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C/ C++ Basics (I) Variables, Expressions and Assignments

C/ C++ Basics (I) Variables, Expressions and Assignments. Berlin Chen 2003. Textbook: Walter Savitch, "Absolute C++," Addison Wesley, 2002. Introduction to C++. Programming Languages Low-level languages Machine, assembly High-level languages B, C, C++, ADA, COBOL, FORTRAN

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C/ C++ Basics (I) Variables, Expressions and Assignments

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  1. 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!)

  4. 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 meanings 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

  5. 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

  6. Basic Characteristics of C/C++ … void main(void) { cout << “Hello World ! \n”; … } High level language -readable -machine independent (portable) C/C++ Language (Text Files) xxx.cpp Compiler 編譯器 VC++, gcc 00100000 01101010 ….. 01101010 Low level language -unreadable -machine dependent Machine Language (Binary Files)

  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 函數名稱 int main(void) { statement_1; statement_2; … statement_n; return 0; //結束函數 } 函數標題 函數主體 敘述(statement)以分號結束 函數定義 註解(comment)以“//”為起始 結束函數

  8. 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; } 標頭檔名及 名稱空間 C++ 程式 Enter the amount of your deposit $ 100 In one year, the deposit will grow to $106.9 程式 執行 過程 輸入

  10. 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)、labels （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)

  11. Identifiers • Valid (合法的) Identifiers: x, x1, x_1, ABC123z7,… • 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

  12. 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

  13. 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 start 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

  14. Variables Memory 1000 numberOfBeans 1004 2108 oneWeight 2112

  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’;

  16. Basic Data Types 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

  17. 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 any expression (運算式) • Example:distance = rate * time;Lvalue: ‘distance’ , Rvalue: ‘rate * time’

  18. 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)

  19. Assigning Data for Variables • Compatibility of data assignments • Type mismatches • General Rule: Cannot place value of one type 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’

  20. 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"; }

  21. Assigning Data for Variables • 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

  22. 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!

  23. Literal Data or Constants (常數) • Naming of constants • Literal constants provide little meaning e.g.: seeing 24 in a program, tells nothing aboutwhat 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

  24. An example of C++ program #include <iostream> using namespace std; 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; } C++ 程式 Enter the amount of your deposit $ 100 In one year, the deposit will grow to $106.9 程式 執行 過程 輸入

  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

  26. 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"; }

  27. Arithmetic Operators (算數運算子) • 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? 使用模數運算子時，必須兩個運算元都是整數才行

  28. 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"; }

  29. 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!

  30. 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 ?

  31. Type Casting (型態轉換 ) • Implicit – also called ‘Automatic’ • Done for you by C++ automatically17 / 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

  32. Shorthand Operators • Increment (++)& Decrement (--) Operators • Just short-hand notation • Increment operator,++ intVar++; is equivalent to intVar = intVar + 1; • Decrement operator, -- intVar--; is equivalent tointVar = intVar – 1;

  33. Shorthand OperatorsPost-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

  34. Shorthand OperatorsPre-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

  35. 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

  36. Console Input/Output • 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

  37. Console Input/Output • 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, youmight get: • The price is $78.500000 or: • The price is $78.5 • We must explicitly tell C++ how to outputnumbers in our programs!

  38. Console Input/Output • 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. Console Input/Output • Input Using cin • ‘>>’ (extraction operator) points opposite • No literals allowed for cin. Must input ‘to a variable’ cin >> num; • Waits on-screen for keyboard entry • Value entered at keyboard is ‘assigned’ to num

  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 toyour 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 definitions #include <iostream>using namespace std; • 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

  48. 如何使用 MS Visual C++ 6.0 之整合程式設計環境 (IDE) • 產生一個 Console mode project (專案)-1 1 選擇 下拉視窗 File 下的 New 選項

  49. 如何使用 MS Visual C++ 6.0 之整合程式設計環境 (IDE) • 產生一個 Console mode project - 2 輸入 project 的名稱 3 2 選擇 Console 模式的 project

  50. 如何使用 MS Visual C++ 6.0 之整合程式設計環境 (IDE) • 產生一個 Console mode project -3 4

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