Strings
Input & Output
GEEN163 Introduction to
Computer Programming
“On two occasions I have been asked
[by members of Parliament]: ‘Pray,
Mr. Babbage, if you put into the
machine wrong figures, will the right
answers come out?’ I am not able
rightly to apprehend the kind of
confusion of ideas that could provoke
such a question.”
Charles Babbage
MyCodeLab
• Do 25 out of the 68 possible questions in
sections 2.1 and 2.2 of MyCodeLab on the
jblearning.turingscraft.com website
• You will earn 4 points for each correct answer
up to a maximum of 100 points
• You can retry incorrect answers
• Due by midnight on Tomorrow, Thursday
Homework
• The first programming assignment has been
posted on Blackboard
• It requires you to write four simple Java
programs
• Programs are due at midnight on Friday
Overwriting Variables
• Whenever a program assigns a value to a
variable, it overwrite any previous value
int wombat;
wombat = 3;
wombat = 5;
// the value 3 has been lost
Integer Division
• Integers can only hold whole numbers
• If division results in a fractional part, the
fraction is dropped and the result is just the
whole number.
8/2 is 4
4 / 3 is 1
8 / 3 is 2
-5 / 4 is -1
Operator Priority
• When an expression has multiple operators the
high priority operations are done first
• When operators are of equal priority, they are
executed left to right
• High priority operators are: * / %
• Lower priority operators are: + • Multiplication and division are done before
addition and subtraction
Evaluation of Expression
dog = 3 + 4 * 5 * 2;
dog = 3 + 20 * 2;
dog = 3 + 40;
dog = 43;
Evaluation of Expression
dog = (3 + 4 * 5) * 2;
dog = (3 + 20) * 2;
dog = 23 * 2;
dog = 46;
Clicking
• Please be sure to register your clicker in
Blackboard
• There are now two GEEN163 sections in the
Reef Polling app. Be sure you are connected
to the correct section
What will be displayed?
int rat = 3, snake =5, spider = 7;
spider = rat * (snake + 2);
System.out.println(spider);
A.
B.
C.
D.
7
17
21
spider
Math Functions
• The Math class provides methods for most of
the usual mathematical functions
• The functions are written as
Math.functionName( value )
• Note the period between Math and the
function name
Math Examples
double fish = 9.0, bird = 3.14159265, frog;
frog = Math.sqrt( fish );
// frog = 3.0
frog = Math.sin( bird / 6.0 );
// frog = 0.5
frog = Math.log( fish ) * bird;
// frog = 6.903
Math Methods
• There are many methods in the Math class
• Some of the popular methods are:
sin
cos
tan
log
log10
abs
asin
acos
atan
• The trigonometric functions take their
arguments in radians, not degrees
Math.pow
• You can compute dog = catgoat
dog = Math.pow( cat, goat );
• Note that all parameters to Math methods are
doubles
• The variable goat should be a double
Attendance Policy
• Students are required to attend all lectures,
recitations and labs
• The lab assignments are usually posted on
Blackboard the night before
• You must be present in Graham 203 to get
credit for the lab
What is Java for the equation?
𝑑𝑜𝑔 =
A.
B.
C.
D.
1 + tan(𝑐𝑎𝑡)
dog = sqrt(1+tan(cat));
dog = Math.sqrt(1.0+Math.tan(cat));
dog = Math.tan(1.0+Math.sqrt(cat));
dog = Math.sqrt * 1.0 + Math.tan * cat;
Math Constants
• The Math class provides two constants that can
be used in equations
Math.PI
𝜋 ratio of circumference to diameter
3.141592653589793
Math.E
e the base of the natural logarithms
2.718281828459045
Math Constant Examples
𝜋
𝑟𝑎𝑑𝑖𝑎𝑛𝑠 = 𝑑𝑒𝑔𝑟𝑒𝑒𝑠 ∗
180
radians = degrees * Math.PI / 180.0;
dog
cow =
cat + 𝑒
cow = dog / (cat + Math.E);
Accuracy
• Some numbers cannot be expressed accurately
with a limited number of digits
1/3 = 0.3333333333333333333333333 etc.
• Some values cannot be stored accurately in a
binary computer
• Depending on the order of operations, some
results may differ in the last few digits
• There are two special double values
o NaN – Not A Number
o Infinity
Not A Number
double
cow = -25.0, goat;
goat = Math.sqrt( cow );
System.out.println(
"Square root of "+cow+" is "+ goat);
This will display
Square root of -25 is NaN
String Variables
• String is a Java class that can hold a bunch of
characters
• String are declared just like simple data types
(i.e. int or double) using the type String
String city;
String county, state;
String name = "Fred";
String Concatenation
• The plus sign ( + ) can be used to concatenate
strings together to make longer strings
String cow = "dog", goat;
goat = "bull" + cow; // goat = “bulldog”
• You can concatenate String variables and String
constants
Mixing Strings and other Data
• If you concatenate a String to a number data
type, it will convert the number to a String and
concatenate them
String cow, goat;
int
myNum = 163;
cow = "GEEN";
goat = cow + myNum;// goat is “GEEN163”
• Any type will be converted to a String
Operator Precedence
• String concatenation has the same priority as
addition
String cow = "answer is " + 5*7;
cow is answer is 35
Be Mindful of Operator Priority
int cat = 3, dog = 4;
String goat, frog;
goat = "animals " + cat + dog;
goat = "animals 3" + dog;
goat = "animals 34";
frog = cat + dog + " animals";
frog = 7 + " animals";
frog = "7 animals";
What is the value of goat?
String
cow, goat;
int
myNum = 160;
cow = "GEEN";
goat= cow + myNum + 3;
A.
B.
C.
D.
cow163
GEEN163
GEEN1603
163
Java Output
There are several ways to display output in Java
JOptionPane.showMessageDialog
JLabel in Java GUI
Console output with System.out.println
Displaying Data on the Screen
• The System class can be used to display
output on the console or the bottom window
of jGRASP
int rat = 5;
System.out.println("Message "+rat);
• This will display
With or Without a New Line
• The method System.out.print("text") displays
a line, but does not go to a new line at the end
System.out.println("first ");
System.out.println("second ");
System.out.print("third ");
System.out.print("fourth ");
Displays
first
second
third fourth
What is displayed?
String cat = “meow”;
String giraffe = “?”;
String cow = “moo”;
cow = cat + giraffe;
System.out.println(cow);
A. moo
B. cow
C. meow?
D. meow+?
Reading Input from the Keyboard
• The java.util.Scanner class can be used to read
values from the keyboard
• At the beginning of your program, you have to
create a Scanner object
java.util.Scanner rabbit = new
java.util.Scanner( System.in );
• I frequently use the variable name keyboard for
a scanner object
Reading an Int
• After you have created a Scanner object, you
can use the nextInt() method to read an int
from the keyboard
int hare;
hare = rabbit.nextInt();
• The program will pause here and wait for
the human to enter a number
Reading a double
• After you have created a Scanner object, you
can use it to read a number with a decimal
point using nextDouble()
double bunny;
bunny = rabbit.nextDouble();
• The program will wait for the human to
enter a number
Reading a String
• You can read letters (up to a space) using
next() or the whole line using nextLine()
String cottontail;
cottontail = rabbit.next();
cottontail = rabbit.nextLine();
Keeping Up
• This class moves quickly. Avoid falling behind.
• Tutors are available in Cherry 124 from
11:00am to 8:00pm
• Tutor schedule in Blackboard course materials
• Read the textbook
• Copy and modify examples
Tutoring Schedule
11:00 AM
12:00 PM
1:00 PM
2:00 PM
3:00 PM
4:00 PM
5:00 PM
6:00 PM
7:00 PM
Monday
Prince Wynn
Kaleb Holley
Kaleb Holley
Ilo
Wynn & Smith
Jordyn Smith
Patrick Grant
Paul Biocco
Paul Biocco
Tuesday
Prince Wynn
Prince Wynn
Kaleb Holley
Kaleb Holley
Jordyn Smith
Jordyn Smith
Holley Jupiter
Paul Biocco
Paul Biocco
Wednesday
Prince Wynn
Hines & Holley
Kaleb Holley
Ilo
Jordyn Smith
Jordyn Smith
Patrick Grant
Paul Biocco
Paul Biocco
Thursday
Prince Wynn
Wynn & Gray
Holley & Staggers
Holley & Staggers
Wynn & Smith
Wynn & Smith
Holley Jupiter
Paul Biocco
Paul Biocco
Friday
Prince Wynn
Hines & Holley
Kaleb Holley
Gray
Jordyn Smith
Jordyn Smith
Jordyn Smith
Paul Biocco
Paul Biocco
Problem Solving
• You have to know how to solve a problem
before you can write a program to do it
• You have to know what it is you want to solve
before you can write a program
Development and Execution
• You write the Java program and make it work
• Somebody else (we call them the “user”) runs
the program
• You, the programmer, do not know the data
values the user will enter
Organizing a Program
• The general flow of a computer program is:
–Read some input data
–Calculate a value from the input
–Display the results
• Some programs will repeat this process many
times
Simple Program Structure
public class MyProg {
public static void main(String[] args) {
data declarations
input needed data
calculate
display results
}
}
Don’t Forget the Comments
• Write the comments as you write the program
• Often writing an explanation of what the
program is supposed to do helps to clarify the
idea
• When writing programs, I often write
comments first identifying what the program
should do at a certain point. Later I write the
Java to make it do that.
Comment Requirements
• All programs must have a comment at the
beginning that gives:
– Your name
– Short explanation of the program
• The declaration of each variable must be
accompanied by a comment explaining its
logical value or purpose
• Programs without these comments will lose
points
Input Values
• What data will the program need to solve the
problem?
• Usually each input value will need a variable
to hold the input
• What data type is the input?
int – if only whole numbers
double – if decimal values are needed
String – holds characters
User Friendly
• A good program will prompt the user for input
• Tell the user what to enter
System.out.println(“Enter the number of years”);
Scanner Class
• The java.util.Scanner class can be used to read
numbers and strings
• You must first create an object of the class
Scanner
java.util.Scanner keyboard = new
java.util.Scanner(System.in);
Scanner Methods
• There are separate methods of the Scanner
class to read different data types
int
cat = keyboard.nextInt();
double dog = keyboard.nextDouble();
String bird = keyboard.next();
Calculations
• The calculations can range from trivial to
extremely complex
• Often you may need additional variables to
hold the results
• Sometimes you will need variables to hold
temporary intermediate results
Displaying Results
• Display the results in an easy to understand
format
System.out.println("mass = " + aNumber);
• You usually do not need additional variable for
displaying the results
Typing the Program
• Most programs are not written from top to
bottom
• You may occasionally need to go back to the
top and declare another variable
MyCodeLab
• Do 25 out of the 68 possible questions in
sections 2.1 and 2.2 of MyCodeLab on the
jblearning.turingscraft.com website
• You will earn 4 points for each correct answer
up to a maximum of 100 points
• You can retry incorrect answers
• Due by midnight on Tomorrow, Thursday
Homework
• The first programming assignment has been
posted on Blackboard
• It requires you to write four simple Java
programs
• Programs are due at midnight on Friday