1. Week 3: Basic Operations
CS 177

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

3. 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

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

5. 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?

6. 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

7. 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;

8. 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;

9. 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;

10. 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;

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

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

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

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

15. 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

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

17. 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;

18. 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;

19. 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;

20. 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)

21. 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

22. 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

23. 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

24. 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 );

25. 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

26. 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

27. 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

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

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

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

31. 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

32. 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

33. 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’

34. 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

35. 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

36. 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”

37. 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 #” ;
// word is “love potion #45”
word = “love potion #” + (4 + 5);
// word is “love potion #9”

38. 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

39. 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

40. 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

41. 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

42. 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!

43. 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”

44. 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!

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

46. 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);

47. 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 = ;
String number = Double.toString(value);

48. 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

49. 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