Second Summer 2002

Iffy Practice Problems: This is textbook chapter 5. Warmups.
Questions:

  1. Find the errors in the following if statements. 
    if quarters > 0 then System.out.println(quarters + "quarters"); 

if (1 + x > Math.pow(x, Math.sqrt(2)) y = y + x;

if (x = 1) y++; else if (x = 2) y = y + 2;

if (x && y == 0) p = new Point2D.Double(x, y); 

if (1 <= x <= 10)
{ System.out.println("Enter y:");
  y = console.readDouble();
}

if (s != "nickels" || s != "pennies"
   || s != "dimes" || s != "quarters")
  System.out.print("Input error!"); 

if (input.equalsIgnoreCase("N") || "NO") 
  return; 

int x = console.readDouble();
if (x != null) y = y + x;

language = "English"; 
if (country.equals("USA"))
  if (state.equals("PR")) language = "Spanish"; 
else if (country.equals("China"))
  language = "Chinese";

  2. Explain the following terms and give an example for each construct:

  3. Explain the difference between an if/else  /else statement and nested if statements. Give an example for each.

  4. Give an example for an if/else /else statement where the order of the tests does not matter. Give an example where the order of the tests matter.

  5. Of the following pairs of strings, which comes first in lexicographic order? 
    "Tom", "Dick"
"Tom", "Tomato"
"church", "Churchill"
"car manufacturer", "carburetor"
"Harry" hairy"
"C++", "Car"
"Tom", "Tom"
"Car", "Carl"
"car", "bar"

  6. Complete the following truth table by finding the truth values of the Boolean expressions for all combinations of the Boolean inputs p, q, and r.

    p q r (p && q) || !r !(p && (q || !r))
    false false false ? ?
    false false true ? ?
    false true false ? ?
    ? ? ? ? ?
    ? ? ? ? ?
    ? ? ? ? ?
    ? ? ? ? ?
    ? ? ? ? ?

  7. True or false: A && B is the same as B && A for any Boolean conditions A and B?

  8. Explain the difference between 
    s = 0;
if (x > 0) s++;
if (y > 0) s++;
    and 
    s = 0;
if (x > 0) s++;
else if (y > 0) s++;

  9. Use De Morgan's law to simplify the following Boolean expressions. 
    !(x > 0 && y > 0)


!(x != 0 || y != 0)


!(country.equals("USA") && !state.equals("HI") && !state.equals("AK"))


!(x % 4 != 0 || !(x % 100 == 0 && x % 400 == 0))

  10. Make up another Java code example that shows the dangling-else problem, using the following statement. A student with a GPA of at least 1.5, but less than 2, is on probation. With less than 1.5, the student is failing.

  11. Explain the difference between the == operator and the equals method when comparing strings.

  12. Explain the difference between the tests 
    r == s
    and 
    r.equals(s)
    where both r and s are of type Rectangle.

  13. What is wrong with this test to see whether r is null? What happens when this code runs? 
    Rectangle r;
...
if (r.equals(null))
  r = new Rectangle(5, 10, 20, 30);

  14. Write Java code to test whether two objects of type Line2D.Double represent the same line when displayed on the graphics screen. Do not use a.equals(b). 
    Line2D.Double a;
Line2D.Double b;

if (your condition goes here)
  g2.drawString("They look the same!", x, y);
    Hint: If p and q are points, then Line2D.Double(p, q) and Line2D.Double(q, p) look the same.

  15. Explain why it is more difficult to compare floating-point numbers than integers. Write Java code to test whether an integer n equals 10 and whether a floating-point number x equals 10.

  16. Give an example for two floating-point numbers x and y such that Math.abs(x - y) is larger than 1000, but x and y are still identical except for a roundoff error.

  17. Consider the following test to see whether a point falls inside a rectangle. 
    Point2D.Double p = ...
boolean xInside = false;
if (x1 <= p.getX() && p.getY() <= x2) 
  xInside = true;
boolean yInside = false; 
if (y1 <= p.getY() && p.getY() <= y2)
  yInside = true; 
if (xInside && yInside) 
  g2.drawString("p is inside the rectangle.", x1, y1);
    Rewrite this code to eliminate the explicit true and false values, by setting xInside and yInside to the values of Boolean expressions.

Last updated: Jun 16, 2002 by Adrian German for A201