Notes

Outline

COM1100 Fundamentals of Computer Science –Winter 2000 Lecture 3 01/11/99

Announcement -- 1 Please visit course web site periodically.    www.ccs.neu.edu/course/com1100    COM1100_Winter_Announcement.html Lecture notes can be found at :  www.ccs.neu.edu/course/com1100/LectureNotes Not all the lecture notes will be posted    Try to take notes.

Announcement -- 2 Sequence info can be found at :  www.ccs.neu.edu/course/com1100/Sequence Talk to the recitation instructor directly if you need to switch the sequence You can go to sequence 8 for recitation    but go to sequence 9 for lab

Announcement -- 3 If you are not a computer science major, you can take the course for Pass/Fail Send me email Name ID Major If I did not get your email, I will assume you want a graded credit. If you are a computer science major, you have to take it for graded credit.

Programming Tool Microsoft Visual C++ 6.0 Service pack 3 It is for Visual Studio (C++) 6.0 Both Windows 95/98 and NT need it Download Winzip 7.0 evaluation version WinZip brings the convenience of using Zip files and other archive and compression formats. http://www.winzip.com/

Reading material this week Textbook Chapter 1 : Introduction Chapter 2 : Fundamental Data Types Package 1 – Patterns Name Use Pattern - Part 1 - Variables and Objects Read-Process-Write Pattern Reading Data Pattern

Review of last lecture First C++ program cout << cin >>

Today’s lecture Identifier Variable declare and use Assignment statements Basic Data Type Arithmetic operations (?)

First C++ program 1 #include <iostream> 2 using namespace std; 3 int main() 4 { 5   cout << "Hello, World\n"; 6 7 return 0; 8 }

Second C++ program 1 #include <iostream> 2 using namespace std; 3 int main() 4 { 5   int num1, num2; 6 cout << “Please enter two numbers: “; 7   cin >> num1 >> num2; 8   cout << “The sum is : “ << num1 + num2; 9 return 0; 10 }

int num1, num2; Declared two integer variables num1, num2 num1 is the name for an integer variable num2 is the name for another integer variable num1 and num2 are called identifiers.

C++ Identifiers -- 1 Identifier give names to variables and functions. Identifier can be made up of any combination of letters, digits, or underscores(_). The first character of the name must be a letter or underscores(_).  Identifier beginning with underscore normally reserved for internal compiler use and for declaring system variable. So it is not recommended.

C++ Identifiers -- 2 Only letters, digits, or underscores may follow the initial letter. Blank spaces are not allowed special characters (*, /, %, (), ?, &, !) are not allowed Use the underscore to separate words in a name consisting of multiple words

C++ Identifiers -- 3 Can not be one of the keywords  A keyword is a word that is set aside by the C++ language for a special purpose and can only be used in a specified manner. Some examples: do if case double return else int short char long unsigned sizeof const float void continue for switch while All keywords in C++ are lowercase.

C++ Identifiers -- 4 Can not consist of more than 31 characters Case-sensitive (upper and lower case letters are different) test and TEST are two different identifiers for variable. int main() and int Main() are two different identifiers for function. C++ program can only be executed from function int main()

Examples of C++ Identifier -- 1 Some valid identifier Count Test32 DeptNo Test_Score _varX FEET_PER_MILE

Examples of C++ Identifier -- 2 Some invalid identifier 2ndQtrSale   // can not start with digit  hi!there // ! not valid  top…ten //. can not be used  case // keyword can not be used  Dept  No // Blank space can not be used

Variable All integers, floating-points, strings and other values used in a program are stored in and retrieved from the memory unit. Conceptually, individual memory locations in the memory unit are arranged like the rooms in a large hotel. Like hotel rooms, each memory location has a unique address (“room number”).  A variable is simply a name (or an identifier) chosen by the programmer that is used to refer to computer storage location. The term variable is used because the value stored in the variable can change, or vary.

Variable ~ Memory location For each variable name the program uses, the computer keeps track of the actual memory address corresponding to that name. In our hotel room analogy, this is equivalent to putting a name on the door of a room and referring to the room by this name, such as BLUE room, rather than using the actual room number.

Variable Declaration Syntax:  data_type variable_name; int x; double y; string myName; When you declare a variable, The system assigned a space in the memory that will be used to store the value of the variable The memory location can be referred by the name of the variable in the future The content of the memory location may be garbage – whatever information has been stored there last time

Variable Definition -- 1 Initialize the value of variable x when it is declared data_type variable_name = initial_value  Examples int x = 5; double y = 7.896; string myName = “Yuhong Yin”;

Variable Definition -- 2 Change the value of a variable anywhere in the program using assignment statement Examples int x;  // variable declaration x = 9; x  = 7 + 8; string herName; // variable declaration herName = “ Elice Brown”;

Variable Definition -- 3 Read user input from the keyboard or input from a file The basic C++ statement that read the input stream from the keyboard (the sequence of keystrokes typed in followed by the return key) is following (using cin >>): int x; cin >> x; string myName; cin >> myName;

Assignment statement Syntax: variable = expression; Evaluate the RHS Put into LHS Assignment statement tells the computer to assign (store) a value into a variable. Assignment always have an equal ( = ) sign and one variable name immediately to the left of this sign. The value on the right of the equal sign is determined first and this value is assigned to the variable on the left of the equal sign.

Invalid Assignment Note : 7 + 8 = x;   // Wrong ! 7 + 8 is not an identifier for any variable LHS of the assignment will not be evaluated

Using a Variable Once an identifier has been properly declared, initialized and assigned, it can be used in computations, used in output and other operations appropriate for its type. Variables of int and double type can take part in arithmetic expressions, can be printed, and can be assigned a new value. Besides reading and writing string, we can manipulate them in a number of ways.

Garbage in, Garbage out If an identifier has not been initialized before it is used, the compiler may notice the error and notify the programmer, or it may ignore the fact and use the garbage that appears in the memory space assigned to the identifier. Garbage in, Garbage out int m=7,n=m, q; cout << "m = " << m << endl; // m = 7 cout << "n  = " << n  << endl; // n = 7 q = q + n;  // q doesn’t have any value cout << "q  = " << q  << endl; // q = -858993453

Variable summary Declare Initialize Use Destroy (Automatically done by the computer)

C++ Basic Data Type Basic data types are character, integer and floating point char int double Exact size and capacity of a type varies with the machine and operating system

C++ Data Type -- char Used to store characters Characters include the letters of the alphabet (both uppercase and lowercase), the 10 digits 0~9, and the special symbols (+ $ &) enclosed by single quote. Examples ‘A’ ‘$’ ‘7’ ‘q’

C++ Data Type -- int Used to store whole numbers (no decimal or fraction), come in three sizes: short int long Examples 13 567 -9 -13

C++ Data Type – floating point Used to store numbers requiring fraction or decimal, come in 3 sizes or precisions float double long double Also called real number Values stored is often not exact (like 2/3 in decimal, have to stop somewhere 0.666667) Special symbols, such as the dollar sign and the comma, are not permitted in real number 5,326.25 $10.99 Most often use double instead of float

signed or unsigned Character and integer types can be declared as either signed or unsigned unsigned are never considered to hold a negative, and so they can hold a large positive value signed is the default and not required to specify. Examples unsigned char ch; unsigned int x;  x = 5; // OK  x = -5; // Wrong int x and signed int x are equal x = 5; // OK x = -5; // OK

Date types – size & capacity

sizeof operator        C++ provides a special operator sizeof which gives the size in bytes of any data type cout << "unsigned char = " << sizeof(unsigned char) << endl;  // unsigned char = 1 cout << "char = " << sizeof(char) << endl;     // char = 1 cout << "short = " << sizeof(short) << endl; // short = 2 cout << "int = " << sizeof(int) << endl; // int = 4 cout << “float = " << sizeof(float) << endl; // float = 4 cout << “double = " << sizeof(double) << endl;   // double = 8

Today’s lecture Identifier Variable declare and use Assignment statements Basic Data Type Arithmetic operations (?)

Thursday’s lecture Arithmetic operations (?) Programming style Safe I /O operations Visual C++ IDE