referred to as the function’s parameters or

arguments.

4.1 FUNCTION BASICS

fun1fun1 fun2fun2 fun3fun3

fun4fun4 fun5fun5 fun6fun6 fun7fun7

mainmain

A function can call others

Recursion

The act of using a function is referred to as an invocating or call （调用） .

•A function can call others. But a programmer-defined function can not cal

l main function.

• A function even call itself, except main function. This process is known a

s recursion （递归） .

4.1 FUNCTION BASICS

A function can can be passed information to perform its task.

• The information is referred to as the function’s parameters or

arguments.

// Determines roots of a quadratic equation

#include <iostream>

#include <string>

#include <math.h>

using namespace std;

int main() {

cout << "Coefficients for quadratic equation: ";

double a, b, c;

cin >> a >> b >> c;

if ((a != 0) && ((b*b - 4*a*c) > 0))

{ double radical = sqrt(b*b - 4*a*c);

double root1 = (-b + radical) / (2*a);

double root2 = (-b - radical) / (2*a);

cout<<"The roots of "<<a<<"x**2 + "<<b<<"x + "<<c

<<" are "<<root1<<" and "<<root2<<endl;

}

else cout<< a<<"x**2 + "<<b<<"x + "<<c<<" does not have two real roots"<<endl;

return 0;

}

// Determines roots of a quadratic equation

#include <iostream>

#include <string>

#include <math.h>


int main() {

cout << "Coefficients for quadratic equation: ";

double a, b, c;

cin >> a >> b >> c;

if ((a != 0) && ((b*b - 4*a*c) > 0))

{ double radical = sqrt(b*b - 4*a*c);

double root1 = (-b + radical) / (2*a);

double root2 = (-b - radical) / (2*a);

cout<<"The roots of "<<a<<"x**2 + "<<b<<"x + "<<c

<<" are "<<root1<<" and "<<root2<<endl;

}

else cout<< a<<"x**2 + "<<b<<"x + "<<c<<" does not have two real roots"<<endl;

return 0;

}

Call library function

The functions interface :

• The type of value ( if any ) that the function will return.

• The function’s name

• A description of function’s parameters

4.1.1 Interface specification 接口声明

For example

int main ()

double sqrt ( double x )

The form for the function interface :


FunctionType FunctionName ( ParameterList

)



FunctionType FunctionName ( ParameterList

)

The type of value that the

function returns



FunctionType FunctionName ( ParameterList )

Identifier name of function




A description of the form

parameters ( if any ) are to

take




ParameterList has the form :

ParameterDeclaration, … , ParameterDeclaration






Description of individual parameters






Value parameter （值参） has the form :

ParameterType ParameterName








Type of value which the parameter represents








Identifier name of parameter


For example

int main ()



For example

int main ()


The type of value


For example

int main ()


The name of function


For example

int main ()


Function’s parameters

4.1.2 Function prototyping 函数原型

• Interface specification is a header of a function.

• A function prototype statement is a interface specification followed a

semicolon （；） .

• At least the interface must be specified before a function can be use. It

can use a function prototype.

• The names of the parameters are not necessary in a function prototype.

4.1.2 Function prototyping 函数原型

For example :

int PromptAndExtract () ;

float CircleArea ( float radius ) ;

int max ( int , int , int ) ;

void ShowOrder ( char , char , char ) ;

FunctionType FunctionName ( ParameterList ) ；

Function prototype statement

4.1.3 Invocation and flow of control 调用和控制流

Calling function form

FunctionName ( ActualParameterList ) ；

For example :

double radical = sqrt ( b*b – 4* a* c) ;

Function definition double sqrt ( double x ){ // … ; return value ;}




For example :



The head of function




For example :


Call function





For example :


Actual Parameter实际参数





For example :

double radical = sqrt ( b*b – 4* a* c) ; Formal Parameter

形式参数





For example :



CallCall




For example :


Function definition double sqrt ( double xdouble x ){ // … ; return value ;}

CallCall

The value of expression




For example :



ReturnReturn




For example :



The value is an expression

ReturnReturn


Calling function in flow of control

For example :

#include<iostream.h>

void main()

{ int a=1 ;

cout<< "a is " << a++ << " , now a is " << a++ << endl ;

return;

}

a is 2 , now a is 1 Output can be :

It is known side effect ( 副作用

). The order of evaluation is left

to the compiler.

It is known side effect ( 副作用

). The order of evaluation is left

to the compiler.


Calling function in flow of control

For example :


void main()

{ int a=1 ;

return;

}

a is 1 , now a is 2 Output :

modifymodify

cout << "a is " << a++ ;

cout << " , now a is " << a++ << endl ;

The preprocessor

• examines file inclusion directives and conditional compilation directives.

检查文件包含指令和条件编译指令。

• Produce the actual file to be compiled.

产生要编译的实际文件。

• The file produced by the preprocessor is called a translation unit.

预处理器产生的文件称为解释单元。

4.2 THE PREPROCESSOR 预处理器

• A file inclusion directive specifies the name of file.

• The file named in the directive replaces the directives itself.

• There are two file inclusion directive forms.

4.2.1 File inclusion directives 文件包含指令

One of forms # include < filename >


• Indicate the file is to be found in one of the standard directories of the

system.

• For PC-based systems, standard directories are within subdirectories

of the directory that contains the compiler.

The second form # include " filename "


Search file:

• If the filename is an absolute pathname ( 绝对路径 ) , then the file is take

n

from that absolute pathname. For example:

#include " c:\example\source\simple.h "



Search file:


n

from that absolute pathname.

For example:

#include " MyFile.dat "

• If an absolute pathname is not specified, then the file is taken from the

user’s current directory ( 当前目录 ).



Search file:


n

from that absolute pathname.

Note:

The preprocessor does not consider whether the lines it is creating are legal

C++ statements. 预处理器不作语法检查

• If an absolute pathname is not specified, then the file is taken from the

user’s current directory ( 当前目录 ).

4.2.2 Conditions compilation 条件编译

#define MacroName String

The MacroName is replaced into String.

Macro directives 宏命令


For example

#include < iostream.h >

#define PI 3.1415926

void main()

{ double r , l , s ;

cout << " Input radius : " ;

cin >> r ;

l = 2 * PI * r ;

s = PI * r * r ;

cout << "L=“ << l << " \nS = " << s << '\n' ;

return ;

}


For example


#define PI 3.1415926

void main()

{ double r , l , s ;

cout << " Input radius : " ;

cin >> r ;

l = 2 * PI * r ;

s = PI * r * r ;111

cout << "L=“ << l << " \nS = " << s << '\n' ;

return ;

}

comparisoncomparison

const PI = 3.1415926 ;


Conditions compilation directives 条件编译命令

Form one

#ifdef Name Statements1#endif

#ifdef Name Statements1#else Statements2 #endif

#ifndef Name Statements1#endif

#ifndef Name Statements1#else Statements2 #endif

Form two

#ifdef ConstExpression Statements1#endif

#ifdef ConstExpression Statements1#else Statements2 #endif

Form three


For example: Object ABC

//abc.cpp


#define T 1

#define ABC void main()\

{cout<<"hello!"<<s<<endl;return;}

#include "abc.h"

//abc.h

#if T

char *s="good morning!";

ABC

#endif

Continuation character (

续行符 )



//abc.cpp


#define T 1



#include "abc.h"

//abc.h

#if T


ABC

#endif

11



//abc.cpp


#define T 1



#include "abc.h"

//abc.h//abc.h

#if T#if T

char *s="good morning!";char *s="good morning!";

ABCABC

#endif#endif


#define T 1



#if T


ABC

#endif

11

22



//abc.cpp


#define T 1



#include "abc.h"

//abc.h

#if T


ABC

#endif


#define T 1

#define ABC void main()\void main()\

{cout<<"hello!"<<s<<endl;return;}{cout<<"hello!"<<s<<endl;return;}

#if T


ABC

#endif

11



void main()


22

global and nonglobal declaration 全局和非全局声明

4.3 USING SOFTWARE LIBQARIES

使用软件库

• A declaration that can be used throughout a program file is called a globa

l declaration （全局声明） .

• The collection of all global declarations is called the global namespace

（全局作用域） .

• Header files that make their declaration globally available have the file e

xtension h .

• Header files that place their declarations within a nonglobal namespace d

o not use any file extension .

NAMESPACE

The C++ standard includes the namespace and using mechanisms that allo

w other namespace to be defined and referenced. Inside a namespace, a col

lection of classes, functions, objects, types, and other namespace can be de

clared.

标准 C++ 包含了 namespace 和 using 机制，它允许定义和引用其他

的名空间（名字作用域）。在名空间中，可以声明类、函数、对象、

类型和其他名空间的集合。

Definitions

The namespaces create virtual packages whose elements can be fully speci

fied to prevent name ambiguity.

名空间创建程序包，它的元素可以被完全指明，防止名字的模糊不清。

namespace name { statements | ExistingName }

The syntax for defining a namespace :

Definitions






Optional identifier name

of namespace

Definitions






Classes, functions, objects,

types, and other namespace

declaration and definitions

Definitions



Reference to an existing nam

espace ( 别名 )




For example

class A{ // … element1;element1; element2; // …};

A.h

B.h

class B{ // … element1;element1; element3; // …};

namespace B

{ class B { // … element1;element1; element3; // … };}

namespace A{ class A { // … element1;element1; element2; // … }; OtherElementA ;OtherElementA ;}

Using namespace

The using statement is often used with namespace. There are two forms:

using namespace name

using namespace name :: element

Using namespace



using namespace name :: elementNamespace whose elements are to b

e part of current namespace( 当前作用域 )

Using namespace



using namespace name :: element

Namespace being reference Element of namespace to be part of current

namespace

# include<iostream.h>

namespace car // 名空间定义{ int model ; int length ; int width ; }

namespace plane{ int model ; namespace size // 名空间可以嵌套 { int length ; int width ; }}

namespace car // 名空间可以随时增加成员 { char * name ; } // 往上面已经定义的 car 空间增加一个成员 char*name;

namespace c = car ; // 名空间可以有别名， c 是 car 的别名

int Time;int Time; // 一个全局变量，它属于一个默认的全局名空间

void main(){ car :: length = 3 ; // 使用另一个空间的变量，要显式说明 // width = 2 ; // 错误， width 不是当前名空间的变量 plane :: size :: length = 70 ; // 嵌套名空间的使用 cout << "the length of plane is " << plane :: size :: length << endl ;

cout << "the length of car is " << car :: length << endl ;cout << "the length of c is " << c :: length << endl ; // 用别名访问名空间

int Time int Time = 1996 ; // main 函数中的临时变量 :: Time = 1997 ; // 全局名空间的变量，有局部变量冲突时要显式说明 using namespace plane ; // 指定以下的变量是属于 plane 空间的变量 model model = 202 ; size :: length size :: length = 93 ; // 无需再显式说明是 plane 空间 cout << "In plane space..." << endl ; cout << model << endl ; cout << size :: length << endl ; using namespace car ; // 指定 car 名空间 name name = "Hello Car" ; //name 是属于 car 空间的变量 , 无需再显式声明 cout << "In space of car..." << endl ; cout << "the name of car is " << name name << endl ;}

C++ I/O libraries provide an interface between the program and the hardware

devices make up the computer system.

The interface spans two levels of abstraction :

• File low level

The stream view allows a program to issue generic I/O requests on flows of

data. Requests are translated by compiler into file-level specific action.

• Stream higher level

The file view captures the important physical characteristics of the actual

device.

4.4 THE IOSTREAM LIBRARY iostream 库

4.4 THE IOSTREAM LIBRARY iostream 库

iosiosiosios

istreamistreamistreamistream fstreambasefstreambasefstreambasefstreambase strstreambasestrstreambasestrstreambasestrstreambase ostreamostreamostreamostream

istream_withassignistream_withassignistream_withassignistream_withassign fstreamfstreamfstreamfstream strstreamstrstreamstrstreamstrstream ostream_withassignostream_withassignostream_withassignostream_withassign

iostreamiostreamiostreamiostream

iostream_withassigniostream_withassigniostream_withassigniostream_withassign

ifstreamifstreamifstreamifstream istrstreamistrstreamistrstreamistrstream ofstreamofstreamofstreamofstream ostrstreamostrstreamostrstreamostrstream constreamconstreamconstreamconstream

4.4.1 Standard streams 标准流4.4.2 Standard error streams object 标准错误流对象

default

cin The object of istream_withassign keyboard

cout The object of ostream_withassign monitor

cerr The object of ostream_withassign monitor

clog The object of ostream_withassign printer

default





Can be redirected to a file


default





• Can not be redirected

• Insertion requests are directed to the monitor

• No buffering, time critical ( No buffering, time critical ( 实时实时 ))


default





• Can not be redirected

• Insertion requests are directed to the monitor

• buffered error streambuffered error stream


// Example


void fn ( int a , int b )

{ if ( b == 0 )

cerr << “Zero encountered. The message connot be redirected.” ;

else

cout << a / b << endl ;

}

void main ( )

{ fn ( 20 , 2 ) ;

fn ( 20 , 0 ) ;

}

Can be redirectedCan be redirected

Can not be redirectedCan not be redirectedHow to redirect ?How to redirect ?

• The program is compiled to a The program is compiled to a

exe fileexe file

• using DOS command that has using DOS command that has

parameter parameter

to run the programto run the program

Such asSuch as ：：

D:\>exampleD:\>example> abc.dat> abc.dat

cout is cout is redirected to redirected to abc.databc.dat

How to redirect ?How to redirect ?

• The program is compiled to a The program is compiled to a

exe fileexe file

• using DOS command that has using DOS command that has

parameter parameter

to run the programto run the program

Such asSuch as ：：

D:\>exampleD:\>example> abc.dat> abc.dat

cout is cout is redirected to redirected to abc.databc.dat

• Some input and output stream manipulators are defined in the iostream li

brary. They are used to adjust the I/O format.

• I/O manipulators can occur as the right operand of either an operator <<

or operator >>.

4.4.3 The iostream manipulators iostream 操作符


Manipulator Purpose

dec 用十进制显示数值

endl 输出一个换行符并清空流

ends 输出一个空字符（ null ）并清空流flush 清空流缓冲器

hex 用十六进制显示数值

oct 用八进制显示数值



void main()

{ int i=10, j=20, k=30;

cout << "Dec: " << i << " " << j << " " << k << endl ;

cout << "Dec: " << dec << i << " " << j << " " << k << "\n" << flush ;

cout << "Oct: " << oct << i << " " << j << " " << k << endl ;

cout << "Hex: " << hex << i << " " << j << " " << k << endl ;

return ;

}

Dec: 10 20 30Dec: 10 20 30Oct: 12 24 36Hex: a 14 1e

Output:

• The iomanip library defines a collection of I/O stream manipulators to mo

dify the behavior of insertions and extractions.

• The iomanip manipulators are defined in the standard header file iomanip.

4.5 THE IOMANIP LIBRARY iomanip 库


#include < iomanip.h >

Manipulator Purposesew(int w) 置显示宽度为 w

setfill(int c) 置填充字符为 c

left 靠左显示right 靠右显示setbase(int b) 置基数为 b 显示fixed 按常用示数法显示浮点数scientific 按科学示数法显示浮点数showpoint 用小数点显示浮点数noshowpoint 仅当浮点数小数部分不为 0 时才显示小数点setprecision(int d) 设显示精度位数为 d

skipws 提取时忽略空白符nokipws 提取包括空白符showpos 显示正数前有 +

noshowpos 正数前没有 +

showbase 显示数带基数符，八进制数前置 0 ，十六进制前置 0x

noshowbase 显示数不带基数符boolalpha 用符号 true 和 false 显示逻辑值noboolalpha 用 1 和 0 显示逻辑值resetiosflags(log f) 按 f 指示的标志设置 0

setiosflags(log f) 按 f 指示的标志设置 1

//Example format_1

#include<iostream>

#include<iomanip>


void main()

{ cout << "Hello" << endl ;

cout << left << setw(10) << setfill('*') << "Hello" << endl ;

cout << right << setw(10) << setfill('#') << "Hello" << endl ;

return ;

}

HelloHello*****#####Hello

Output:

//Example format_2

#include<iostream>

#include<iomanip>


void main()

{ double x = 20.0/7 ;

cout << x << endl ;

cout << setprecision(0) << x << endl

<< setprecision(1) << x << endl



<< setprecision(4) << x << endl ;

cout << setiosflags(ios::fixed) << setprecision(8) << x << endl ;

cout << setiosflags(ios::scientific) << 0.000000000000123 << endl ;

return ;

}

2.857142.8571432.92.862.8572.857142861.23e-013

Output:

//Example format_3

#include<iostream>

#include<iomanip>


void main()

{ int i = 10, j = 20, k = 30 ;

cout << showbase ;

cout << "Dec: " << i << " " << j << " " << k << endl ;

cout << setbase(8) ;

cout << "Oct: " << i << " " << j << " " << k << endl ;

cout<< setbase(16) ;

cout << "Hex: " << i << " " << j << " " << k << endl ;

return ;

} Dec: 10 20 30Oct: 012 024 036Hex: 0xa 0x14 0x1e

Output:

//Example format_4

#include < iostream >

#include < iomanip >

using namespace std ;

void main()

{ bool T = true , F = false ;

cout << T << "\t" << F << endl ;

cout << boolalpha << T << "\t" << F << endl ;

cout << noboolalpha << T << "\t" << F << endl ;

return ;

}

1 0true false1 0

Output:

//Example format_5

#include < iostream >

#include < iomanip >

using namespace std ;

void main()

{ char c1, c2, c3, c4 ;

cin >> c1 ;

cout << '[' << c1 << ']' << endl ;

cin >> noskipws >> c2 ;

cout << '[' << c2 << ']' << endl ;

cin >> skipws >> c3 ;

cout << '[' << c3 << ']' << endl ;

cin >> c4 ;

cout << '[' << c4 << ']' << endl ;

return;

}

a b c d[a][ ][b][c]

Input:

Output:

4.6 THE FSTREAM LIBRARY fstream 库

iosiosiosios

istreamistreamistreamistream fstreambasefstreambasefstreambasefstreambase strstreambasestrstreambasestrstreambasestrstreambase ostreamostreamostreamostream

istream_withassignistream_withassignistream_withassignistream_withassign fstreamfstreamfstreamfstream strstreamstrstreamstrstreamstrstream ostream_withassignostream_withassignostream_withassignostream_withassign

iostreamiostreamiostreamiostream

iostream_withassigniostream_withassigniostream_withassigniostream_withassign

ifstreamifstreamifstreamifstream istrstreamistrstreamistrstreamistrstream ofstreamofstreamofstreamofstream ostrstreamostrstreamostrstreamostrstream constreamconstreamconstreamconstream

// Program 5.2 Calculates average of file mydata.nbr#include <iostream>

#include <fstream>

#include <string>


int main()

{ ifstream fin("mydata.nbr");

int ValuesProcessed = 0; float ValueSum = 0; float Value;

while (fin >> Value)

{ ValueSum += Value; ++ValuesProcessed; }

if (ValuesProcessed > 0)

{ ofstream fout("average.nbr");

float Average = ValueSum / ValuesProcessed;

fout << "Average: " << Average << endl;

}

else cerr << "No list to average" << endl;

return 0;

}

ModifyInput a string that is a

file name

// Program 5.3 Prompts user for a file and then calculates // the average of the values in that file// …int main() { cout << "File of values to be averaged: "; string FileName; cin >> FileName; ifstream fin(FileName.c_str()); if (! fin) { cerr << "Cannot open " << FileName<< " for averaging."<< endl; exit(1); } int ValuesProcessed = 0; float ValueSum = 0; float Value; while (fin >> Value) { ValueSum += Value; ++ValuesProcessed; } if (ValuesProcessed > 0) { float Average = ValueSum / ValuesProcessed; cout << "Average of values from " << FileName<< " is " << Average << endl; } else { cerr << "No values to average in "<< FileName << endl; exit(1);} return 0;}

The C++ stdlib library ( stdio.h ) provides two functions that are uesful in

generating pseudorandom number sequences.

4.6 RANDOM NUMBERS 随机数

rand() no parameters, returns a uniform pseudorandom from inclusive

interval 0 to RAND_MAX.

srand() expects an unsigned int parameter. The parameter is used to set

the seed ( 启动值 ) for generating the first pseudorandom numbe

r. time() returns a value of type time_t , which is an integral type . It

defined in the time library ( time.h ) .

time(0) returns the current time.

//Program 4.5: Display ten pseudorandom numbers


#include<stdlib.h>

#include<time.h>

void main()

{ cout<<"Random number seed (number):";

unsigned int seed;

cin>>seed;

srand(seed);

for(int i=1;i<=10;i++)

cout<<rand()<<endl;

return;

}

Input the value of seed

//Program 5.6: Display numbers


#include<stdlib.h>

#include<time.h>

void main()

{ srand( (unsigned int) time(0) ) ;

for ( int i = 1; i <= 10 ; i++ )

cout << rand() << endl ;

return;

}

The value of function time() is used as seed

//uniform.h

#ifndef RANDOM_H

#define RANDOM_H

void InitializeSeed();

int Uniform(int Low, int High);

#endif

//using uniform.cpp//uniform.cpp#include <iostream>#include <stdlib.h>#include <string>#include <time.h>#include "uniform.h"using namespace std;void InitializeSeed() { srand((unsigned int) time(0)); }int Uniform(int Low, int High) { if (Low > High) {cerr << "Illegal range passed to Uniform\n"; exit(1); return 0; } else { int IntervalSize = High - Low + 1; int RandomOffset = rand() % IntervalSize; return Low + RandomOffset; }}

//Rand.cpp#include<iostream.h>#include "uniform.h"void main(){ int k,i; for(i=1;i<=10;i++) { k=Uniform(i,i*100); cout<<k<<endl; }}

• assert ( 调试声明 ) is useful during program development, it is declar

ed

in assert library.

• The assert macro expects an integral expression as its parameter.

• If the expression is:

Nonzero the program continues.

Zero the program produces message to the standard error, and then ,

the program terminates.

4.8 THE ASSERT LIBRARY assert 库

// Program 5.7: Computes quotient and remainder of two inputs#include <cassert>

#include <iostream>

#include <string>


int main()

{ int Numerator;

cout << "Enter numerator: ";

cin >> Numerator;

int Denominator;

cout << "Enter denominator: ";

cin >> Denominator;

assert(Denominator); // really should be if test

int Ratio = Numerator / Denominator;

int Remainder = Numerator % Denominator;

cout << Numerator << "/" << Denominator << " = "

<< Ratio << " with remainder " << Remainder << endl;

return 0;

}

// Program 5.7: Computes quotient and remainder of two inputs#include <cassert>

#include <iostream>

#include <string>


int main()

{ int Numerator;

cout << "Enter numerator: ";

cin >> Numerator;

int Denominator;

cout << "Enter denominator: ";

cin >> Denominator;

assert(Denominator); // really should be if test

int Ratio = Numerator / Denominator;

int Remainder = Numerator % Denominator;

cout << Numerator << "/" << Denominator << " = "

<< Ratio << " with remainder " << Remainder << endl;

return 0;

}

modifyif (Denominator) { cerr << " The Denominator is fail. \n" ; return 1; }

chapter 4 function basic 函数基础 introduction functions improve clarity and enable software. a...

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