Week 3: Basic Operations CS 177

What did we talk about last week? Data representation Built-in types int double boolean char String Literals Declaration of variables Assignment

Basic operations In Java, each data type has a set of basic operations you are allowed to perform It is not possible to define new operations or change how the operations behave Some programming languages allow this, but not Java

Operations for each type We are going to consider the basic operations for numerical types: int double

The + Operator for int Use the + operator to add two ints together int a; int b; a = 5 + 6; // a contains 11 b = a + 3; // b contains 14 a + b; // not allowed, does nothing a = a + 1; // a contains 12, and b?

Shortcuts Some expressions are used so often that Java gives us a short cut x = x + y; can be written x += y; x = x + 1; can be written x++; int x; x = 6; // x contains 6 x += 4; // x contains 10 x++; // x contains 11

The - Operator for int Exactly like + except performs subtraction int a; int b; a = 6 - 5; // a contains 1 b = 3 - a; // b contains 2 a -= 10; // shortcut for a = a – 10; a--; // shortcut for a = a – 1;

The * Operator for int The * operator performs multiplication int a; int b; a = 5 * 6; // a contains 30 b = a * 3; // b contains 90 a *= 2; // shortcut for a = a * 2;

The / Operator for int The / operator performs integer division when used with two ints Not the same as regular division The factional part is dropped, not rounded int a; int b; a = 9; // a contains 9 b = a / 2; // b contains 4 a /= 2; // shortcut for a = a / 2;

The % Operator for int The % operator is the mod operator It returns the remainder after division This operator is a good way to find out if a number is even or odd int a; int b; a = 38; // a contains 38 b = a % 5; // b contains 3 a %= 2; // shortcut for a = a % 2;

Area example Compute the area of a rectangle width Area length area = width * length; width Area length

Conversion example If you run 26 miles, how many feet is that?

The + Operator for double Exactly the same as + for int, except now you can have fractional parts double a; double b; a = 3.14159; // a contains 3.14159 b = a + 2.1; // b contains 5.24159 a += 1.6; // shortcut for a = a + 1.6; a++; // shortcut for a = a + 1.0;

The – and * Operator for double No surprises here They do subtraction and multiplication double a; double b; a = 3.14159; // a contains 3.14159 b = a - 2.1; // b contains 1.04159 a = b * 0.5; // a contains 0.520795

The / Operator for double Unlike int, this division does have fractional parts Can you explain this mystery? double a; double b; a = 9; // a contains 9.0 b = a / 2; // b contains 4.5 b = 9 / 2; // b contains 4.0

Area example Compute the area of a triangle height Area base area = 1.0/2.0 * base * height; height Area base

TF = (9/5)TC + 32 Conversion example Given a temperature in Celsius, what is the equivalent in Fahrenheit? TF = (9/5)TC + 32 tempF = 9.0/5.0 * tempC + 32;

Complex expressions How complex can expressions get? What’s the value of a? 18! int a = 31; int b = 16; int c = 1; int d = 2; a = b + c * d – a / b / d;

Complex expressions Order of operations holds like in math You can use parentheses to clarify or change the precedence Now a is 46 int a = 31; int b = 16; int c = 1; int d = 2; a = (((b + c) * d) – a / b) / d;

Casting You cannot directly store a double value into an int variable However, you can cast the double value to convert it into an int Casting tells the compiler that you want the loss of precision to happen You can always store an int into a double int a = 2.6; // fails! int a = (int)2.6; // succeeds! (a = 2)

Rounding In Java, the conversion of a double into an int does not use rounding As in the case of integer division, the value is always rounded down You can think of this as using the floor function from math If you want to round normally, you can simply add 0.5 before the cast double x = 2.6; int a = (int)(x + 0.5); // rounds

Adding is great, but… There are operations beyond +, -, *, /, and % that you probably want to do with numbers Java has those built-in because the computer can do those directly A number of other operations can be done by calling methods Methods will end up being very important to us later

Methods A method is a collection of Java code that has been packaged up so that you can use it over and over Usually a method will take some input and give some output System.out.println() is an example of a method Using a method (calling a method) always requires parentheses

Method example with sin() The sin() method allows you to find the sine of an angle (in radians) This method is inside the Math class The answer that it gives back is of type double To use it, you might say the following: double value = Math.sin( 2.4 );

result = class.method( input ); Method syntax Unless the method is inside your class, you must supply a class name and a dot If your method takes input, you put it inside the parentheses, if not, you leave them empty result = class.method( input ); You can store the result of the method, as long as the variable matches the type that the method gives back Next, you must give the method name that you are calling

Other Math methods Return type Name Job double sin( double theta ) Find the sine of angle theta cos( double theta ) Find the cosine of angle theta tan( double theta ) Find the tangent of angle theta exp( double a ) Raise e to the power of a (ea) log( double a ) Find the natural log of a pow( double a, double b ) Raise a to the power of b (ab) long round( double a ) Round a to the nearest integer random() Create a random number in [0, 1) sqrt( double a ) Find the square root of a

Operations on booleans The boolean type seems so simple What on earth would we want to do with it? Just like numerical types, we can combine booleans in various ways You might be familiar with these operations if you have taken a course in logic

The ! Operator The NOT operator Changes a true into a false or a false into a true x !x true false

The && Operator The AND operator It gives back true only if both things being combined are true x y x && y true false

The || Operator The OR operator It gives back true if either or both things being combined are true x y x || y true false

The ^ Operator The XOR operator is what some people mean when they say “or” in English It gives back true if one but not both things are true x y x ^ y true false

Short circuit evaluation In some circumstances, Java doesn’t check the whole expression: (true || (some complicated expression)) Ignores everything after || and gives back true (false && (some complicated expression)) Ignores everything after && and gives back false

What kinds of operations would you expect on chars? Multiplication and division don’t seem to make sense We can increment and decrement a char char letter; letter = ‘f’; // letter contains ‘f’ letter++; // letter contains ‘g’ letter++; // letter contains ‘h’

Sometimes it’s useful to know the number It is possible to convert a char into an int It can often be more useful to get the offset from a starting point int number; number = ‘a’; // letter contains 97 char letter = ‘r’; int number; number = letter - ‘a’ + 1; //number is 18

Escape sequences Remember that we use single quotes to designate a char literal: ‘z’ What if you want to use the apostrophe character ( ‘ )? apostrophe: ‘\’’ What if you want to use characters that can’t be printed, like tab or newline? tab: ‘\t’ newline: ‘\n’ The backslash is a message that a special command called an escape sequence is coming

Concatenation The only operator that we will use directly with Strings is the + (concatenation) operator This operator creates a new String that is the concatenation of the two source Strings As with numerical types, the + operator does not change the two Strings being concatenated String word; word = “tick” + “tock”; // word is “ticktock”

Concatenation with other types Concatenation is a great tool for merging lots of different types into a String Be careful about the order: String word; word = 99 + “ problems”; // word is // “99 problems” String word; word = “love potion #” + 4 + 5; // word is “love potion #45” word = “love potion #” + (4 + 5); // word is “love potion #9”

Strings are objects Objects have data inside of them but also have the ability to do things with methods Among other things, Strings can: Compare themselves with other Strings Report their length Say which character is located at position i Report a substring

String comparison To see if two Strings are identical, use the equals() method: If they are the same (including case), the method equals() will return true If they are not, the method will return false String word1 = “lettuce”; String word2 = “let us”; boolean same = word1.equals(word2); // false

String comparison To see which String goes first in the dictionary, use the compareTo() method: If word1 comes first, value will be a negative number If word2 comes first, value will be a positive number If they are the same, value will be 0 String word1 = “hard work”; String word2 = “success”; int value = word1.compareTo( word2 ); // < 0

String length To find the length of a String, use the length() method: It is possible to have a String of length 0: String word = “a mile long”; int length = word.length(); // length = 11 String nothing = “”; int length = nothing.length(); // length = 0

char at position i To find the char at position i in a String, use the charAt() method: Be careful not to ask for a character out of range: String word = “walnut”; char c = word.charAt(3); // c = ‘n’ String word = “short”; char c = word.charAt(10); // ouch!

Getting a substring To get a substring of a String, use the substring() method: The first int tells which char to start on, the second int says which char to stop before String word1 = “disco fever”; String word2 = word1.substring(3,7); //word2 = “co f”

Classes and objects are useful There are certain things that are difficult to do with the operations we’ve shown you For example, how do you turn a String representation of a number like “847” into the actual int 847? Wrapper classes!

Wrapper classes Each primitive data type in Java has a wrapper class We will focus on 3: Integer Double Character

Integer class The main uses of the Integer class are converting ints to and from Strings To convert a String to an int, use the parseInt() method To convert an int to a String, use the toString() method String number = “345”; int value = Integer.parseInt(number); int value = 543; String number = Integer.toString(value);

Double class The Double class is much like the Integer class To convert a String to a double, use the parseDouble() method To convert a double to a String, use the toString() method String number = “0.4581”; double value = Double.parseDouble(number); double value = 6.0223; String number = Double.toString(value);

Charcter class The Character class is mostly useful for getting information about a particular char For example, you can find out whether a char is a digit, is a letter, is uppercase, or is lowercase by calling the isDigit(), isLetter(), isUpperCase(), or isLowerCase() methods, respectively char c = ‘8’; boolean value = Character.isDigit(c); //true

Where we are headed Next week we will talk about conditional execution if-statements switch-statements Conditionals will allow you to write a program that makes choices With choices, your program can do different things based on input