Discussion Section Handout 1 - Java Basics

This is the first discussion section handout. (DiSH. Being in the military for 8 years gave me a love for acronyms.) These will normally be handed out on Fridays. The DiSHes serve many purposes. 

1. Give you more practice in the topics the course covers.

2. Serve as a guide for discussion sections, although there is far more material here than can be covered in 30 minutes (10 minutes of discussion section for quiz, 10 for questions on assignment, 30 for some DiSH problems.)

3. Give you material to work on in preparation of exams. The best way to prepare for exams is by being active. Doing problems instead of looking at your notes. 

It is okay share work on the DiSHes. 

The goal of discussion section this week is  to get more practice with basic Java syntax and problem solving.

1.    Create a method that prints the representation of its integer parameter as a Roman numeral.  Thus, if the parameter is 1998, the output is MCMXCVIII.

2. Write a method that accepts two arguments, an array of integers, and a target number. The method shall print out all combinations of exactly three values in the array that when added together equal the target number. Print out the index and values for each combination. Consider this example. The array consists of the following values {1, 3, 5, 8, 10, -2, 5, 0} and the target is 13.

Target: 13
Combinations:
Indexes: 1 2 6 Values: 3 5 5
Indexes: 1 4 7 Values: 3 10 0
Indexes: 2 3 7 Values: 5 8 0
Indexes: 2 4 5 Values: 5 10 -2
Indexes: 3 6 7 Values: 8 5 0
Indexes: 4 5 6 Values: 10 -2 5

3. Write a method to determine the number of punctuation marks in a String.

4. A checksum is the 32-bit integer that is the sum of all Unicode characters in a String or file (we allow silent overflow ). Two identical Strings or files have the same checksum. Write a program to compute the checksum of a String that is supplied as a parameter. What number of ASCII characters would it take to cause an overflow in the checksum value?

5. Write a method that simulates picking numbers for a lottery. The possible numbers in the lottery are between 1 and N, a parameter to the method. The method shall pick S, another parameter to the method, numbers from the range 1 to N. No number may be picked more than once for the given set of numbers S. What is the required relationship between N and S so the method will always be able to find a solution? The method should return an array if ints with a length of S, containing the numbers picked. The first number picked will stored in the element with index 0, the second number picked will be stored in the element with index 1 and so forth.

You may use the Random class from the Java standard library. Random is contained in the java.util package.

6. Write out one tenth (1/10) in base 10. Now do it in base 2.

7. The following problems are from Rich Pattis, a lecturer at CMU. I thank him for the permission to use them here:

 Classify each of the following as a legal or illegal identifier. If it is illegal, propose a legal identifier that can take its place (a homophone (something that sounds the same but is spelled differently) or homoglyph (something that looks the same but is spelled differently))

8. Classify each of the following numeric literals as int, or double, or illegal (neither); write the equivalent value of each double without using E notation; for each illegal literal, write a legal one with the "same" value.

9. What is the difference between 5, 5., five, '5', and "5"? What is the difference between true and "true"?

10. Write a String literal that includes the characters I've said a million times, "Do not exaggerate!"

11. Choose an appropriate type to represent each of the following pieces of information

• the number of characters in a file
• a time of day (accurate to 1 second)
• the middle initial in a name
• whether the left mouse button is currently pushed
• the position of a rotary switch (with 5 positions)
• the temperature of a blast furnace
• an indication of whether one quantity is less than, equal to or greater than another
• the name of a company

12. Write prototypes for each of the following methods. Infer the information needed in the prototype from each function's description.

• countPrimesBetween returns the number of primes occurring between two integers: e.g., countPrimesBetween(11,24) evaluates to 5, because 11, 13, 17, 19, and 23 are prime.
• distance returns the Euclidean distance between two points in the plane (each point is represented by two doubles).
• inSameQuandrant returns whether or not two points lie in the same quadrant (each point is represented by two doubles).
• shuffle returns the interleaved characters from its operands: shuffle("abcd","efgh") returns "aebfcgdh".

13. What are the results of each of the the following operators?

    7/10   7./10   7/10.   7./10.
    7/10   57/10   157/10   2157/10
    7%10   57%10   157%10   2157%10

  false == false          'a' < 'Z'          'a'-'A'
  true  != true           true && false      true || false
  x++                     ++x                1 = x
  x = 1                   x == 1             x = 'A'
  Math.abs(-3.5)          Math.abs(3.5)      Math.max(3.5, 8)
  Math.sqrt(-1.)          Math.pow(-3.,2.)   Math.pow(-3.,.5)

  s = System.out.println("Hi");

15. What is printed in each of the following method calls.

  System.out.println("" + 1 + 1 + 1);
  System.out.println(1 + 1 + 1 + "");
  System.out.println('1' + 1 + "");

16. Assume that we declare double a,b; evaluate the expression (a+b - Math.abs(a-b))/2. when

• a stores 3. and b stores 5.
• a stores 5. and b stores 3.

17. Translate each of the following mathematical formulas (7 on the top line, 3 on the middle and bottom lines) into Java expressions. Assume that all variables (some have subscripts) are declared to be of type double. Writing |x| means the absolute value of x.

18. Suppose that we declare int attendance = 3000, capacity=10000; the number of fans attending an event at a stadium and the maximum number of fans possible at that stadium respectively. Which of the following Java expressions evaluates to 30, the percentage of fans in the stadium? What do the "incorrect" expressions evaluate to?

        attendance/capacity
        100*attendance/capacity
        100*(attendance/capacity)
        attendance/capacity*100        

19. Assume that we declare int x; Write an expression whose result is X rounded to the nearest 10: e.g., if x were 1432 the result would be 1430; if x were 1437 the result would be 1440; if x were 1435 round up so the result would be 1440 also. Use only the standard arithmetic operators and casting.

20. Assume that we declare double x; Write an expression whose result is the int value closest to x: e.g., if x were 3.2 the result would be 3; if x were 3.9 the result would be 4; if x were 3.5 round up so the result would be 4 also. Just casting doesn't work, because of truncation: 3.9 would be cast to 3. (no if statements)

21. Assume that we declare int x,y,z; Write an expression that computes whether. (no if statements)..

• ...x is odd
• ...x is a multiple of 20 (e.g., 20, 40, 60, ...)

• ...x and y are both positive
• ...x and y have the same sign (both are positive or both are negative)
• ...x and y have different signs (one is positive and one is negative)

• ...all three variables (x, y, and z) store the same value
• ...all three variables (x, y, and z) store different values (none the same)
• ...two variables store the same value, but the third one is different

22. Assume that we specify two points in the place with the declaration double x1,y1, x2,y2; Write an expression that computes...

• ...the distance between these points
• ...the slope of the line from the first point to the second
• ...whether both points lie on the same line from the origin
• ...whether the first point is above the second
• ...what quadrant the first point lies in (1st, 2nd, 3rd, or 4th)
• ...whether the two points lie in the same quadrant

23. Assume that specify an interval by a pair of int values (the ones at the beginning and end of the interval in the place with the declaration): 5 and 8 would specify the interval containing the numbers 5, 6, 7, and 8 inclusive. We declare int b1,e1, b2,e2; to represent the beginning and end of two intervals, and int x; so represent some value. Note that we will guarantee that the intervals are "well formed": b1 <= e1 and b2 <= e2.

• Write an expression that computes the number of values in an interval beginning with b1 and ending with e1.
• Write an expression that computes whether...
  • ...x is inside the first interval
  • ...x is not inside the first interval
  • ...x is inside the first interval but not the second
  • ...x is inside either the first or second interval (or both)
  • ...x is inside either the first or second interval (but not both)
• Write an expression that computes whether...
  • ...the first interval is the same as the second
  • ...the first interval ends before the second one begins
  • ...the first interval ends on the same value as the second one begins
  • ...the first interval is inside the second one
  • ...the first interval and the second interval overlap (at least one common value)
  • ...the first interval and the second interval do not overlap (no common values)

24.  Nitty-Gritty Java. If you just can't get enough here are some more syntactical snippets. I would not consider these very important for evaluation purposes, but some people just love to get into the language. 

Will method foo compile? Why or why not?

public int foo(int x)
{  int foo;
   foo = x;
   if(foo % 2 == 0)
      return 100;
   if(foo % 2 == 1)
      return 200;
}

What is the output of the following Java statements?

float f1 = 1.121223;
System.out.println(f1);

double d1 = 1.12f;
System.out.println(d1);

double d2 = 12345678901111l; // that's a lower case l at the end
System.out.println(d2);

int i1 = 4000000000;
System.out.println(i1);

int i2;
char c1 = 'I';
i2 = c1;
System.out.println(i2);

byte b1 = 125;
int i3 = b1 * 10;
System.out.println(i3);

byte b2;
int i4 = -1000;
b2 = (byte)i4;
System.out.println(b2);

boolean bo1 = 131 % 7 == 0;
int i5 = 12;
bo1 = i5;
System.out.println(bo1);

bo1 = (boolean)i5;
System.out.println(b01);

int i6 = 12;
if( i6 )
    System.out.println(i6);