Exam 1 Review

Problem X1r.1.

Tabulate the values taken on by each variable as the program fragment below executes.

int n;
int k;
int i;
n = 32;
k = n + 1;
i = 2;
while(i * i <= n) {
    if(n % i == 0) {
        k = k + i + (n / i);
    }
    i = i + 1;
}

Problem X1r.2.

Suppose we have a vertical line segment from (lx,ly0) to (lx,ly1) (where ly0 < ly1). What should go into an if statement's condition to test whether a point (px,py) lies on that line?

if(

                                                                   ) {
  print("point on line");
}

Problem X1r.3.

Complete the below program fragment so that the ball changes its color between red and green every second. Your program should use an if statement (and a while loop) to accomplish this.

public class AlternateColor extends GraphicsProgram {
    public void run() {
        GOval ball = new GOval(25, 25, 50, 50);
        ball.setFilled(true);
        add(ball)

        boolean isRed = true;
        while(true) {
            if(isRed) {
                ball.setFillColor(new Color(0, 255, 0));
                isRed = false;
            } else {
                ball.setFillColor(new Color(255, 0, 0));
                isRed = true;
            }
            pause(1000);
        }
    }
}

Problem X1r.4.

The java.awt package contains a class called Point for representing points on the two-dimensional plane. It contains the following methods.

double getX()

Returns the x-coordinate of this point.

double getY()

Returns the y-coordinate of this point.

void translate(double dx, double dy)

Translates this point by a distance of dx horizontally and by a distance of dy vertically.

Suppose that the Point variable query already refers to a point somewhere on the plane. Write a program fragment that reflects the point so that its x-coordinate is positive. That is, if the point's x-coordinate is negative, then its x-coordinate should become |x| (its absolute value). The only Point methods you should use are those listed above.

if(query.getX() < 0) {
    query.translate(2 * -query.getX(), 0.0);
}

Problem X1r.5.

Each variable declared within Java has a scope. Explain what scope means, and explain how the Java compiler determines a variable's scope from a program.

Problem X1r.6.

Complete the following program fragment so that it repeatedly reads values from the user until the user enters an integer between 0 and 100. (Both 0 and 100 should be accepted.) The program should then display A if the number is 90 or above, B if the number is 80 or above (but less than 90), and worse if the number is below 80.

import acm.program.*;

public class GetScore extends Program {
    public void run() {
        int score = readInt("Grade: ");

        while(score < 0 || score > 100) {
            score = readInt("Grade: ");
        }
        if(score >= 90) {
            println("A");
        } else if(score >= 80) {
            println("B");
        } else {
            println("worse");
        }
    }
}

Problem X1r.7.

What does the below program fragment display?

int n = 5;
String sum = "";
while(n > 0) {
    sum = sum + n;
    n--;
}
println(sum);

Problem X1r.8.

What are the first and last lines displayed by the below program fragment?

int i = 1234;
while(i < 4321) {
    String s = "" + i;
    if(s.substring(1, 2).equals("5")) {
        println(s);
    }
    i++;
}

Problem X1r.9.

The below fragment is meant to continue reading and echoing strings typed by the user until the user enters the word stop. However, the program never actually stops. Explain why, and describe how to repair the program to work as intended.

String line = this.readLine();
while(line != "stop") {
    this.println(line);
    line = this.readLine();
}

Problem X1r.10.

Suppose a user executes the following code fragment.

String str = "arboreteum";
int j = 0;
int i = 0;
while(i < str.length()) {
    if(str.substring(i, i + 1).equals("r")) {
        print(str.substring(j, i));
        j = i + 1;
    }
    i++;
}
println(str.substring(j));

Show all the values taken on by the variables i and j as this fragment executes. Also, what does the fragment print?

i  0  1  2  3  4  5  6  7  8  9 10
j  0  2        5

Problem X1r.11.

Tabulate the values taken on by the variables i and j as the program fragment below executes. Also, what will the program print?

String str = "barber";
int i = 0;
while(i < str.length()) {
    String c = str.substring(i, i + 1);
    int j = 0;
    while(j < i && !str.substring(j, j + 1).equals(c)) {
        j++;
    }
    if(j == i) {
        print(c);
    }
    i++;
}

Problem X1r.12.

Complete the following program so that it displays the number of lower-case r's in the string entered by the user.

import acm.program.*;

public class CountRs extends Program {
    public void run() {
        String line = readLine("Line of text: ");

        int count = 0;
        int i = 0;
        while(i < line.length()) {
            if(line.substring(i, i + 1).equals("r")) {
                count++;
            }
            i++:
        }
        println(count);
    }
}

Problem X1r.13.

Complete the program below so that it computes another string containing every other letter from the string typed by the user. For example, if the user enters blackberry, the program should compute baker, and if the user enters steadfastness, the program should compute sedates.

Note that your program should build up its result in the out variable rather than display letters as it goes through the string.

public class ShowEveryOther extends Program {
    public void run() {
        String in = this.readLine("Give me a line: ");
        String out = "";

        // your solution here

        this.println("Result is: " + out);
    }
}

        int i = 0;
        while(i < in.length()) {
            out = out + in.substring(i, i + 1);
            i += 2;
        }

Problem X1r.14.

Complete the below program so that it reads a line from the user; but as long as the user types an empty line, it displays Please enter your name. and reads another line from the user. But when the user enters a line that isn't empty, where name is the line typed by the user, the program should display Hello, name. and then exit.

public class Solution extends Program {
    public void run() {

        String name = readLine("Please enter your name. ");
        while(name.equals("")) {
            name = readLine("Please enter your name. ");
        }
        println("Hello, " + name + ".");
    }
}

Problem X1r.15.

Complete the following program so that after reading an integer n from the integer, it displays a square of asterisks with n rows and n columns. You may assume that n is not negative.

Square size? 4
****
****
****
****

import acm.program.*;

public class StarSquare {
    public void run() {
        int n = readInt("Square size? ");

        for(int i = 0; i < n; i++) {
            for(int j = 0; j < n; j++) {
                print("*");
            }
            println();
        }
    }
}

Problem X1r.16.

Translate the following code fragment into an equivalent fragment without any for loop.

    int sum = 1;
    for(int i = n; i > 1; i = i / 2) {
        sum = sum + i;
    }

    int sum = 1;
    int i = n;
    while(i > 1) {
        sum = sum + i;
        i = i / 2;
    }