c/c++ basics (i) variables, expressions and assignments berlin chen 2003 textbook: walter savitch,...

C/C++ Basics (I)

Variables, Expressions and Assignments

Berlin Chen 2003

Textbook: Walter Savitch, "Absolute C++," Addison Wesley, 2002

2

Introduction to C++

• Programming Languages– Low-level languages

• Machine, assembly– High-level languages

• B, C, C++, ADA, COBOL, FORTRAN– Object-Oriented-Programming in C++

• C++ Terminology– Programs and functions– Basic Input/Output (I/O) with cin and cout

3

Introduction to C++

• C++ is thought of C with classes (and othermodern features added), is a high level language

• C is first developed by Dennis Ritchie of AT&T in the 1970s for the UNIX system

• Bjarne Stroustrup of AT&T developed C++ in the early 1980s

• Most C is a subset of C++, and so most C programs are also C++ programs (The reverse is not true!)

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C++ and Object-oriented Programming (OOP)

• The main characteristics of OOP– Encapsulation ( 資料封裝 )

• A form of information hiding and abstraction

– Inheritance ( 繼承 )• Has to do with writing reusable code

– Polymorphism ( 多形 )• Refers to a way that a single name can have multiple meanin

gs in the context of inheritance

– OOP provides classes, a kind of data type combining both data and algorithms( 演算法 )

– C++ tempers its OOP features

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Outline• Basic Characteristics• Identifiers

– Keywords or Reserved Words– Variables

• Data types– Assigning Data for Variables

• Literal Data or Constants • Console Input/Output• Escape Sequences• Arithmetic Operators• Type Casting

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Basic Characteristics of C/C++…void main(void){ cout << “Hello World ! \n”; …}

0010000001101010

…..

01101010

C/C++ Language(Text Files)

xxx.cpp

Compiler 編譯器VC++, gcc

Machine Language(Binary Files)

High level language-readable-machine independent (portable)

Low level language-unreadable-machine dependent

7

Basic Characteristics of C/C++

• C/C++ is case-sensitive ( 注意大小寫 )

• A program composed of one to several functions ( 函數 )– 每個 C/C++ 都需有一個稱為 main() 的函數

int main(void){ statement_1; statement_2; …

statement_n; return 0; // 結束函數}

函數標題

函數主體

函數定義

函數名稱

結束函數註解 (comment) 以“ //” 為起始

敘述 (statement) 以分號結束

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Basic Characteristics of C/C++

• Program Style - Readability– 儘可能每行寫一個敘述 (statement)– 函數開始與結束之左右大括號各佔一行– 函數主體之敘述從大括號處往內 ( 右 ) 縮– 函數名稱與之後的左右括號不需要有空白

9

An example of C++ program#include <iostream>

using namespace std;

void main (void)

{

const double RATE=6.9;

double deposit;

cout << "Enter the amount of your deposit $";

cin >> deposit;

double newBalance;

newBalance= deposit+ deposit*(RATE/100.0);

cout << "In one year, the deposit will grow to\n" <<"$"<< newBalance;

}

Enter the amount of your deposit $ 100In one year, the deposit will grow to$106.9

輸入

C++ 程式

程式執行過程

標頭檔名及名稱空間

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Identifiers ( 識別字 )• Identifiers are items defined/used in a program

– Keywords ( 關鍵字 )or Reserved Words ( 保留字 )– Variables ( 變數 ) 、 function names ( 函數名稱 ) 、 la

bels （標記）

• Identifiers must start with either a letter (A-Z, a-z) or the under score symbol(_), and the remaining characters must be letters (A-Z, a-z), digits(0-9), or the underscore symbol(_)

• Identifiers are case sensitive and have no limit to their length ( 大小寫有別、長度不受限 )

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Identifiers• Valid ( 合法的 ) Identifiers: x, x1, x_1, ABC123z

7,…

• Invalid ( 不合法的 ) Identifiers: 12, 3x, %change, data-1, myfirst.c

• Identifier started with underscore are informally reserved for system (should avoid using it)– 以底線為首的命名方式通常保留給系統使用，因此我們在變數、函

數等命名時儘量不要使用

• Identifier are usually spelled with their first letter in lowercase

• topSpeed, bankRate1, bankRate2, timeOfArrival, …儘量使用有意義的名稱

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Keywords or Reserved Words

• A special class of identifiers which have predefined meaning in C/C++ and cannot be used as names for variables or anything else– 關鍵字或保留字是 C/C++ Compiler 本身所使用的– 如 : break case char float

do goto long double if else .. … ….

– 我們只能使用它們，不能重新定義它們

• Some predefined words, such as cin and cout are not keywords, because they are not the core of C/C++ language

在某些編輯環境下保留字會以特別的顏色顯示

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Variables

• A memory location to store data for a program

• Must declare all data (variables) before use in program– Tell the compiler/computer what kind of data you will

be storing in variables int numberOfBeans; double oneWeight, totalWeight;

• Variables can be declared any place– Declared either just before they are used or at the sta

rt of a block “{” (or a function/procedure)– 通常在程式或每個函式開頭宣告，也可以在程式任意

位置用到時才宣告

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Variables

599

399.14

1000 numberOfBeans

1004

2108

2112

oneWeight

Memory

15

Variables

• Declaring ( 宣告 ) and Defining ( 定義 )– Declaring: the name of a variable is introduced

int numberOfBeans; // 整數宣告 char nKey; // 字元宣告

– Defining: the storage for the named variable is allocated

numberOfBeans=10000;

nKey=‘A’;

• A variable declaration both declares and defines the variable

int numberOfBeans=10000;

char nKey=‘A’;

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Basic Data Types

TYPE NAME

MEMORY

USED

SIZE RANGE PRECISION

short 2 bytes -32768 ~ +32767 -

int 4 bytes -2,147,483,648 ~ +2,147,483,647

-

float 4bytes ±10-38 ~ ± 1038 7digits

double 8 bytes ± 10-308 ~ ± 10308 15digits

char 1byte All ASCII characters

-

bool 1 byte true, false -

整數型態

浮點數型態

字元或字串型態

布林數型態

有效位數及指數範圍不同

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Assigning Data for Variables • Initializing data in declaration statement

int myValue = 0; or int myValue(0);

• Assigning data during execution myValue = 0;

– Lvalues (left-side) & Rvalues (right-side)– Lvalues must be variables, Rvalues can be a

ny expression ( 運算式 )– Example:

distance = rate * time;Lvalue: ‘distance’ , Rvalue: ‘rate * time’

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Assigning Data for Variables

• Assigning data during execution (cont.)

– 將等號 (=) 右邊的運算式的值指定給左邊的變數

VarName= Expression; ( 變數＝運算式 ;) 敘述 (a statement)

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• Compatibility of data assignments– Type mismatches– General Rule: Cannot place value of one typ

e into variable of another type

e.g.:

int intVar;

…

intVar = 2.99; // 2 is assigned to intVar!• Only integer part ‘fits’, so that’s all that goes• Called ‘implicit’ or ‘automatic type conversion’

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Assigning Data for Variables• Example: char( 字元 ) vs. short ( 整數 )

#include <iostream>using namespace std;void main() //Relation Between char and short{ char testChar='X'; short testShort=testChar;

cout << "The ASCII for "<<testChar<<" is "<<testShort<<"\n"; testChar=testChar+1; testShort=testChar; cout << "The ASCII for "<<testChar<<" is "<<testShort<<"\n"; testChar=122; cout << "New testChar is "<<testChar<<"\n";}

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• bool – C++ assigns nonzero values to true and zero

value to false bool start= true;

bool start= false;

bool start= 100 ; //start assigned true

bool start= 0 ; //start assigned false

int ans = true ; //start assigned 1

int ans = false; //start assigned 0

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Literal Data or Constants ( 常數 )

• Cannot change values during execution Examples:

2 // Literal constant int 整數575.34 // Literal constant double 浮點數5.7534e2 // Literal constant double 浮點數3.4e-2 // Literal constant double (0.034)‘Z’ // Literal constant char 字

元“Hello World” // Literal constant string 字串

• Called ‘literals’ ( 實字 ) because you ‘literally typed’ them in your program!

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Literal Data or Constants ( 常數 )• Naming of constants

– Literal constants provide little meaning e.g.: seeing 24 in a program, tells nothing about

what it represents– Use named constants instead

• Meaningful name to represent data const double = 6.9;– Called a ‘declared constant’ or ‘named

constant’

• Now use it’s name wherever needed in program – Added benefit: changes to value result in one fix

若程式中數個地方用到同一個常數，要修改其值，只要修改符號定義一處即可

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An example of C++ program#include <iostream>


void main (void)

{

constant double RATE=6.9;

double deposit;

cout << "Enter the amount of your deposit $";

cin >> deposit;

double newBalance;

newBalance= deposit+ deposit*(RATE/100.0);

cout << "In one year, the deposit will grow to\n" <<"$"<< newBalance;

}

Enter the amount of your deposit $ 100In one year, the deposit will grow to$106.9

輸入

C++ 程式

程式執行過程

25

Escape Sequences ( 逸出順序、控制碼 )

• Backslash,( 背斜線 ) \ preceding a character ( 字元 )

• Instructs compiler: a special ‘escapecharacter’ ( 逸出字元 ) is coming– 逸出字元 : 字串及字元中無法顯示之字元

ESCAPE SEQUENCES MEANING

\n 換行 / 新列 (New line)

\' 單引號 (single quote)

\" 雙引號 (double quote)

\a 響鈴 (bell)

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Escape Sequences

#include <iostream>using namespace std;

void main() //escape sequence{ cout <<'\a'<<"This is a test for escape sequences\n"; cout <<"How about you \"computer course\"?\n";}

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Arithmetic Operators ( 算數運算子 )

Arithmetic Operators MEANING

+ 相加 - 相減* 相乘/ 相除% 餘數 ( 模數 )

使用模數運算子時，必須兩個運算元都是整數才行

• 5 arithmetic operators

• Precedence ( 優先權 ) rules – standard rules如 *,/,% 優先於 +,- ; 由左至右之結合律 (associative)

float logs=120/4*5; //150 ? 6?

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Arithmetic Operators ( 算數運算子 )#include <iostream>using namespace std;

void main(){ int numA, numB, finalResult;

cout << " 請輸入第一個整數 : "; cin >> numA; cout << " 請輸入第二個整數 : "; cin >> numB;

finalResult=numA+numB; cout << " 相加的結果 : " <<"$"<< finalResult<<"\n";

finalResult=numA-numB; cout << " 相減的結果 : " <<"$"<< finalResult<<"\n"; finalResult=numA*numB; cout <<" 相乘的結果 : " <<"$"<< finalResult<<"\n"; finalResult=numA/numB; cout <<" 相除的結果 : " <<"$"<< finalResult<<"\n"; finalResult=numA%numB; cout <<" 相除的餘數 : " <<"$"<< finalResult<<"\n";}

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Arithmetic Precision ( 算數運算精確度 )

• ‘Highest-order operand’ determines typeof arithmetic ‘precision’ performed

– Common pitfall!

• Examples:– 17 / 5 evaluates to 3 in C++! (Both operands are integers)

• Integer division is performed!

– 17.0 / 5 equals 3.4 in C++!• Highest-order operand is ‘double type’• Double ‘precision’ division is performed!

– int intVar1 =1, intVar2=2; intVar1 / intVar2 equals 0 in C++!

• Performs integer division!

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Arithmetic Precision

• Calculations done ‘one-by-one’– 1 / 2 / 4.0 / 8 performs 3 separate divisions

• First 1 / 2 equals 0• Then 0 / 4.0 equals 0.0• Then 0.0 / 8 equals 0.0!

– What if 1 / 4.0/ 2 / 8 ?

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Type Casting ( 型態轉換 )

• Implicit – also called ‘Automatic’– Done for you by C++ automatically

17 / 5.5 ‘implicit type cast’ to take place, casting the 17 17.0

• Explicit type conversion– Programmer specifies conversion with cast operator

(double)17 / 5.5; or double(17 )/ 5.5;

(double) myInt / myDouble;

old C syntax old C++ syntax

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Shorthand Operators

• Increment (++)& Decrement (--) Operators– Just short-hand notation

• Increment operator,++ intVar++; is equivalent to

intVar = intVar + 1;

• Decrement operator, -- intVar--; is equivalent to

intVar = intVar – 1;

33

Shorthand Operators Post-Increment in Action

• Post-Increment in Expressions: int n = 2, valueProduced;

valueProduced = 2 * (n++); cout << valueProduced << "\n"; cout << n << "\n";

– This code segment produces the output:43

– Since post-increment was used

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Shorthand Operators Pre-Increment in Action

• Now using Pre-increment:

int n = 2, valueProduced;

valueProduced = 2 * (++n); cout << valueProduced << "\n"; cout << n << "\n";

– This code segment produces the output:63

– Because pre-increment was used

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Console Input/Output

• I/O objects: cin, cout

• Defined in the C++ library called <iostream>

– Must have these lines (called pre-processor directives) near start of file

#include <iostream> using namespace std;

– Tells C++ to use appropriate library so we canuse the I/O objects cin, cout

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• Console Output– What can be outputted?– Any data can be outputted to display screen

• Variables• Constants or Literals• Expressions (which can include all of above)

– cout << numberOfGames << " games played.";

(2 values are outputted: ‘value’ of variable numberOfGames, literal string “ games played.”)

• Cascading: multiple values in one cout

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• Formatting Output– Formatting numeric values for output– Values may not display as you’d expect!

cout << " The price is $ " << price << "\n";– If price (declared double) has value 78.5, you

might get:• The price is $78.500000 or:• The price is $78.5

– We must explicitly tell C++ how to outputnumbers in our programs!

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• Formatting Output– ‘Magic Formula’ to force decimal sizes:

cout.setf(ios::fixed); cout.setf(ios::showpoint); cout.precision(2);

– These statements force all future continued values:

• To have exactly two digits after the decimal place

– Example: cout << “The price is $” << price << endl;

– Now results in the following: The price is $78.50

39


• Input Using cin– ‘>>’ (extraction operator) points opposite– No literals allowed for cin. Must input ‘to a vari

able’

cin >> num;

– Waits on-screen for keyboard entry– Value entered at keyboard is ‘assigned’ to nu

m

40

Program Style

• Bottom-line ( 底線 ): Make programs easy to read and modify

• Comments, two methods:– // Two slashes indicate entire line is to be ignored– /*Delimiters indicates everything between is ignored*/

• Identifier naming– ALL_CAPS for constants– lowerToUpper for variables– Most important: MEANINGFUL NAMES!

41

Libraries

• C++ Standard Libraries #include <Library_Name> ( 標頭檔 )– Directive to ‘add’ contents of library file to

your program– #include is called ‘preprocessor directive’ ( 前端

處理程式指令 )– Executes before compiler, and simply ‘copies’

library file into your program file

• C++ has many libraries– Input/output, math, strings, etc.

42

Namespaces ( 名稱空間 )

• Namespaces defined:– Collection of name definitions

• Examples:– Includes entire standard library of name definiti

ons #include <iostream>


– Can specify just the objects we want #include <iostream> using std::cin;

using std::cout;

43

Summary 1

• C++ is case-sensitive

• Use meaningful names– For variables and constants

• Variables must be declared before use– Should also be initialized

• Use care in numeric manipulation– Precision, parentheses, order of operations

• #include C++ libraries as needed

44

Summary 2

• Object cout– Used for console output

• Object cin– Used for console input

• Use comments to aid understanding ofyour program– Do not overcomment

45

ASCII 碼

46

擴充字元集 (Extended Character Set)

47

如何使用 MS Visual C++ 6.0 之整合程式設計環境 (IDE)

• 打開 Microsoft Visual C++ 6.0

打開 MS Visual C++ 6.0

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• 產生一個 Console mode project ( 專案 )-1

1選擇下拉視窗 File 下的 New 選項

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• 產生一個 Console mode project - 2

3

2

選擇 Console 模式的 project

輸入 project 的名稱

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• 產生一個 Console mode project -3

4

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5

新增檔案到 Project

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選擇新增檔案的類型為 .cpp 即 C++ 程式

輸入新增檔案為 .cpp 即 C++ 程式

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8

9

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按兩下即可在右邊視窗編輯

選擇程式視窗

編輯你的程式

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編輯你的程式

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產生程式的執行檔

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13Compile 的結果

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執行程式

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執行程式

c/c++ basics (i) variables, expressions and assignments berlin chen 2003 textbook: walter savitch,...

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