# CSCI A201/A597 and I210 Homework One Solutions

Second semester 2000-2001

Here are example of solutions to the problems in this first set.

 1. Write a program that displays the squares, cubes, and fourth powers of the numbers 1-5. Here's a sample run of such program: ```frilled.cs.indiana.edu%java One First five powers of 1: 1 1 1 1 1 First five powers of 2: 2 4 8 16 32 First five powers of 3: 3 9 27 81 243 First five powers of 4: 4 16 64 256 1024 First five powers of 5: 5 25 125 625 3125 frilled.cs.indiana.edu%```

```/* This is the solution to the first problem of the first problem
set: it displays the first five powers of the first five numbers. This
is a bit of an overkill, but that's only because the problem is very
simple. Notice then that the third, fourth, and fifth powers are being
computed from previously calculated values. */

public class One {

public static void main(String[] args) {

int number = 1;
int square = number * number;
int fourthPower = square * square;

System.out.print("First five powers of " + number + ": ");

System.out.println( number              + " " +
square              + " " +
number * square     + " " +
fourthPower         + " " +
number * fourthPower );
// done with 1.

number += 1;
square = number * number;
fourthPower = square * square;

System.out.print("First five powers of " + number + ": ");

System.out.println( number              + " " +
square              + " " +
number * square     + " " +
fourthPower         + " " +
number * fourthPower );
// done with 2.

number += 1;
square = number * number;
fourthPower = square * square;

System.out.print("First five powers of " + number + ": ");

System.out.println( number              + " " +
square              + " " +
number * square     + " " +
fourthPower         + " " +
number * fourthPower );
// done with 3.

number += 1;
square = number * number;
fourthPower = square * square;

System.out.print("First five powers of " + number + ": ");

System.out.println( number              + " " +
square              + " " +
number * square     + " " +
fourthPower         + " " +
number * fourthPower );
// done with 4

number += 1;
square = number * number;
fourthPower = square * square;

System.out.print("First five powers of " + number + ": ");

System.out.println( number              + " " +
square              + " " +
number * square     + " " +
fourthPower         + " " +
number * fourthPower );
// done with 5

// -- finished

}
}
```

 2. Write a program that prompts the user for two integers and then prints The sum The difference The product The average The distance (absolute value of the difference) The maximum (the larger of the two ) The minimum (the smaller of the two) Here's a sample run with your program: ```frilled.cs.indiana.edu%java Two Please enter your first integer number, then press Enter. 3 Please enter your second integer number, then press Enter. 6 3 + 6 = 9 3 - 6 = -3 3 * 6 = 18 avg(3, 6) = 4.5 dist(3, 6) = 3 max(3, 6) = 6 min(3, 6) = 3 frilled.cs.indiana.edu%```
```/* This is the solution to the second problem in the first problem set.
Note that you need ConsoleReader, which is available from the second set
of lab notes (Lab Two) off the class notes page or from the book. Once you
get the Consolereader class off the web notes place it in a file of its
own, with the name ConsoleReader.java in the same directory with Two.java
whose source code is written below, then compile Two.java. For more details
see http://www.cs.indiana.edu/classes/a201/spr2001/labs/Two.html */

public class Two {
public static void main(String[] args) {

System.out.println("then press Enter.");

System.out.println("then press Enter.");

int sum = n1 + n2;
System.out.println(n1 + " + " + n2 + " = " + sum);

int diff = n1 - n2;
System.out.println(n1 + " - " + n2 + " = " + diff);

int prod = n1 * n2;
System.out.println(n1 + " * " + n2 + " = " + prod);

double avg = sum / 2.0;
System.out.println("avg(" + n1 + ", " + n2 + ") = " + avg);

int dist = Math.abs(n1 - n2);
System.out.println("dist(" + n1 + ", " + n2 + ") = " + dist);

long max = Math.round(avg + dist / 2.0);
// neat trick; can you explain it?

System.out.println("max(" + n1 + ", " + n2 + ") = " + max);

long min = Math.round(avg - dist / 2.0);
// neat trick; can you explain it?

System.out.println("min(" + n1 + ", " + n2 + ") = " + min);

}
}```

 3. Write a program that prompts the user for a measurement in meters and then converts it into miles, feet and inches. Here's a sample run with your program: ```frilled.cs.indiana.edu%java Three Please enter the measurement in meters: 100 Your original measurement of 100.0 meters has been converted. 0 miles, 328 feet, and 1.0078719999998889 inches. frilled.cs.indiana.edu%java Three Please enter the measurement in meters: 16000 Your original measurement of 16000.0 meters has been converted. 9 miles, 4983 feet, and 7.1495919999956445 inches. frilled.cs.indiana.edu%```
```/* This is the solution to the third problem on the first problem set. It
also needs the ConsoleReader class which can be taken from lab notes 2 (see
problem 2 above for more details). This is not a hard problem the difficulty
(if any) is *only* in the conversion formulas. I decided to use three named
constants to avoid magic numbers and I express miles and feet as integers,
while the number of inches is reported as a double. */

public class Three {
public static void main(String[] args) {
// create a console reader for user input, call it console

System.out.println("Please enter the measurement in meters: ");

// get the measurement in meters from the user

// define three constants needed in the conversion
final double METERS_PER_MILE = 1609;
final double METERS_PER_FOOT = 0.3048;
final double METERS_PER_INCH = 1/39.37;

// first compute the number of miles
int miles = (int) (measurement / METERS_PER_MILE);

// take those miles away from the measurement
// first make a copy of the original value of the measurement
double original = measurement;
// then take the miles away
measurement -= miles * METERS_PER_MILE;

// then compute the number of feet from the remaining distance
int feet = (int) (measurement / METERS_PER_FOOT);

// take the feet away from the measurement
measurement -= feet * METERS_PER_FOOT;

// then compute the number of inches in the remaining distance
double inches = (measurement / METERS_PER_INCH);

// print title of report
System.out.print(original);
System.out.print(" meters has been converted.\n");

// print conversion results
System.out.println("  " + miles + " miles,");
System.out.println("  " + feet + " feet, and");
System.out.println("  " + inches + " inches.");

}
} ```

 4. Write a program that prompts the user for a radius and then prints The area and circumference of the circle with that radius The volume and surface area of the sphere with that radius Here's a sample run with your program: ```whitetip.cs.indiana.edu%java Four Please enter value for radius then hit enter. 100 Thank you. The radius is 100.0 Here are the computed values. Area of the circle: 31415.926535897932 Circumference: 628.3185307179587 Area of a sphere: 125663.70614359173 Volume of sphere: 41887.90204786391 whitetip.cs.indiana.edu%java Four Please enter value for radius then hit enter. 1 Thank you. The radius is 1.0 Here are the computed values. Area of the circle: 3.141592653589793 Circumference: 6.283185307179586 Area of a sphere: 12.566370614359172 Volume of sphere: 4.1887902047863905 whitetip.cs.indiana.edu%```
```/* This is the solution to problem four in first problem set. Note
that just as in problem 2 you need to have the ConsoleReader class in
a file in the same directory before compiling this class */

public class Four {
public static void main(String[] args) {

// get a connection to the keyboard

// get the value

// echo value to user

// apply formulas
double circumferenceCircle  = 2 * Math.PI * radius;
double volumeSphere         = 4 * Math.PI * radius * radius / 3;

// as you can see I have an error here in the volume formula!

// report the computed values
System.out.println("Here are the computed values.");

System.out.println("Area of the circle: " + areaCircle);
System.out.println("Circumference: "      + circumferenceCircle);
System.out.println("Area of a sphere: "   + areaSphere);
System.out.println("Volume of sphere: "   + volumeSphere);

}
}```

 5. Write a program that asks the user for the lengths of the sides of a rectangle. Then print The area and perimeter of the rectangle The length of the diagonal (use the Pythagorean theorem) Here's a sample run with your program: ```whitetip.cs.indiana.edu%java Five Please enter the value for the first side. 3 Thanks. Side one is 3.0 Please enter the value for the second side. 4 Thanks. Side two is 3.0 Area is: 12.0 Perimeter is: 14.0 Diagonal is: 5.0 whitetip.cs.indiana.edu%java Five Please enter the value for the first side. 1 Thanks. Side one is 1.0 Please enter the value for the second side. 1 Thanks. Side two is 1.0 Area is: 1.0 Perimeter is: 4.0 Diagonal is: 1.4142135623730951 whitetip.cs.indiana.edu%```
```/* This is the solution for the fifth problem in the first set. Like in
problem 2 you need to have ConsoleReader.java in the same directory before
compiling this class (Five.java). The problem gets the values from the user
and then applies formulas and reports the computed values. */

public class Five {
public static void main(String[] args) {

// get a connection to the keyboard

// greet the user, ask for first value
System.out.println("Please enter the value for the first side.");

// echo value to user
System.out.println("Thanks. Side one is " + side1);

System.out.println("Please enter the value for the second side.");

// echo second value
System.out.println("Thanks. Side two is " + side1); // side2!!

// apply formulas, compute required quantities
double area = side1 * side2;
double perimeter = (side1 + side2 ) * 2;
double diagonal = Math.sqrt(side1 * side1 + side2 * side2);

// report all computed value
System.out.println("Area is: " + area +
"\nPerimeter is: " + perimeter +
"\nDiagonal is: " + diagonal);

}
}```

 6. (Giving change) Implement a program that directs a cashier how to give change. The program has two inputs: the amount due and the amount received from the customer. Compute the difference, and compute the dollars, quarters, dimes, nickels, and pennies that the customer should receive in return. Hint: First transform the difference into an integer balance, denominated in pennies. Then compute the whole dollar amount. Subtract it from the balance. Compute the number of quarters needed. Repeat for dimes and nickels. Display the remaining pennies. Here's a sample run with your program: ```frilled.cs.indiana.edu%java Six Type the amount due then press enter. 3.72 Type the amount received then press enter. 5 Give 1.28 in change as follows: 5 quarters 0 dimes 0 nickels 3 cents frilled.cs.indiana.edu%java Six Type the amount due then press enter. 0.08 Type the amount received then press enter. 0.5 Give 0.42 in change as follows: 1 quarters 1 dimes 1 nickels 2 cents frilled.cs.indiana.edu%```
```/* This is the solution to problem six in first problem set. ConsoleReader is
needed, use it as described in problem 2. This is a typical problem in which you
carefully need to cover roundoff errors, so use Math.round after you transform
the problem in whole units (cents). */

public class Six {
public static void main(String[] args) {

// get a connection to the keyboard

// ask the user for the amount due
System.out.println("Type the amount due then press enter.");

System.out.println("Type the amount received then press enter.");

// assume received is bigger than due and compute difference
double difference = (received - due);
// you need to return this as change so make it a whole number
// of cents regardless of how many decimals the user has entered
int diff = (int)(Math.round(difference * 100));

// tell the user what change you are processing
System.out.println("Give " + diff / 100.00 + " in change as follows: ");

// number of quarters; integer division
int quarters = diff / 25;
// report it
System.out.println("   " + quarters + " quarters");

// adjust the remaining change (modulo)
diff = diff % 25;

// compute the number of dimes
int dimes = diff / 10;
// report it
System.out.println("   " + dimes + " dimes");

// adjust remaining cents (notice shortcut operator)
diff %= 10; // notice anything compared to the previous assignment?

// these are the cents
int cents = diff;
// report them too
System.out.println("   " + cents + " cents");

}
}```

 7. Write a program that asks the user to input The number of gallons The fuel efficiency The price Then print how far the car can go with the gas in the tank and print the cost per 100 miles. Here's a sample run with your program: ```frilled.cs.indiana.edu%java Seven Please enter the number of gallons then press enter. 32 Please enter the fuel efficiency (miles/gallon) then press enter. 16 Please enter the price per gallon, then press enter. 1.54 With the gas in the tank you can go 512.0 miles, at a cost of 9.625 per 100 miles. frilled.cs.indiana.edu%java Seven Please enter the number of gallons then press enter. 2.8 Please enter the fuel efficiency (miles/gallon) then press enter. 18.5 Please enter the price per gallon, then press enter. 1.48 With the gas in the tank you can go 51.8 miles, at a cost of 8.0 per 100 miles. frilled.cs.indiana.edu%```
```/* This is the solution to problem seven in the first problem set.
Need ConsoleReader in the same directory before you can compile it,
just like in problem 2. Formulas used are straightforward, easy. */

public class Seven {
public static void main(String[] args) {

// get a connection to the keyboard

// greet the user, ask for number of gallons
System.out.println("Please enter the number of gallons then press enter.");

System.out.println("Please enter the fuel efficiency " +
"(miles/gallon) then press enter. ");

// ask for price per gallon
System.out.println("Please enter the price per gallon, then press enter.");

// computer how far the user can go with that much gas
double howFar = efficiency * gallons;

// compute how much that will cost
double totalCost = price * gallons;

// divide this by howFar and multiply by 100 to get price per 100 miles
double pricePer100Miles = totalCost / howFar * 100;

System.out.println("With the gas in the tank you can go " +
howFar + " miles, \nat a cost of "     +
pricePer100Miles + " per 100 miles.");

}
}```

 8. Write a program that reads a number greater than or equal to 1000 from the user and prints it out with a comma separating the thousands Here's a sample run with your program: ```frilled.cs.indiana.edu%java Eight Please enter an integer >= 1000: 2000 2,000 frilled.cs.indiana.edu%java Eight Please enter an integer >= 1000: 2000000 2000,000 frilled.cs.indiana.edu%java Eight Please enter an integer >= 1000: 2000000000 2000000,000 frilled.cs.indiana.edu%java Eight Please enter an integer >= 1000: -3000 -3,000 frilled.cs.indiana.edu%java Eight Please enter an integer >= 1000: 20 Exception in thread "main" [...] ```
```/* This is the solution to problem eight in the first set of problems. You
need ConsoleReader in the same directory before compiling, as detailed at
problem 2. Essentially you read a number as string, and write it back as
required, with a comma separating the thousands (the last three digits). As
before, no error checking is done, user is assumed to be program-friendly. */

public class Eight {
public static void main(String[] args) {

// get a connection to the keyboard

// ask the user to enter a number
System.out.print("Please enter an integer >= 1000: ");

// read the number, assume the user types the right thing

// report the number with the comma separating the thousands
System.out.println(// first write digits up to thousands
number.substring(0, number.length() - 3) +
"," + // then write the comma
// and the remaining part (the thousands)
number.substring(number.length()-3));
}
}```

 9. Write a program that reads a number greater than or equal to 1000 from the user, where the user enters a comma in the input. Then print the number without a comma. Hint: Read the input as a string. Measure the length of the string. Suppose it contains n characters. Then extract substrings consisting of the first n - 4 characters and the last three characters. Here's a sample run with your program: ```frilled.cs.indiana.edu%java Nine Please enter an integer between 1,000 and 999,999: 123,456 123456 frilled.cs.indiana.edu%java Nine Please enter an integer between 1,000 and 999,999: 1,000 1000 frilled.cs.indiana.edu%java Nine Please enter an integer between 1,000 and 999,999: 1000000,000 1000000000 frilled.cs.indiana.edu%```
```/* This is the solution to the nineth problem in the first problem
set. You need ConsoleReader in the same directory, as explained in
problem 2, before you can compile this program. This program assumes
that the user types in a number that contains a comma separating the
thousands from the rest of the number. */

public class Nine {
public static void main(String[] args) {

// get a connection to the keyboard

// ask the user for input
System.out.print("Please enter an integer between 1,000 and 999,999: ");

// get the first part (no comma or thousands)
String firstPart = number.substring(0, number.length() - 4);
// get last three digits
String lastThreeDigits = number.substring(number.length() - 3);

// print the two without the comma
System.out.println(firstPart + lastThreeDigits);

}
}```

 10. (Printing a grid) Write a program that prints the following grid to play tic-tac-toe. ```+--+--+--+ | | | | +--+--+--+ | | | | +--+--+--+ | | | | +--+--+--+``` Of course, you could simply write seven statements of the form `System.out.println("+--+--+--+")` You should do it a smarter way, though. Define string variables to hold two kinds of patterns: a comb-shaped pattern and the bottom line. Print the comb three times and the bottom line once. Here's a sample run with your program: ```frilled.cs.indiana.edu%java Ten +--+--+--+ | | | | +--+--+--+ | | | | +--+--+--+ | | | | +--+--+--+ frilled.cs.indiana.edu%```
```/* This is the solution to the tenth problem in the first problem
set. Given the hint it's straightforward. We define two patterns and we
print them accordingly to obtain the desired output. */

public class Ten {
public static void main(String[] args) {

// first pattern, comb-shaped
String comb   = "+--+--+--+\n" +
"|  |  |  |\n";

// second pattern, bottom line
String bottom = "+--+--+--+\n";

// print the grid for the user
System.out.println(comb + comb + comb + bottom);

}
}```

 11. Write a program that reads an integer and breaks it into a sequence of individual digits. For example the input 16384 is displayed as `1 6 3 8 4` You may assume that the input has no more than five digits and is not negative. Hint: There are two ways of solving this problem. You can use integer arithmetic and repeatedly divide by 10, or you can convert the number into a string and extract the digits from the string. Here's a sample run with your program: ```frilled.cs.indiana.edu%java Eleven Please enter a number between 0 and 99999: 123 1 2 3 frilled.cs.indiana.edu%java Eleven Please enter a number between 0 and 99999: 0 0 frilled.cs.indiana.edu%java Eleven Please enter a number between 0 and 99999: 83021 8 3 0 2 1 frilled.cs.indiana.edu%java Eleven Please enter a number between 0 and 99999: 123456 2 3 4 5 6 frilled.cs.indiana.edu%```
```/* This is the solution to problem eleven of the first problem set. You
read from the user a string of digits and then make sure the resulting
string is at least 5 characters long. Once you have that, just print the
characters one by one separated by blank spaces. We just print the last
five, and we know we have at least five characters in the string because
we have taken care to pad it with a string of five blanks from the start */

public class Eleven {
public static void main(String[] args) {

// open a connection with the keyboard

// tell the user to enter a number
System.out.print("Please enter a number between 0 and 99999: ");
// read the number the user types

// pad the number with five spaces
number = "     " + number; // thus string has at least 5 characters

int i = number.length() - 1; // index of last char in string

// print the last five digits of the number
// note the order in which we print the chars and their indices
System.out.print(number.substring(i-4, i-3) + " ");
System.out.print(number.substring(i-3, i-2) + " ");
System.out.print(number.substring(i-2, i-1) + " ");
System.out.print(number.substring(i-1, i)   + " ");
System.out.print(number.substring(i)        + " \n");
}
}```

 12 The following program prints the values of sine and cosine for 0 degrees, 30 degrees, 45 degrees, 60 degrees, and 90 degrees. Rewrite the program for greater clarity by factoring out common code. ```public class Twelve { public static void main(String[] args) { System.out.println("0 degrees: " + Math.sin(0) + " " + Math.cos(0)); System.out.println("30 degrees: " + Math.sin(30 * Math.PI / 180) + " " + Math.cos(30 * Math.PI / 180)); System.out.println("45 degrees: " + Math.sin(45 * Math.PI / 180) + " " + Math.cos(45 * Math.PI / 180)); System.out.println("60 degrees: " + Math.sin(60 * Math.PI / 180) + " " + Math.cos(60 * Math.PI / 180)); System.out.println("90 degrees: " + Math.sin(90 * Math.PI / 180) + " " + Math.cos(90 * Math.PI / 180)); } } ```
```/* This is the solution to problem twelve in the first problem set.
We first compute the constant for the conversion from degrees to radians
and then we repeatedly use the value of this constant, stored as x.  */

public class Twelve {
public static void main(String[] args) {

double x = Math.PI / 180; // degreesToRadians constant

System.out.println
(" 0 degrees: " + Math.sin( 0    ) + " " + Math.cos( 0    ));

System.out.println
("30 degrees: " + Math.sin(30 * x) + " " + Math.cos(30 * x));

System.out.println
("45 degrees: " + Math.sin(45 * x) + " " + Math.cos(45 * x));

System.out.println
("60 degrees: " + Math.sin(60 * x) + " " + Math.cos(60 * x));

System.out.println
("90 degrees: " + Math.sin(90 * x) + " " + Math.cos(90 * x));

}
}
```

 13. Write a program that prints out a message "Hello, my name is Hal!" Then, on a new line, the program should print the message "What is your name?" Next the program should read the user's name and print "Hello, user name. I am glad to meet you." Then, on a new line, the program should print a message "What would you like me to do?" Then it is the user's turn to type in an input. Finally the program should ignore the user input and and print the message "I am sorry, user name. I cannot do that." Here's a sample run with your program: ```frilled.cs.indiana.edu%java Thirteen Hello, my name is Hal! What is your name? Dave Hello, Dave. I am glad to meet you. What would you like me to do? Clean up my room, if you'd like. I am sorry, Dave. I cannot do that. frilled.cs.indiana.edu%java Thirteen Hello, my name is Hal! What is your name? Dave Hello, Dave. I am glad to meet you. What would you like me to do? Get lost. I am sorry, Dave. I cannot do that. frilled.cs.indiana.edu%```
```/* This is the solution to problem thirteen in the first problem set.
You need ConsoleReader in the same directory, as explained in problem two,
before you can compile and run this program. */

public class Thirteen {
public static void main(String[] args) {

// open a keyboard connection

// greet the user and ask for input
System.out.println("Hello, my name is Hal!");

// get user input store it in String variable called name

// echo the data to the user and ask for request
System.out.println("Hello, " + name + ". I am glad to meet you.");
System.out.println("What would you like me to do?");

// get the request

// end the conversation politely
System.out.println("I am sorry, " + name + ". I cannot do that.");

}
}```

 14. This is somewhat tricky so there are lots of hints in the text. You don't know yet how to program decisions, but it turns out that there is a way to fake them using `substring`. Write a program that asks a user to input The number of gallons of gas in the tank The fuel efficiency in miles per gallon The distance the user wants to travel Then print out `You will make it` or `You will not make it` The trick here is to subtract the desired distance from the number of miles the user can drive. Suppose that the number is `x`. Suppose further that you find a way of setting a value `n` to 1 if `x` >= 0 and to 0 if `x` < 0. Then you can solve your problem: ```String answer = " not "; // note the spaces before and after not System.out.println( "You will" + answer.substring(0, 5 - 4 * n) + // sometimes not! "make it"); ``` Hint: Note that `x + |x|` is 2`x` if `x` >= 0, and 0 if `x` < 0. Then divide by `x`, except that you need to worry about the possibility that `x` is zero so use: ```// compute x, the difference between what you can and what you want double x = efficiency * gallons - distance; // avoid division by zero with this, can you see how? double epsilon = 0.00001; // make n 1 if x >= 0 and 0 otherwise long n = Math.round( ( (x + Math.abs(x) ) * x + epsilon) / ( 2 * x * x + epsilon) ); // why can't n be int? what do I need to do if I want it to be an int? // report the correct answer System.out.println("You will" + answer.substring(0, 5 - 4 * (int)n) + "make it."); // why do I need to convert n from the long it is to an int here? ``` Here's a sample run with your program: ```frilled.cs.indiana.edu%java Fourteen Please enter number of gallons then press enter. 10 Please enter fuel efficiency in miles per gallon. 20 Please enter distance in miles you want to cover. 199 You will make it. frilled.cs.indiana.edu%java Fourteen Please enter number of gallons then press enter. 5 Please enter fuel efficiency in miles per gallon. 20 Please enter distance in miles you want to cover. 101 You will not make it. frilled.cs.indiana.edu%```
```/* This is the solution to problem fourteen in the first problem set. You
need ConsoleReader in the same directory, as explained in problem two, to
compile and run this program. Note that the hint in the text is almost all
you need, minus one trick that makes sure that we can never divide by zero
and also produces an n of 1 when x is zero (avoiding division by zero too). */

public class Fourteen {
public static void main(String[] args) {

// get a connection to the keyboard

// greet user, ask for input
System.out.println("Please enter number of gallons then press enter.");
// get number of gallons

System.out.println("Please enter fuel efficiency in miles per gallon.");
// get it from user

System.out.println("Please enter distance in miles you want to cover.");
// get distance

String answer = " not ";

// compute x, the difference between what you can and what you want
double x = efficiency * gallons - distance;

// avoid division by zero with this, can you see how?
double epsilon = 0.00001;
// make n 1 if x >= 0 and 0 otherwise
long n = Math.round( ( (x + Math.abs(x) ) * x + epsilon) /
(              2 * x * x + epsilon)
);
// why can't n be int? what do I need to do if I want it to be an int?

System.out.println("You will" +
answer.substring(0, 5 - 4 * (int)n) +
"make it.");
// why do I need to convert n from the long it is to an int here?

}
}```

 15. Write a program that reads two times in military format (0900, 1730) and prints the number of hours and minutes between the two times. Here is a sample run. User input is in color. ```Please enter the first time: 0900 Please enter the second time: 1730 8 hours 30 minutes``` Extra credit if you can deal with the case that the first time is later than the second time: ```Please enter the first time: 1730 Please enter the second time: 0900 15 hours 30 minutes``` Here's a sample run with your program: ```frilled.cs.indiana.edu%java Fifteen Please enter the first time: 0920 Please enter the second time: 1025 1 hours 5 minutes frilled.cs.indiana.edu%java Fifteen Please enter the first time: 1025 Please enter the second time: 0920 22 hours 55 minutes frilled.cs.indiana.edu%```
```/* This is the solution to problem fifteen in the first problem set.
You need to have ConsoleReader in a file in the same directory before
you can compile and run this. This problem adds 24 hours to the hours
difference to avoid negative values and takes the remainder with 24
to avoid values bigger than 24 (number of hours in a day). It also
transforms the hours into minutes to absorb possible negative values
from the difference between given minutes. */

public class Fifteen {
public static void main(String[] args) {

// get a connection to the keyboard

// greet the user, ask for time
System.out.print("Please enter the first time: ");
// get the first time as a string (in military format)

// compute the number of hours from first two characters
int hours1 = Integer.parseInt(first.substring(0, 2));
// compute minutes from next two characters
int minutes1 = Integer.parseInt(first.substring(2, 4));

System.out.print("Please enter the second time: ");
// get it

// compute hours, same as before
int hours2 = Integer.parseInt(second.substring(0, 2));
// compute minutes
int minutes2 = Integer.parseInt(second.substring(2, 4));

// compute the difference between hours, add 24 then divide
// by 24 and take the remainder to express the hour difference
int difHours  = (hours2 + 24 - hours1) % 24;
// make these hours into minutes
int hrsToMins = difHours * 60;
// compute straight difference between given minutes, could be < 0
int difMins   = minutes2 - minutes1;
// add this to total count of minutes
hrsToMins    += difMins;

// report total count of minutes in hours and minutes
System.out.println(
hrsToMins / 60 + " hours " + hrsToMins % 60 + " minutes"
);

}
}
```

 16 Run the following program, and explain the output you get. ```public class Sixteen { public static void main(String[] args) { ConsoleReader console = new ConsoleReader(System.in); int total = 0; System.out.println("Please enter a positive number:"); int x1 = Integer.parseInt(console.readLine()); System.out.println("total = " + total); total = total + 1 / x1; System.out.println("total = " + total); System.out.println("Please enter a positive number:"); int x2 = Integer.parseInt(console.readLine()); total = total + 1 / x2; System.out.println("total = " + total); total = total * x1 * x2 / 2; System.out.println("total = " + total); System.out.println("The average is " + total); } }``` Note the trace messages, which are inserted to show the current contents of the `total` variable. Then fix up the program, run it with the trace messages in place to verify that it works correctly, and remove the trace messages.
```/* This is the solution to problem number sixteen in the first problem
set. The program attempts to use a different way of computing the average
of two numbers a and b by computing (1/a + 1/b)*a*b/2 in stages. It is OK
if the numbers entered are restricted to integers, but to correctly compute
at all stages in the program we need to circumvent integer division which
can result in a loss of precision. So we turn everything in float and all
is well after that. You need ConsoleReader as explained in problem 2. */

public class Sixteen {
public static void main(String[] args) {
// one change below
float total = 0;
// second change below
System.out.println("total = " + total);
total = total + 1 / x1;
System.out.println("total = " + total);
// third change below
total = total + 1 / x2;
System.out.println("total = " + total);
total = total * x1 * x2 / 2;
System.out.println("total = " + total);
System.out.println("The average is " + total);
// that's it, three changes overall
}
}```

 17. Writing large letters. A large letter `H` can be produced like this: ```* * * * ***** * * * *``` It can be declared as a string constant like this: ```public static final String LETTER_H = "* *\n* *\n*****\n* *\n* *\n";``` Do the same for the letters `E`, `L`, and `O`. Then write the message ```H E L L O``` Here's a sample run with your program: ```frilled.cs.indiana.edu%java Seventeen * * * * ***** * * * * ***** * ***** * ***** * * * * ***** * * * * ***** *** * * * * * * *** frilled.cs.indiana.edu%```
```/* This is the solution to problem seventeen in the first set of problems.

The text gives a suggestion that is enough to solve the problem, although
in this particular case you can't do it directly as suggested. But reading
the section on constants in the book you can come up with the following
absolutely obvious solution: */

public class Seventeen {
public static void main(String[] args) {

// define the letters as strings
final String LETTER_H = "*   *\n*   *\n*****\n*   *\n*   *\n\n";
final String LETTER_E = "*****\n*    \n*****\n*    \n*****\n\n";
final String LETTER_L = "*    \n*    \n*    \n*    \n*****\n\n";
final String LETTER_O = " *** \n*   *\n*   *\n*   *\n *** \n\n";

// print them one after another
System.out.println(
LETTER_H + LETTER_E + LETTER_L + LETTER_L + LETTER_O
);

}
}```

 18. Write a program that transforms numbers `1`, `2`, `3`, ..., `12` into the corresponding month names `January`, `February`, `March`, ..., `December` Hint: Make a very long string `"January February March......."` in which you add spaces such that each month name has the same length. Then use `substring` to extract the month you want. Here's a sample run with your program: ```frilled.cs.indiana.edu%java Eighteen Please enter a month number from 1 to 12. 2 February frilled.cs.indiana.edu%java Eighteen Please enter a month number from 1 to 12. 12 December frilled.cs.indiana.edu%java Eighteen Please enter a month number from 1 to 12. 1 January frilled.cs.indiana.edu%java Eighteen Please enter a month number from 1 to 12. 14 Exception in thread "main" [...] frilled.cs.indiana.edu%```
```/* Solution to problem eighteen in the first problem set. Use ConsoleReader
from lab notes as explained in problem set 2. The trick here (as hinted in the
text) is to transform a number for a month in the position in the string where
the month name is starting, all names being made of the same length, and then
concatenated together in one final string. */

public class Eighteen {
public static void main(String[] args) {

String monthNames = "January   " +
"February  " +
"March     " +
"April     " +
"May       " +
"June      " +
"July      " +
"August    " +
"September " + // longest
"October   " +
"November  " +
"December  "    ;

// open a connection with the keyboard

// greet the user, and ask for input
System.out.println("Please enter a month number from 1 to 12.");
// get month name
// report the name of the month
System.out.println(
monthNames.substring("September ".length() * (month-1),
"September ".length()*month));

// formula uses the length of the longest name
}
}```

 19. Change the password program (textbook, page 80) to make it generate more secure passwords. Use the random numner generator `Random` in the `java.util` package to generate a random number as follows: `int r = new Random().nextInt(1000);` Multiply the age by the random number. Then concatenate the initials with the last four digits of the product. Here's a sample run with your program: ```frilled.cs.indiana.edu%java Nineteen Your password is: WJC0068 frilled.cs.indiana.edu%java Nineteen Your password is: WJC9494 frilled.cs.indiana.edu%java Nineteen Your password is: WJC2042 frilled.cs.indiana.edu%```
```/* This is solution to problem nineteen in the first set. We start from the
MakePassword problem on page 79 in the text. Following the hint the rest
becomes straightforward, including how you get the last four digits of the
product between the random number and the age, to concatenate to initials.
Note that you need to import the Random class from the java.util package. */

import java.util.Random; // need to import this to work with random numbers

public class Nineteen {
public static void main(String[] args) {
String firstName  = "William";
String middleName = "Jefferson";
String lastName    = "Clinton";

// extract initials

String initials =
firstName.substring (0, 1) +
middleName.substring(0, 1) +
lastName.substring  (0, 1);

// append age

int age = 54;

int r = new Random().nextInt(1000);

// multiply age by random number

String product = (age * r) + "";

// concatenate initials with last four digits of product

String password = initials + product.substring(product.length() - 4);