Variables

9. Variables and Names - Link

Task
Output
Question
FAQ

You can print things out with System.out.println and you can do math. The next step is to learn about variables. In programming a variable is nothing more than a name for something so you can use the name rather than the something as you code. Programmers use these variable names to make their code read more like English, and because programmers have a lousy ability to remember things. If they didn’t use good names for things in their software they’d get lost when they came back and tried to read their code again.If you get stuck with this exercise, remember the tricks you’ve been taught so far for finding differences and focusing on details:

Write a comment above each line explaining to yourself what it does in English.
Read your .java file backwards.
Read your .java file out loud, saying even the punctuation and symbols.

VariablesAndNames.java

public class VariablesAndNames
{
    public static void main( String[] args )
    {
        int cars, drivers, passengers, cars_not_driven, cars_driven;
        double space_in_a_car, carpool_capacity, average_passengers_per_car;

        cars = 100;
        space_in_a_car = 4.0;
        drivers = 30;
        passengers = 90;
        cars_not_driven = cars - drivers;
        cars_driven = drivers;
        carpool_capacity = cars_driven * space_in_a_car;
        average_passengers_per_car = passengers / cars_driven;

        System.out.println( "There are " + cars + " cars available." );
        System.out.println( "There are only " + drivers + " drivers available." );
        System.out.println( "There will be " + cars_not_driven + " empty cars today." );
        System.out.println( "We can transport " + carpool_capacity + " people today." );
        System.out.println( "We have " + passengers + " to carpool today." );
        System.out.println( "We need to put about " + average_passengers_per_car + " in each car." );
    }
}

The _ in space_in_a_car is called an underscore character. Find out how to type it if you do not already know. We use this character a lot to put an imaginary space between words in variable names.

U:\My Documents\CompSci\>java VariablesAndNames
There are 100 cars available.
There are only 30 drivers available.
There will be 70 empty cars today.
We can transport 120.0 people today.
We have 90 to carpool today.
We need to put about 3.0 in each car.

U:\My Documents\CompSci\>

Assignments turned in without these things will not receive any points.

I used 4.0 for space_in_a_car, but is that necessary? What happens if it’s just 4?
Answer : It’s not necessary. If it’s just 4, the result will be the same.
Remember that 4.0 is a “floating point” number. Find out what that means.
Answer : A floating-point number is a number that has either a decimal point or an exponent or both.

Write comments above each of the variable assignments.

Answer :

VariablesAndNames.java

cars = 100; // Assign 100 to cars
space_in_a_car = 4.0; // Assign 4.0 to space_in_a_car
drivers = 30; // Assign 30 to drivers
passengers = 90; // Assign 90 to passengers
cars_not_driven = cars - drivers; // Assign cars - drivers to cars_not_driven
cars_driven = drivers; // Assign drivers to cars_driven
carpool_capacity = cars_driven * space_in_a_car; // Assign cars_driven * space_in_a_car to carpool_capacity
average_passengers_per_car = passengers / cars_driven; // Assign passengers / cars_driven to average_passengers_per_car

Make sure you know what = is called (equals) and that it’s making names for things.
Answer : The = is called an assignment operator. It’s used to assign a value to a variable.
Remember _ is an underscore character.
Answer : An underscore character is a character that is used to put an imaginary space between words in variable names.

10. More Variables and Printing - Link

Task
Output
Question
FAQ

Now we’ll do even more typing of variables and printing them out. Every time you put ” (double-quotes) around a piece of text you have been making a string. A string is how you make something that your program might give to a human. You print them, save them to files, send them to web servers, all sorts of things.Strings are really handy, so in this exercise you’ll learn how to make strings that have variables embedded in them.As usual, just type this in even if you don’t understand it and make it exactly the same.

VariablesAndNames.java

public class MoreVariablesAndPrinting
{
    public static void main( String[] args )
    {
        String myName, myEyes, myTeeth, myHair;
        int myAge, myHeight, myWeight;

        myName = "Zed A. Shaw";
        myAge = 35;     // not a lie
        myHeight = 74;  // inches
        myWeight = 180; // lbs
        myEyes = "Blue";
        myTeeth = "White";
        myHair = "Brown";

        System.out.println( "Let's talk about " + myName + "." );
        System.out.println( "He's " + myHeight + " inches tall." );
        System.out.println( "He's " + myWeight + " pounds heavy." );
        System.out.println( "Actually, that's not too heavy." );
        System.out.println( "He's got " + myEyes + " eyes and " + myHair + " hair." );
        System.out.println( "His teeth are usually " + myTeeth + " depending on the coffee." );

        // This line is tricky; try to get it exactly right.
        System.out.println( "If I add " + myAge + ", " + myHeight + ", and " + myWeight
            + " I get " + (myAge + myHeight + myWeight) + "." );
    }
}

U:\My Documents\CompSci\>java MoreVariablesAndPrinting
Let's talk about Zed A. Shaw.
He's 74 inches tall.
He's 180 pounds heavy.
Actually that's not too heavy.
He's got Blue eyes and Brown hair.
His teeth are usually White depending on the coffee.
If I add 35, 74, and 180 I get 289.

U:\My Documents\CompSci\>

Assignments turned in without these things will not receive any points.

Change all the variables so there isn’t the my in front. Make sure you change the name everywhere, not just where you used = to set them.

Answer :

MoreVariablesAndPrinting.java

public class MoreVariablesAndPrinting
{
    public static void main( String[] args )
    {
        String name, eyes, teeth, hair;
        int age, height, weight;

        name = "Zed A. Shaw";
        age = 35;     // not a lie
        height = 74;  // inches
        weight = 180; // lbs
        eyes = "Blue";
        teeth = "White";
        hair = "Brown";

        System.out.println( "Let's talk about " + name + "." );
        System.out.println( "He's " + height + " inches tall." );
        System.out.println( "He's " + weight + " pounds heavy." );
        System.out.println( "Actually, that's not too heavy." );
        System.out.println( "He's got " + eyes + " eyes and " + hair + " hair." );
        System.out.println( "His teeth are usually " + teeth + " depending on the coffee." );

        // This line is tricky; try to get it exactly right.
        System.out.println( "If I add " + age + ", " + height + ", and " + weight
            + " I get " + (age + height + weight) + "." );
    }
}

Try to write some variables that convert the inches and pounds to centimeters and kilos. Don’t just type in the measurements, but work out the math inside your Java program.

Let's talk about Zed A. Shaw.
He's 74 inches (or 187.96 cm) tall.
He's 180 pounds (or 81.6466266 kg) heavy.
Actually that's not too heavy.
He's got Blue eyes and Brown hair.
His teeth are usually White depending on the coffee.
If I add 35, 74, and 180 I get 289.

Answer :

MoreVariablesAndPrinting.java

public class MoreVariablesAndPrinting
{
    public static void main( String[] args )
    {
        String name, eyes, teeth, hair;
        int age, height, weight;
        double heightCm, weightKg;

        name = "Zed A. Shaw";
        age = 35;     // not a lie
        height = 74;  // inches
        weight = 180; // lbs
        eyes = "Blue";
        teeth = "White";
        hair = "Brown";

        heightCm = height * 2.54;
        weightKg = weight * 0.453592;

        System.out.println( "Let's talk about " + name + "." );
        System.out.println( "He's " + height + " inches (or " + heightCm + " cm) tall." );
        System.out.println( "He's " + weight + " pounds (or " + weightKg + " kg) heavy." );
        System.out.println( "Actually, that's not too heavy." );
        System.out.println( "He's got " + eyes + " eyes and " + hair + " hair." );
        System.out.println( "His teeth are usually " + teeth + " depending on the coffee." );

        // This line is tricky; try to get it exactly right.
        System.out.println( "If I add " + age + ", " + height + ", and " + weight
            + " I get " + (age + height + weight) + "." );
    }
}

Can I make a variable like this: 1 = "Zed Shaw"?

No, 1 is not a valid variable name. They need to start with a letter, so a1 would work, but 1 will not, and neither would 1a.

How can I round a floating point number?

You can use the round() function like this: Math.round(1.7333).

Why does this not make sense to me?

I’m not sure, but you can try changing the numbers in this program to your own information (your height and weight, your eye color, etc). It’s weird, but talking about yourself will make it seem more “real”.

11. Using Variables - Link

Task
Answer
Output

Write a program that creates three variables: an int, a double, and a String.Put the value 113 into the first variable, the value 2.71828 into the second, and the value "Computer Science" into the third. It does not matter what you call the variables… this time.Then, display the values of these three variables on the screen, one per line.

This is room # 113
e is close to 2.71828
I am learning a bit about Computer Science

Your program SHOULD NOT look like this:

UsingVariables.java

System.out.println( "This is room # 113" );
System.out.println( "e is close to 2.71828" );
System.out.println( "I am learning a bit about Computer Science" );

You must use three variables. Your program will probably have nine lines of code inside the curly braces of main().

UsingVariables.java

public class UsingVariables
{
    public static void main( String[] args )
    {
        int room;
        double e;
        String learning;

        room = 113;
        e = 2.71828;
        learning = "Computer Science";

        System.out.printf( "This is room # %d\n", room );
        System.out.printf( "e is close to %f\n", e );
        System.out.printf( "I am learning a bit about %s\n", learning );
    }
}

U:\My Documents\CompSci\>java UsingVariables
This is room # 113
e is close to 2.71828
I am learning a bit about Computer Science

U:\My Documents\CompSci\>

12. Still Using Variables - Link

Task
Answer
Output

Write a program that stores your name and year of graduation into variables, and displays their values on the screen.Make sure that you use two variables, and that the variable that holds your name is the best type for such a variable, and that the variable that holds the year is the best type for that variable.Also make sure that your variable names are good: the name of a variable should always relate to its contents.

My name is Juan Valdez and I'll graduate in 2010.

Notice that in the example above, the values “Juan Valdez” and 2010 have been stored in variables before printing.You’re doing it wrong if your program looks like this:

StillUsingVariables.java

System.out.println( "My name is Juan Valdez and I'll graduate in 2010." );

You must use three variables. Your program will probably have nine lines of code inside the curly braces of main().

StillUsingVariables.java

public class StillUsingVariables
{
    public static void main( String[] args )
    {
        String name =  "Muhammad Muqoffin Nuha";
        int graduation = 2027;

        System.out.printf( "My name is %s and I'll graduate in %d.\n", name, year );
    }
}

U:\My Documents\CompSci\>java StillUsingVariables
My name is Muhammad Muqoffin Nuha and I'll graduate in 2027.

U:\My Documents\CompSci\>

13. Your Schedule - Link

Task
Answer
Output

Use several variables to store the names of your classes and their teachers. Then, display a nice little table displaying your schedule. Just FYI, my column of courses has a width of 26 characters, and the teacher column has a width of 15. The first and last lines are a plus sign, fifty dashes (a.k.a. minus signs) and another plus sign.Your table doesn’t need to look exactly like this, or even line up. I used a total of sixteen variables (course1, course2, … course8, teacher1, teacher2, etc.). You should do the same.

+------------------------------------------------------------+
| 1 |                          English III |       Ms. Lapan |
| 2 |                          Precalculus |     Mrs. Gideon |
| 3 |                         Music Theory |       Mr. Davis |
| 4 |                        Biotechnology |      Ms. Palmer |
| 5 |           Principles of Technology I |      Ms. Garcia |
| 6 |                             Latin II |    Mrs. Barnett |
| 7 |                        AP US History | Ms. Johannessen |
| 8 | Business Computer Infomation Systems |       Mr. James |
+------------------------------------------------------------+

YourSchedule.java

public class YourSchedule
{
    public static void main( String[] args )
    {
        String course1, course2, course3, course4, course5, course6, course7;
        String teacher1, teacher2, teacher3, teacher4, teacher5, teacher6, teacher7;
        course1 = "PKN";
        course2 = "BIG";
        course3 = "MATDIS";
        course4 = "Aljabar Linear";
        course5 = "Statistika dasar";
        course6 = "Pemrograman 1";
        course7 = "Sistem Operasi";
        teacher1 = "Pak Ikhsan";
        teacher2 = "Bu Isti";
        teacher3 = "Bu Ngatini";
        teacher4 = "Bu Puji";
        teacher5 = "Pak Arif";
        teacher6 = "Bu Fiqo";

        System.out.println("+------------------------------------------------------------+");
        System.out.println("| 1 |                                 "+course1+" |       "+teacher1+" |");
        System.out.println("| 2 |                                 "+course2+" |          "+teacher2+" |");
        System.out.println("| 3 |                              "+course3+" |       "+teacher3+" |");
        System.out.println("| 4 |                      "+course4+" |          "+teacher4+" |");
        System.out.println("| 5 |                    "+course5+" |       "+teacher3+" |");
        System.out.println("| 6 |                       "+course6+" |         "+teacher5+" |");
        System.out.println("| 7 |                      "+course7+" |          "+teacher6+" |");
        System.out.println("+------------------------------------------------------------+");
    }
}

U:\My Documents\CompSci\>java YourSchedule
+------------------------------------------------------------+
| 1 |                                 PKN |       Pak Ikhsan |
| 2 |                                 BIG |          Bu Isti |
| 3 |                              MATDIS |       Bu Ngatini |
| 4 |                      Aljabar Linear |          Bu Puji |
| 5 |                    Statistika dasar |       Bu Ngatini |
| 6 |                       Pemrograman 1 |         Pak Arif |
| 7 |                      Sistem Operasi |          Bu Fiqo |
+------------------------------------------------------------+

U:\My Documents\CompSci\>

Persiapan Belajar

Materi

Task

Programming by Doing

Compiler

9. Variables and Names - Link

10. More Variables and Printing - Link

11. Using Variables - Link

12. Still Using Variables - Link

13. Your Schedule - Link

Persiapan Belajar

Materi

Task

Programming by Doing

Compiler

​9. Variables and Names - Link

​10. More Variables and Printing - Link

​11. Using Variables - Link

​12. Still Using Variables - Link

​13. Your Schedule - Link

9. Variables and Names - Link

10. More Variables and Printing - Link

11. Using Variables - Link

12. Still Using Variables - Link

13. Your Schedule - Link