# ASSIGNMENT 1 COMP-202 solved

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Part 1 (0 points): Warm-up
Do NOT submit this part, as it will not be graded. However, doing these exercises might help you to do the
second part of the assignment, which will be graded. If you have difficulties with the questions of Part 1, then
we suggest that you consult the TAs during their office hours; they can help you and work with you through
the warm-up questions. You are responsible for knowing all of the material in these questions.
Warm-up Question 1 (0 points)
Create a file called HelloWorld.java, and in this file, declare a class called HelloWorld. This class
should define only one method called main(). In the body of this method, use System.out.println()
to display “Hello world!”. You can find such a class in the lecture slides; make sure you can compile
and run it properly.
Warm-up Question 2 (0 points)
Create a file called A.java, and in this file, declare a class called A. This class should define only one
method called main(). In the body of this method, use System.out.println() to display the following
pattern:
A
A A
AAAAA
A A
A A
Warm-up Question 3 (0 points)
Practice with Binary:
Humans usually operate in base 10, probably because most of us have ten fingers. When operating in
base 10, we think of numbers as having a ones column, a tens column, a 100s column, etc. These are
all the powers of 10.
There is nothing special about 10 though. This can in fact be done with any number. In base 2, we
have each column representing (from right to left) 1,2,4,8,16,etc. In base 3, it would be 1,3,9,27, etc.
1. In base 10, what is the largest digit that you can put in each column? What about base 2? Base
3? Base n?
2. Represent the number seven in base 7.
3. Represent the number seven in base 2.
4. What binary number is equal to the sum of these two binary numbers? 11001101 + 100101010
5. What is the number from the previous part in base 10?
6. What is the binary number for 11010010 – 11000101?
7. And what is the number from the previous part in base 10?
Warm-up Question 4 (0 points)
Background: As discussed in the lectures, there are different types of values in programming languages
such as Java. For instance, we can have integers like the number 5, real numbers like the number 5.1,
boolean values like true and false, characters like ‘m’, or more complex types like Strings e.g. “we all
live in a yellow submarine”. We can compare, contrast, display and apply operators to values of these
different types.
Recall also that we can assign values to variables that are declared with the appropriate type: char
for storing characters, int for integers, double for real numbers, boolean for logical values, and several
others.
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When you declare a variable in Java, you need to specify the type. This involves thinking about a real
world piece of data and figuring out what type you should use.
For each of the following, what type would best represent the data? Are there any that cannot be
represented in Java?
1. The name of a day in the week.
4. The name of the planet that your COMP 202 teacher is from.
5. The temperature outside.
6. The mathematical number π
7. The name of the author who wrote the Hitchhiker’s Guide to the Galaxy?
8. The average number of brain cells in a human.
9. The average number of brain cells of a McGill student after three years of parties.
10. The (estimated) number of molecules in the universe.
11. The result of multiplying two integers together
12. The result of dividing an integer by another integer.
13. ”Is the date today Sept 11?”
14. Does this string ”i am happy” start with the character ’i’?
Warm-up Question 5 (0 points)
Logic
1. What does the following logical expression evaluate to?
(False or False) and (True and (not False))
2. Let a and b be boolean variables. Is it possible to set values for a and b to have the following
expression evaluate as False?
a or (((not b) or (not b)) or (b or (not a)))
Warm-up Question 6 (0 points)
Write a method swap which takes as input two int values x and y. Your method should do 3 things:
1. Print the value of x and y
2. Swap the values of the variables x and y, so that whatever was in x is now in y and whatever was
in y is now in x
3. Print the value of x and y again.
For example, if your method is called as follows: swap(3,4) the effect of calling your method should be
the following printing
inside swap: x is:3 y is:4
inside swap: x is:4 y is:3
Now, create 2 int (integer) variables in the main method. Call them x and y. Assign values to them and
call the swap method you wrote in the previous part.
After calling the swap() method—inside the main method— print the values of x and y. Are they
different than before? Why or why not?
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Warm-up Question 7 (0 points)
Consider the following 2-d matrix:

a b
c d
Write a Java program that first reads 4 doubles, representing a,b,c, and d from the keyboard. It then
outputs to the screen the determinant of the matrix.
For a 2×2 matrix, the determinant is always equal to a × d − b × c
Warm-up Question 8 (0 points)
Now consider the same question except on a 3×3 matrix:

a b c
d e f
g h i

Write a Java program that first reads 9 doubles, representing the 9 letters above from the keyboard. It
then outputs to the screen the determinant of the matrix.
For a 3×3 matrix, the determinant is always equal to
a × (i × e − f × h) − b × (d × i − f × g) + c × (h × d − e × g)
Part 2
The questions in this part of the assignment will be graded.
Question 1: My first (graded) Java program (30 points)
The following should go inside a class called MakingChange and thus a file called MakingChange.java
Write a Java program that takes as input an integer number and outputs the minimum number of coins
that can be used to make change. For the purposes of this question, assume that we have the following
coin denominations available: 1 cent, 5 cents, 10 cents, 25 cents, 1 dollar (100 cents) and 2 dollars (200
cents). We have no bills.
The program should obtain its input from String[] args in the main method. This is a different way
to get input than Scanner. Do not use Scanner on this part. That means that the input will
initially be of type String. The input will be in the first position in a list of Strings. You can access
this String-type list element and convert the it to an integer using the following command:
int money = Integer.parseInt(args[0]);
When making change, if we want to use the smallest possible number of coins, we will do this using
something that in (advanced) computer science courses you will learn is a greedy algorithm. First, use
the largest possible number of 2 dollar coins. Then, for the remaining amount, use the largest possible
number of 1 dollar coins. Then, for the remaining amount, the largest possible number of 25 cent coins,
etc.
For example, if we would like to make change for 1.46\$, we would do the following:
• We can fit zero 2-dollar coins in 146 cents 146 − (0 × 200) = 146.
• We can fit one 1-dollar coin in 146 cents. 146 − (1 × 100) = 46.
• We can fit one 25-cent coin in 46 cents. 46 − (1 × 25) = 21.
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• We can fit two 10-cent coins in 21 cents. 21 − (2 × 10) = 1
• We can fit zero 5-cent coins in 1 cent. 1 − (0 × 5) = 1
• We can fit one 1-cent coin into 1 cent. 1 − 1 × 1 = 0.
So given an input of 146, the program should output:
Change for 146 cents is:
Number of toonies: 0
Number of loonies: 1
Number of quarters: 1
Number of dimes: 2
Number of nickels: 0
Number of pennies: 1
You can assume that the input is valid: it is a non-negative integer number, where the last two digits
are cents and any other digits denote dollars.
Question 2: Using Methods to Understand Leap Years (30 points)
The goal of this question is to have you write several methods that are all very similar in nature in
order to experiment with the different sorts of methods. You should put all of your code into a class
LeapYearCalculator and thus a file LeapYearCalculator.java.
Leap years occur ever 4 years, except when it’s the turn of the century and the year is divisible by 100
(e.g. 2100). However if the year is divisible by 400, it’s suddenly a leap year again. This is because the
Earth’s revolution around the sun is not precisely 365.25 days.
So 1900 is NOT a leap year (although 1900 is divisible by 4, it is also divisible by 100) but 2000 IS a
leap year (although it is divisible by 100, it is also divisible by 400).
2a)Void method that prints something
Write a method printIsLeapYear() that takes as input an int argument representing a year and prints
whether or not the year that is input is a leap year or not. You should include the year as part of the
message. For example, your message could be “1994 is not a leap year” or “2000 is a leap year”. Note that
for full marks, this must be written on one line. (Remember that one way to do this is the + operator.
Another way is to recall the difference between System.out.print() and System.out.println()).
Hint: To test your method, you can write a main method like you did in question one. The
main method will not be graded, but without it, you won’t know whether your method
works or not! Your main method should call this method and verify the results. For
example, your main method could be as follows. You should think of other cases to test!
public class LeapYearCalculator {
public static void main(String[] args) {
printIsLeapYear(1000);
printIsLeapYear(4);
}
public static void printIsLeapYear(int year) {
}
}
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2b)Method that returns something
The previous method that you wrote is not very general. Remember that one of the key ideas of methods
is to write a general piece of code that can be re-used.
What stops it from being general? Well, your method is only useful if you want to print whether
something is a leap year. Suppose you wanted to check whether something was a leap year and take
action based on that? For example, a calendar program may wish to check if it is currently a leap year,
not for the purpose of printing it to the screen, but so that it knows whether to show 29 or 28 days in
February.
We will now make another method inside the same LeapYearCalculator class that is more general.
Write a method isLeapYear inside of LeapYearCalculator that, like the previous method, takes as
input an int representing the year to check, but returns a boolean value of true if it is a leap year and
false if it is not a leap year. Your method should not print anything.
To test your method, you will need to call it from your main method. Think about how you can call the
method and get it to display whether it is a leap year or not.
2c)A method calling another method!
Now that you have a more general leap year method, you can use it as part of a more complicated
operation.
Write a method subsequentLeapYear that takes as input an int representing a year and returns an int
representing the next leap year. If the input year is leap year, do not return the current year, but find
the next leap year and return that one.
For example, if the input to the method is 1996 it should return 2000 since that is the next leap year.
Hint: You may choose to use a loop for this question, but it is not required. One way to do
it is by a series of several if block. (In that case, think about what is the largest possible
gap between leap years?) Another way to do it is by clever use of the modulus operator
to determine the first multiple of 4 after the input number.
Hint 2: Remember that you can and should use your method isLeapYear in this section!
Question 3: DNA strand computations (40 points)
Background: Our DNA is double stranded consisting of the Watson strand and the Crick strand (after
the two co-discoverers of the double-helical structure of DNA). Each strand can be viewed as a string
over a four letter alphabet A, C, G, T. The Watson and Crick strands are paired up. The first letter of
the Watson string matches with the first letter of the Crick string etc. Finally, the Watson and Crick
strands are complementary: when the Watson strand contains an A, the Crick strand contains at T at
that position (and vice versa); when the Watson strand contains a C, the Crick strand contains a G (and
vice versa). This is called the Watson-Crick complement.
For example, if the Watson strand is ”ACCAGACTAG”, then the Crick strand will be ”TGGTCTGATC”.
When analyzing a DNA sequence, the DNA symbols are generally processed in groups of 3 letters.
The Watson-Crick complement of a 3 letter sequence is calculated by taking each individual letter
of the 3 letter sequence and calculating the complement of it. This new 3 letter String can then be
used to figure out a corresponding Amino Acid. For more information on this process, see https:
//en.wikipedia.org/wiki/Genetic_code.
In this question, you should define a Java class called DnaUtilities. As a part of doing that, you will
learn the very valuable skill of exploring the built-in Java libraries so that you do not need to “reinvent
the wheel.”
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Question 3.1 Checking if something is a valid base.
Write a method isValidBase that accepts as input a value of type char and returns a value of type
boolean. The method checks whether the value of the character is either A, C, G, T. So there are 4
possible char values that will return true.
Example invocation and output:
System.out.println(isValidBase(‘A’));//should print true
System.out.println(isValidBase(‘a’)); //should print false
System.out.println(isValidBase(‘X’) ); // should print false
For testing purposes, you should write a main method and call your method with various inputs. The
main method will not be marked by TAs and so you can use it to test out your code thoroughly. The
best way to make sure that your code works is to try really, really hard to break it.
Question 3.2 Finding the Watson Crick complement
Write a method watsonCrickComplement that accepts as input a value of type char and returns a char.
The method should call isValidBase to make sure it is valid. If it is valid, it should find the WatsonCrick complement of the parameter (e.g. A becomes T, T becomes A, C becomes G, G becomes C), and
return it as a char. If it is not valid, return the same character you started with.
1. watsonCrickComplement(‘A’) returns ‘T’
2. watsonCrickComplement(‘t’) returns ‘t’
3. watsonCrickComplement(‘4’) returns ‘4’
To test this question, you may add more code to the main method.
Question 3.3 Finding the Watson Crick complement of a 3 letter String
Write a method watsonCrickTripletComplement that accepts as input a value of type String, called
dnaSequence and returns a String-type value.
The method should do the following things:
1. First, it should verify that dnaSequence has exactly 3 characters in it. (Hint: Use a method from
the String class to accomplish this. See http://docs.oracle.com/javase/7/docs/api/java/
lang/String.html) and examples below.
2. Next verify that each of the 3 characters are valid Watson-Crick complements. For this method,
you should also use the documentation and the hint below.
3. If either of the above two steps fail, the method should return an empty String (e.g. “”).
4. Finally, convert each of the 3 characters to their Watson-Crick complement, and return the String
Here are a few examples of the expected output:
1. watsonCrickTripletComplement(“AGA”) returns TCT
2. watsonCrickTripletComplement(“agA”) returns the empty String.
3. watsonCrickTripletComplement(“AAAA”) returns the empty String.
Hint: There are two methods that you may find very useful for this question. You are not required to
use them, but we recommend doing so.
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• length() : This method will tell you the number of characters in a String.
• charAt(int i) : This method will get the specific character at position i of the string, where i is
an int. Note that the first letter is actually position 0. The 2nd letter is position 1, etc.
String message = “good-bye”;
int length = message.length(); // stores 8, since 8 characters total.
char firstCharacter = message.charAt(0); // stores ’g’, the first character.
char fifthCharacter = message.charAt(2 * 2); // stores ’-’, the 5th character.
char outOfRange = message.charAt(100); // results in a run time error, program crashes.
What To Submit
You have to submit one zip file with all your files in it to MyCourses under Assignment 1. If you do not