More Practice with Nested Ifs

If statements, especially when they are nested or involve logical connectives, can be tricky to follow. Additional exercises may prove beneficial.

EXERCISE 11.13:    Consider the following code segment containing a nested if-else statement. if (x >= y) { if (x*10 < 100) { document.write("ONE<br />"); } else { document.write("TWO<br />"); } } else { document.write("THREE<br />"); }

Predict the output sequences that would be produced by the code given the following assignments.

x = 0; y = 5; x = 0; y = -5; x = 9; y = 9; x = 22; y = 21;

EXERCISE 11.14:    Consider the following code segment containing a cascading if-else statement. if (x > y) { document.write("GREATER<br />"); y = x; } else if (x == y) { document.write("EQUAL<br />"); x = 100; } document.write("x = " + x + ", y = " + y); Predict the output sequences that would be produced by the code given the following assignments. x = 0; y = 5; x = 0; y = -5; x = 9; y = 9; x = 22; y = 21;

EXERCISE 11.15:    Create a Web page named classify.html that reads in a number entered by the user and then determines whether that number is positive, negative, or zero. The page should contain a text box in which the user can enter the number and a button for initiating the analysis. When the user inputs the desired number and clicks the button, a function should be called to identify the number's type and display the result in an alert box. Be sure to parseFloat the contents of the box before testing the value.

Load your page and verify that it correctly identifies positive, negative, and zero values.

EXERCISE 11.16:    Imagine that variables roll1, roll2 and roll3 represent three die rolls (each storing a value between 1 and 6). Examine the following Boolean expressions, then describe in English what each represents.

(roll1 == 1 && roll2 == 1) ((roll1 + roll2 == 11) || (roll1 + roll2 == 12)) (roll1 > 2 && roll1 < 5) (roll3 < 3 || roll3 > 4) (roll1 == roll2 && roll2 == roll3) (roll1 == 1 && roll2 == 1 && roll3 == 1)

Does the following expression correctly represent the condition that all three die rolls are different? That is, does this expression evaluate to true if all three variables have unique values, but evaluate to false in all other cases? If you believe that the expression accurately represents this condition, justify your claim. If not, provide a correct expression that performs the task.

(roll1 != roll2 && roll2 != roll3)

Example: Designing an ESP Test

Although no scientific evidence supports the existence of Extra-Sensory Perception (ESP), many people believe in its legitimacy. A standard test for determining whether someone has ESP involves a deck of cards with various shapes on them. The test subject is shown the backs of cards and asked to guess the shapes on the reverse sides. If the subject is able to guess the correct shape more frequently than would be expected, then a claim of ESP might be made.

For example, suppose the cards in the deck displayed four different shapes. Guessing at random, you would expect a person to be able to pick the correct shape 25% of the time. If the subject were able to consistently guess correctly more than 25% of the time, then some people might view this as evidence of ESP.

In Chapter 7 (EXERCISE 7.8), you wrote a Web page that performed a rudimentary of such an ESP test. Your program selected a random number, prompted the user for a guess, and then displayed the number. Now that you know how to write an if statement, you can go the next step and actually compare the user's guess with the selected number to see if they were right. In addition, you can utilize a counter to keep track of how many times the user is right out of a sequence of guesses.

EXERCISE 11.17:    Design and implement a Web page named esp.html that allows the user to conduct an ESP test. Your page should look something like the one in Figure 11.10. The rectangle containing a question mark represents the back of a card about which the user must guess. After contemplating which shape appears on the card, the user clicks the button corresponding to the shape he or she believes is on the card (here, four possibilities are assumed). After each guess, the question mark should be replaced by the shape randomly selected by the page, and an alert box should notify the user as to whether the guess was correct. Statistics concerning the number and percentage of correct guesses should be maintained in text boxes. After the statistics have been updated and the alert box has been closed, the page should restore the question-mark image, enabling the user to enter another guess.

Although you can create your own images for the cards, you can also use the images provided for you in the www.creighton.edu/~csc107/Images directory. This directory contains images of four shapes -- square.gif, triangle.gif, circle.gif, and star.gif -- as well as mystery.gif which displays the question mark.

Figure 11. 10: Sample appearance of ESP test page.

EXERCISE 11.18:    Using your esp.html page, test yourself for ESP. After 10 guesses, what is your percentage of correct guesses? After 20 guesses? After 50 guesses?

Example: Designing a Dot Race

A recent trend at sporting events is the running of dot races on the scoreboard. In a dot race, dots of different colors speed around a track, and members of the crowd are encouraged to cheer for different dots. Every few seconds, each dot moves forward a different random amount; the dot that reaches the finish line first is the winner.

EXERCISE 11.19:    Design and implement a Web page named dotrace.html that allows the user to simulate a race between two dots. Your page should look something like the one below. The positions of the two dots (initially 0) are to be displayed in text boxes, as is the length of the race (i.e., the goal distance). Each time the user clicks on the button labeled "Take a Step", each of the dots should move forward a random amount, either 1, 2, or 3 units. When one of the dots crosses the finish line, i.e., their position is greater than or equal to the goal distance, an alert box should appear to identify the winner. Note that it is possible for the dots to tie if both cross the finish line on the same step. The page should recognize this event and display an appropriate message in the alert box.

Figure 11. 11: Sample appearance of dot race page.

EXERCISE 11.20:    Augment your dotrace.html page so that that users can initiate repeated races without having to reload the page. You can accomplish this by adding a button labeled "New Race"; when a user clicks this button, the positions of the dots should be reset to 0. If the user clicks the "Take a Step" button after the current race is complete, an alert wndow should appear directing the user to click the "New Race" button and begin a new race.