Algorithms and OOD (CSC 207 2014F) : Assignments

Assignment 4: Playful Parsing in Java

Due: 10:30 p.m., Tuesday, 16 September 2014

Summary: In this assignment, you will explore strings and the parsing of strings. Along the way, you will consider the design and implementation of some simple classes, some issues in numeric computation, and the use of arrays in Java.

Purposes: To help you think about types in Java. To give you experience reading the Java API. To improve your skills at string processing (in Java and beyond). To continue exploring the relationships between C and Java.

Collaboration: You must do this assignment in groups of two, three, or four students. You may form a group yourself or you may ask me to randomly assign you to a group. If you form your group yourself, you may not choose partners who you worked with on the previous assignment. You may discuss this assignment with anyone, provided you credit such discussions when you submit the assignment.

Wrapper (Prologue): Individually read through this assignment and make sure that you understand what is required. Then use the form available at to indicate (a) how long you think this assignment will take and (b) what you think will be the most challenging aspect of this assignment.

Wrapper (Epilogue): When you are done with the assignment, fill out the form available at to indicate (a) how long the assignment took, (b) what the most challenging part of the assignment was, and (c) something important you learned from doing the assignment. If you find that the assignment took much less or much more time than you expected, also include (d) a note as to what might have led to that difference.

Submitting: Please put all of your work in a GitHub repository named csc207-hw4. Email the address of that repository to Please use a subject of “CSC207 2014F Assignment 4 (Your Name)”.

Warning: So that this assignment is a learning experience for everyone, we may spend class time publicly critiquing your work.


Since we'll be testing array procedures in this lab, you may find the assertArrayEquals is helpful. You'll find that the assertArrayEquals has the expected parameters. You can read more at


a. Create a new Eclipse project for this assignment. You can name the project whatever you like, provided it's not in bad taste.

b. Create a new package in the project. You may choose the name (again, provided it's in good taste). I would suggest edu.grinnell.csc207.username.utils.

c. Copy the Fraction class from a recent lab into your project. Make sure to add a note to the top of the class citing the original authors of the class and the peers who helped you update the class.

d. Create two utility classes, and


Part A: Fixing Fractions

Your fraction class will need the following constructors and methods, many of which you may have created in a previous laboratory. Add any that it does not yet include.

  • A Fraction(String str) constuctor. Your constructor should accept all reasonable strings that describe fractions, including strings that do and do not include a slash (e.g., new Fraction("1/2") should create the fraction 1/2 and new Fraction("17") should create the fraction 17/1). You need not handle strings that represent decimal values. (You are aslo free to handle such strings, if you'd like.)
  • A negate() method that returns a new fraction which is the additive inverse of the original fraction.
  • A multiply(Fraction multiplicand) method that returns the new fraction that results of multiplying the fraction by the multiplicand.
  • A subtract(Fraction subtrahend) method that returns the new fraction that results from subtracting the subtrahend from the fraction.
  • A divide(Fraction divisor) method that returns the new fraction that results from dividing the fraction by the divisor.
  • A pow(int expt) method that returns the new fraction that results from raising the given fraction to the given exponent, which may be positive, negative, or zero.

Part B: Further Fixing Fractions

Suppose we decide that fractions are always represented in simplest form. That is, the numerator and denominator have no common fractors, the denominator is always positive, and if the numerator is 0, then the denominator is 1.

Update the Fraction class so that it follows this policy.

Part C: Segmenting Strings

Programmers often find it convenient to store compound data in a text file with one line per entry. To separate the components of the entry, they use some designated symbol, such as a colon. For example, we might store movie ratings in a form like the following:


Write and test a procedure, StringUtils.splitAt, that takes as input a string and a character to use in splitting the string and returns an array of the appropriate substrings. For example,

assertArrayEquals(new String[] { "a", "b", "c" },
                  StringUtils.splitAt("a:b:c", ':'));
assertArrayEquals(new String[] { "a", "b", "c" },
                  StringUtils.splitAt("a b c", ' '));
assertArrayEquals(new String[] { "a:b:c" },
                  StringUtils.splitAt("a:b:c", ' '));
assertArrayEquals("one field", new String[] { "a" },
                  StringUtils.splitAt("a", ':'));
assertArrayEquals("empty inner field", new String[] { "a", "", "c" },
                  StringUtils.splitAt("a::c", ':'));
assertArrayEquals("leading empty field", new String[] { "", "a" },
                  StringUtils.splitAt(":a", ':'));
assertArrayEquals("trailing empty field", new String[] { "a", "" },
                  StringUtils.splitAt("a:", ':'));

In implementing splitAt you may not use use the split method of java.util.String. (Yes, that's right, you have to implement a simpler version of that method by hand. Don't worry, it's good experience.)

Part D: Segmenting Strings, Supplemented

One disadvantage of the “split at this character” strategy is that you cannot easily use the separator within one of the fields. Data designers have chosen a variety of approaches to handle the issue. One of the most common is to say “if the separator appears within text surrounded by double quotation marks, take it as part of the field, rather than as a separator”.

For example, consider the following input file.

Reilly,Computer Science,2014
  • The first line represents three fields. The first is “Reilly”. The second is “Computer Science”. The third is “2014”.
  • The second line represents four fields. The first is “Smith”. The second is “Reilly”. The third is “Math”. The fourth is “2015”.
  • The third line represents three fields. The first is “Smith, Reilly”. The second is “GenSci/CS”. The third is 2016.

But even that approach has its own potential problems. For example, what if we want both the separator and a quote in a field. Once again, there is a common solution: “if two double quotation marks appear in sequence, treat them as a single character, rather than as the end of a string”.

For example, consider the following input file.

a,the first letter of the English alphabet
',a single quotation mark
"",the empty string
",",a comma
"""",a double-quotation mark
""",""",a comma surrounded by double-quotation marks

Write and test a procedure, StringUtils.splitCSV, that splits a string using those policies, with a comma as the separator, and returns an array of the fields. For example,

assertArrayEquals(new String[] { "a", "b", "c" },
assertArrayEquals(new String[] { "a,b", "c" },
assertArrayEquals(new String[] { "a", "b,b\"", "c" },
assertArrayEquals(new String[] { "a", "b\"b", "c" }, 

(Yeah, aren't backslash quotation marks a wonder in C-like languages?)

You may assume that the input is in correct CSV format. (So you don't need to deal with strings that have an odd number of quotation marks.) Of course, those who want to do an exceptional job probably deal with erroneous input in a sensible way, reporting where the error occurred.

If you're not sure how a line of CSV should be parsed, save it in a file with a .csv suffix and open it with a spreadsheet application such as LibreOffice, Google Spreadsheets, Microsoft Excel, or Excel365.

Part E: A Simple Calculator

As you know, one of the primary uses of computers is to compute with numbers - that is, to calculate. A typical computer calculator needs to read and parse textual input, do the computation, and return a result. In this part of the assignment, we will simulate that behavior.

Write a method, Calculator.eval0, that takes as input a string that represents an arithmetic expression over the integers and returns the result of evaluating that expression (as a BigInteger). You may assume that numbers and operators are separated by spaces. You should use the “naive” approach to calculation: Do operations in the order they appear, rather than paying attention to precedence. You should support the operations +, -, *, /, and ^ (exponentation). For example,

assertEquals(BigInteger.valueOf(0), Calculator.eval0("0"));
assertEquals(BigInteger.valueOf(2), Calculator.eval0("1 + 1"));
assertEquals(BigInteger.valueOf(4), Calculator.eval0("1 + 2 + 1"));
assertEquals(BigInteger.valueOf(9), Calculator.eval0("1 + 2 * 3"));

Your calculator must support arbitrarily large integers. For example, you should get the correct answer for Calculator.eval0 ("2 ^ 64"). You can achieve this result by using java.lang.BigInteger for your data values.

Part F: A Fractional Calculator

Write a method, Calculator.eval1, that takes as input a string that represents an arithmetic expression over fractions and returns the result of evaluating that expression (as a Fraction). You may assume that numbers and operators are separated by spaces. You should use the “naive” approach to calculation: Do operations in the order they appear, rather than paying attention to precedence. You should support the operations +, -, *, /, and ^ (exponentation).


The string segmentation problems stem from a laboratory on strings that I wrote in the distant past.

The first two fraction problems were inspired by the lab on fractions.

I've assigned variants of the calculator problems over the years. The first is copied directly form an assignment given in a previous semester. The second is new (at least in the current form), although the idea is not.