COMP-202: Foundations of Programming. Lecture 2: Variables, and Data Types Sandeep Manjanna, Summer 2015

Transcription

1 COMP-202: Foundations of Programming Lecture 2: Variables, and Data Types Sandeep Manjanna, Summer 2015

2 Announcements Midterm Exams on 4 th of June (12:35 14:35) Room allocation will be announced soon Final Exams on 26 th of June. Timing and Rooms will be announced soon McGill ID is necessary for taking exams. Assignment 1 released. Due Date : 17 th of May (@ 23:30)

3 Review Programming language Data representation in computers Compilers and Interpreters Errors and Debugging

4 Review public class Hello { public static void main(string[] args) { System.out.println("Hello, there!!"); } } What is the Class Name? Importance of main method? Can I replace main with some other word in this example? Which part of this program is a statement? What does this program do? Every Statement should end with a semi-colon ( ; ).

5 This Lecture Variables Data Types Strings

6 Handling Output

7 Output (More on Printing) System.out.println( Hello, world. ); The phrases appearing in are called Strings (string of characters). println is a short form for print line. This makes the next statement go to a newline.

8 Output (More on Printing) Guess the output of : public class Hello { public static void main(string[] args) { System.out.print( Hello, there!! ); System.out.println( How are you doing? ); System.out.println( What are you up to? ); } } Hello, there!!how are you doing? What are you up to?

9 Escape Characters Use an escape sequence for punctuations or spacing. \n NewLine \t Tab \ To have double quotes inside a string \\ To escape the escape character!!!!!

10 Output (More on Printing) Guess the output of : public class Hello { } public static void main(string[] args) { } System.out.println( Hello, there!! \t How are you doing? \nwhat are you up to? ); Hello, there!! How are you doing? What are you up to?

11 Comments Comments are the annotations that can be included in your code about what it does so that it is easy to understand. Examples: Single-line Comments // This will be ignored by the compiler Multi-line Comments /* So will this, if you need to write a comment that spans multiple lines. */ /* It is important to comment, especially in tricky parts of the code. But don't go overboard and comment every single line! */

12 Variables

13 Input To take input from the user We need, 1. A place to store it for further computations 2. A way to take input from the user

14 What are Variables? Remember this from high-school algebra? From geometry: In plots below: = 2 What are a and b? Area of a circle = What is r? What are x and y?

15 Variables in Java A variable is a place in memory reserved for storing a value. In Java, we give each variable a name, to make it easier to identify and a type to keep track of the kind of data we are storing. String myname; int counter; Essentially, you could say a variable is a typed label for a space of memory.

16 Variable Declaration These statements declare variables of various types. What are the types and the names? String myname; int myage; double myheight; boolean isstudent; You can name a variable anything that you want, as long as it only contains letters, numbers, and _, and doesn t start with a number. But you need to choose a clear, simple and appropriate name for readability (and full marks!) With variable declaration, you are declaring to the compiler that you will be using a variable with specified name in the program.

17 Keywords There is a set of words that have specific meaning in Java programming. These words are referred to as Keywords. The Keywords contribute to the syntax of the Java program. Example : class, public, int, char, etc. Hence, we cannot use these keywords as names for variables or classes or any other identifier.

18 Setting the Value of a Variable The assignment operator = sets the variable on the left-hand side to the value on the right-hand side. String myname; myname = Sandeep"; Or combined (in single line): String myname = Sandeep"; The expression can be complicated, so it will be evaluated for its value first. While expressing the variable and its value on paper, usual convention is to write: myname Sandeep

19 Assignment Examples int x; int y; x = 10; y = ; x = y + 3; What are the values of x and y at each line above? What happens if we try to use a variable we haven t declared, like z? Assignment operation always has a variable on LHS and an expression or a constant on RHS. 3 = x + 2 is a wrong usage of assignment.

20 Declaring and Assigning Declaring a variable: <type> <var_name>; int numstudents; Assigning a value to the variable: <var_name> = <value>; numstudents = 125; Doing both at the same time: <type> <var_name> = <value>; int numstudents = 125;

21 Printing Variables To print a variable with System.out.println: int numstudents = 125; System.out.println(numStudents); System.out.print( Number of Students per TA = ); System.out.println(numStudents/3);

22 Try it out Write a piece of code that swaps the value of two int numbers x, and y. So, if x is 5 and y is 4 before the piece of code is run, x is 4 and y is 5 after. Fill in the : int x = 5; int y = 4; // by here, x is 4 and y is 5

23 Data representation in Binary

24 Data in Computers Remember that all information in a computer is represented as a number??? Text as numbers: T h i s [space]. 84, 104, 105, 115, 32,.. Images as numbers: Store (R,G,B) values of each pixel. A pixel could be 37-red, 76-green and 125-blue and it would then look like this - Music as numbers: Store the amplitude of the sound wave over time. But the numbers used to represent these data are not in Decimal system. Instead Binary numbers are used to store these data.

25 Decimal Numbers What does writing the number 2015 mean? Ones: 5 Tens: 1 Hundreds: 0 Thousands: 2 So, 2015 =

26 Binary Numbers Base is 2, not = To convert to decimal, just compute the sum! = = = =106

27 Decimal to Binary Easiest way: parity checks and divisions by 2 e.g., What is 53 in binary? = 26 1 =13 0 =6 1 =3 0 =1 1 = =110101

28 Try it out Perform the following conversions Binary to decimal: Decimal to binary:

29 Why Binary??? Binary number system has two possible digits: 0 and 1. A switch in a circuit can take two states : ON and OFF. In computer domain, each such switch is referred to as a bit. ON is interpreted as 1 and OFF is interpreted as 0. Thus with two bits, we can have 4 permutations, (Decimal) OFF OFF OFF ON ON OFF ON ON How many bits needed to represent 8 numbers? In general, with n bits one can represent : 2 numbers. 3

30 Basic Data Types

31 Some Basic Variable Types int long float double boolean char Integer values Long integer values Real numbers Real numbers, with more precision True or false One character

32 More Types byte Can only store integers between -128 and 127 short Can only store integers between and Difference between byte, short, int, long is the number of bits (binary digits) used. Difference between float and double is also because of the number of bits.

33 Some Basic Variable Types Primitive Data Types Size in bytes Range byte < x < 127 short 2-32,768 < x < 32,767 int < x < long < x < float 4 (approx) +/-3.4 * double 8 (approx) +/-1.8 * boolean Representsonly 1-bit information. Size is not precisely defined. true and false char 2 0 < x <

34 Mismatching Types If you try to store something of type X in something of type Y, the Java compiler will complain. int x; x = "Hello"; What are the types of "Hello" and x?

35 When we write int x; Why Does Java Care? Java needs to set aside enough memory to store one integer value. If you try to store a String, it doesn't know whether it will fit or not! Also, ensuring type consistency helps us debug code. x = "Hello"; in the previous slide is obviously wrong! The same sequence of bits that encode an int would be meaningless if decoded as a String.

36 int The type int represents an integer in range of: [ 2,2 1], or [ , ]. An int takes up 32 bits = 4 bytes in memory (1 byte has 8 bits). So, there are 2 possible values that can be represented. Half of these are for negative numbers, and one more spot is taken up by zero.

37 double The type double represents a real number with fractional values. They cannot store an infinite number of digits (e.g., pi). They can only store to a limited precision. double almostpi = ; // can only store // some of the digits If you write.0after an integer constant, it will be stored as a double. int x = 3.0; int x = 3; double x = 3.0;

38 Operations between an int and a double A Mathematical operation between an int and a double will result in a double value. 2.0 * 5 = 10.0 A floating point multiplied with an integer will result in a double. 1 / 2 = 0 Because both 1 and 2 are integers. The result will be an integer (the floating point will be truncated). 1.0 / 2 = 0.5 One of the operands is a floating point. Hence the resultant will be floating point. (double)3 + 5 = 8.0 One can convert an integer into double by using a type cast technique as shown above.

39 Trial Examples What is the type and value of the Result? 2.5 * 2 = Value Type 5.0 double 5 / 2 = 5.0 / 2 = = 2 int 2.5 double 3.0 double

40 Casting You can force a conversion to the type you want, even if you lose information! This is called type casting or just casting. Write (data_type) before the expression to convert. int x = (int) 7.5; int y = (int) -7.5; 40

41 Casting is Temporary Just as double x = 3.5; double y = -x; does not change x, neither does double x = 3.5; int y = (int) x; x is still a double.

42 char A char occupies 2 bytes (16 bits). A character set is an ordered list of characters, where each character corresponds to a unique number. A char stores a single character from the Unicode character set. char gender; gender = 'M';

43 Java Unicode As mentioned before, all the characters have a number associated with them. This number is called Unicode. Look up at : Thus char datatype behaves like an integer as it stores an integer. Typecasting Example: char addition: int x = 65; char a = (char) x; char a = 'A'; char b = 'B'; char d = (char)(c+b);

44 boolean A boolean stores things that are either true or false. boolean areyouonfacebook; areyouonfacebook = false;

45 Strings and String Operations

46 String A String stores a sequence of char's. It is a more complex type than the others mentioned. Why? How many bits do you need to store a String? We've seen how to assign text to a String: String helloworld = "Hello, World!";

47 String Operations String comparison Are two strings the same? String concatenation Put together multiple Strings String length Getting the length of a String

48 String Comparison Check whether two Strings are equal: somestring.equals(otherstring) Examples: "Abc".equals("Abc") "Abc".equals("A") "ABC".equals("Abc") Can also compare String variables String myname = Marley"; String yourname = "Bob"; System.out.println(myName.equals(myName)); System.out.println(myName.equals(yourName));

49 String Concatenation Put together two Strings with the + operator. firststring + secondstring System.out.println("This is " + "one sentence."); String secondstring; secondstring = "COMP-202"; System.out.println("This is " + secondstring); secondstring = Not bad"; System.out.println("This is " + secondstring);

50 String Concatenation We can also concatenate numbers and variables with strings. Hello + 23 Hello23 Hello Numbers Hello123Numbers int a = 5; double b = 2.3; boolean c = false; String s = Hello + a + b + c; System.out.println(s); Hello52.3false

51 Get the length of a String: somestring.length( ); Examples: "Abc".length( ) String Length String myname = Marley"; String yourname = "Bob"; int bothlength = myname.length() + yourname.length(); There are more examples of methods provided by Java so that we don t need to write our own methods to do these common operations. Please refer :

52 Warm-Up What type of variable would you use to store the following? The number of school days in a year. The cost of a meal at a restaurant. Name of the course instructor. A PIN for an ATM card. Whether it snowed on January 3 rd. Grade of a student.

53 Input How do we take input from the user? We need, 1. A place to store it for further computations 2. A way to take input from the user

54 Inputs from the user How to read input from the user? Use the following magical (for now) incantation: At the top of the file: import java.util.scanner; In the main method: Scanner scanner_variable = new Scanner(System.in); Then, call this method to read a line: scanner_variable.nextline(); Problem: need to store it in a variable!

55 Inputs from the user import java.util.scanner; public class Repeater { public static void main(string[] args) { System.out.println("Say something!"); // create a Scanner to read some input Scanner scanner = new Scanner(System.in); String s = scanner.nextline(); System.out.println(s); } } We will see more of Scanner in classes and assignments. How do I look up about Java standard libraries like Scanner??? Java docs are very useful to look up about any standard library in java.

56 Inputs from the user Now we know how to read a String from standard input. How to read other data types??? For the basic data types, use.next<typename>: scanner.nextboolean() scanner.nextbyte() scanner.nextint() scanner.nextdouble()

57 Try it out Write a program that asks for a temperature in Fahrenheit, then computes and displays the equivalent in Celsius. celsius = (fahrenheit 32.0) * 5.0 / 9.0 Hint: break it down into the following steps: 1. Set up main method in a class 2. Get the Scanner working 3. Implement the conversion formula 4. Display the output

58 Try it out import java.util.scanner; public class TempConverter { public static void main(string[] args) { System.out.println("Enter temperature in F:"); Scanner scanner = new Scanner(System.in); // TODO: read temperature and convert System.out.println("The temperature in C is:"); // TODO: display new temperature } } celsius = (fahrenheit ) * 5.0 / 9.0

59 Summary More printing options Variable (Declaration and Assignment) Binary Numbers Data Types Strings Reading input (Scanner)