CSCI A201/A597 Lab Notes Three Second Summer 2000

Note that Takehome Quiz Two and Problem Problem Set Two have been posted last week and are due on Tuesday and Wednesday respectively.

You should get together with your group (find out who has your problem) and start working on the assignment.

To give you a bit of help here's a list of hints, one per problem.

Questions:

Hints are presented in blue.

1. Write the following mathematical expressions in Java.

   
   
   
   

   This should not present any problem, but please be careful with default order of evaluation, and use methods from class Math for square root, cosine, also remember that class Math contains a few constants defined in it.

2. Write the following Java expressions in mathematical notation.

   dm = m * ((Math.sqrt(1 + v / c) / Math.sqrt(1 - v / c)) - 1);
   volume = Math.PI * r * r * h;
   volume = 4 * Math.PI * Math.pow(r, 3) / 3; 

   This is the reverse of the previous problem. To just give you a pointer in the text I will mention here page 71, but of course much of chapter 2 is on this topic.

3. What is wrong with this version of the quadratic formula?

   x1 = (-b - Math.sqrt(b * b - 4 * a * c)) / 2 * a; 
   x2 = (-b + Math.sqrt(b * b - 4 * a * c)) / 2 * a;

   This is the same as saying that there is a difference between

   2 / 3 * 1000

   and

   2 / (3 * 1000)

   In fact, is there a difference between the two?

4. Give an example of integer overflow. Would the same example work correctly if you used floating-point?

   Give an example of a floating-point roundoff error. Would the same example work correctly if you used integers? When using integers, you would of course need to switch to a smaller unit, such as cents instead of dollars or milliliters instead of liters.

   Roundoff and overflow errors are mentioned in the book. Note that the questions asks more than to just find the sections in your text where these topics are mentioned (pp. 62, 53, etc.)

5. Let n be an integer and x a floating-point number. Explain the difference between

   n = (int)x;

   and

   n = (int)Math.round(x);

   For what values of x do they give the same result? For what values of x do they give different results? What happens if x is negative?

   To start thinking about this problem read common error 2.2 in your text.

6. Find at least five syntax errors in the following program.

   public class WarmUpSix
   { public static void main(String[] args) 
     { System.out.print("This program adds two numbers.),
       x = 5; 
       int y = 3.5;
       System.out.print("The sum of " + x + " and " + y " is: "); 
       System.out.println(x + y)
     }
   }

   Syntax errors are those that a compiler would detect. Come to think of it, if you're completely lost you could try to compile the program above. As soon as you notice an error write it down, fix it, and recompile. By the time your program compiles without error you will have the answer to this problem.

7. Find at least three logic errors in the following program.

   public class WarmUpSeven 
   { public static void main(String[] args) 
     { ConsoleReader console = new ConsoleReader(System.in);
       int total = 1; 
       System.out.println("Please enter a number:"); 
       int x1 = Integer.parseInt(console.readLine()); 
       total = total + x1; 
       System.out.println("Please enter another number:"); 
       int x2 = Integer.parseInt(console.readLine()); 
       total = total + x1; 
       double average = total / 2;
       System.out.println("The average of two numbers is " 
         + average); 
     }
   }

   For this you need to assume that the program should compute the average of two numbers obtained from the user of the program.

8. Explain the difference between 2, 2.0, "2", and "2.0".

   This is easy: the four constants are of exactly three different types. How many of these types are primitive types?

9. Explain what each of the following two program segments computes:

   x = 2; 
   y = x + x;

   and

   s = "2";
   t = s + s; 

   You need to describe what y and t contain (or point to) at the end. Some of this is explained in the book on page 77 or thereabouts.

10. Uninitialized variables can be a serious problem. Should you always initialize every variable with zero? Explain the advantages and disadvantages of such a strategy.

    I'll leave this one up to you. By the way, what is 3/0?

11. True or false? (x is an int and s is a String)
    • Integer.parseInt("" + x) is the same as x

    • "" + Integer.parseInt(s) is the same as s

    • s.substring(0, s.length()) is the same as s

    For this you need to think of what kind of argumenr parseInt expects, and how substring and length work on Strings. Then, try to come up with the answers. Finally, you can write a short program to test your answers on a few selected cases.

12. Give two ways for converting a number to a string. What is the advantage of each of these ways?

    Section 2.6 in the book might contain useful information on that (perhaps not from the very beginning, but it's a long section).

13. How do you get the first character of a string? The last character? How do you remove the first character? The last character?

    Section 2.6.2 will be useful here. Remember though that Strings are immutable objects, and be creative, using the tools (methods) that the section describes.

14. How do you get the last digit of a number? The first digit? That is, if n is 23456, how do you find out 2 and 6? Do not convert the number to a string. Hint: %, Math.log

    To get the last digit should be easy. For the first digit you need to remember that taking the logarithm in base 10 of a number will give you the number of digits in that number minus 1 (this is an immediate consequence of the definition of the logarithm function).

15. What is a final variable? Can you define a final variable without supplying its value?

    The book has the answer in section 2.4 somewhere.

16. What are the values of the following expressions? In each line, assume that

    double x = 2.5;
    double y = -1.5;
    int m = 18;
    int n = 4;
    String s = "Hello";
    String t = "World"; 

    Remember that you can check each one of your answers by writing a program to print the values of each one of the expressions above. While this is somewhat reassuring it should also mean that you're encouraged to "think first, and experiment later".
    • x + n * y - (x + n) * y

    • m / n + m % n

    • 5 * x - n / 5

    • Math.sqrt(Math.sqrt(n))

    • (int)Math.round(x)

    • (int)Math.round(x) + (int)Math.round(y)

    • s + t

    • s + n

    • 1 - (1 - (1 - (1 - (1 - n))))

    • s.substring(1, 3)

    • s.length() + t.length()

1.	Write a program that displays the squares, cubes, and fourth powers of the numbers 1-5.

Don't do any calculations yourself, make the computer do them. You have to compute the squares, cubes, and fourth powers of 5 numbers. You can use Math functions.

First get the integers from the user, then do the calculations.

You need to know the rate of conversion from meters to miles. It would be OK if you were to transform meters in yards, feet and inches. One meter is 1.093 yards, and is 39.37 inches.

Once you get the radius you need to apply the formulas.

You need to apply three formulas here. Writing the program is easy once you know the formulas.

Integer division and using the modulus operator (%) operator to get the remainder should prove their usefulness in this problem.

Here you need to come up with your own formulas for the distance that the car can travel with the gas it has and then for the car's mileage cost, given the price of the gasoline.

Working with Strings and ConsoleReader so review 2.6 and 2.7 from the book if you don't know how to work with any them.

Read the problem carefully. You need to add only one comma (just one) in the representation of the number, to separate the thousands. The problem that follows is somewhat related too.

This is the reverse of the preceding problem. Assume that the user is program-friendly and types the comma in the right place. Just as in the hint to the previous problem you need to remove just one comma, that the user typed to separate the thousands.

One or another this is a simple problem.

The problem's hint is already more than needed, as far as how you go about solving this problem, so I will only make a comment on the interpretation of it, what the problem asks: essentially in your problem assume that you know the number already, and that it does not come from some user at run-time. So initialize a variable with the number of your choice and then compute the individual digits and print them.

There's a "quality tip" somewhere in the book about this.

Lab notes two (of last time, thursday) and section 2.7 in the book contain material related to what you need to accomplish here.

All you have to do is to set n to 1 or 0 depending on the sign of x (see the hint above).

I think Francisco's section (the lab that starts at 5pm) worked this problem out last Thursday in class -- so you will find it, or most of it, in the posted notes somewhere.

This is a debugging question.

This problem is easy, and we solved a similar one in class, when we wrote a large ">".

If all month names are of the same length some will have to be padded with spaces, for all to match the longest name. What will the common length be after the extra padding with spaces? What will the total length of the resulting string be? How did you compute that? Use the same strategy to find the place where all the previous 11 substrings end.

The password program is explained in section 2.6.4 in the textbook.