Summary: Here you will find some sample solutions to
the exercises from Homework 2.
Contents
a. Extend the Fraction class so that it permits multiplication
of two fractions.
An Answer
/**
* Multiply this fraction by another fraction.
*/
public Fraction multiply(Fraction other)
{
// a/b * c/d = (a*c)/(b*d)
return new Fraction(this.numerator.multiply(other.numerator),
this.denominator.multiply(other.denominator));
} // multiply(Fraction)
b. Test your code.
An Answer
i. Using the original fractions:
Hmmm ... perhaps I should simplify.
ii. Using fractions whose product should be 1.
iii. Using a variety of negative fractions.
-1/4 * 3/11 = -3/44
1/4 * -3/11 = -3/44
-1/4 * -3/11 = 3/44
Seems okay.
iv. Using some really big numerators and denominators
1111111111/4 * 1111111111/11 = 1234567900987654321/44
4123/1111111111 * 11/1111111111 = 45353/1234567900987654321
Still seems fine. (Yeah, I should have chosen something easier to check.)
As you may know, we can represent every non-negative rational number as a whole
number plus a fractional value no smaller than 0 and no bigger than 1.
a. Write a method of the Fraction class, fractional,
that identifies and returns this fractional value. Your procedure need
only work for positive numbers.
For example,
Fraction f = new Fraction(11,3);
pen.println(f.fractional());
// Prints 2/3
f = new Fraction(1,2);
pen.println(f.fractional());
// Prints 1/2
f = new Fraction(4,2);
pen.println(f.fractional());
// Prints 0/2 or something similar
An Answer
Well, to find the fractional part, what I really want is the remainder
when I divide the numerator by the denominator. I can then return the
ratio of that remainder and the denominator.
A quick check of the documentation reveals that BigInteger helpfully
provides a remainder(BigInteger divisor) method, so I can
just use that.
/**
* Compute the fractional part of the fraction. That is, if we
* think of the fraction n/d as (a*d+b)/d, where 0 <= b < d,
* returns d/b.
*/
public Fraction fractional()
{
return new Fraction(numerator.remainder(this.denominator),this.denominator);
} // fractional()
b. Test your method.
An Answer
I'll try four tests:
(i) a fraction with no whole part,
(ii) a fraction with no fractional part,
(iii) a fraction with a small whole part, and
(iv) a fraction with a large whole part.
The fractional part of 1/3 is 1/3
The fractional part of 6/3 is 0/3
The fractional part of 12/5 is 2/5
The fractional part of 1231152344523342/10 is 2/10
Write and test a third constructor for the Fraction class.
This constructor should accept a string as a parameter, parse
that
string, and generate the appropriate fraction. For example,
Fraction f = new Fraction("1/4");
pen.println(f.doubleValue());
// Prints 0.25
f = new Fraction("120/3");
pen.println(f.doubleValue());
// Prints 40.0
You can expect that
the string will have two positive integers separated by a slash. You
may find it useful to reflect on the indexOf method of the
java.lang.String class and on various methods of the
java.lang.Integer class.
An Answer
To convert a string of the form "num/denom" to the appropriate fields,
it seems that I need to figure out two things: (i) how to separate the
big string into two smaller strings (one for the numerator and one for
the denominator) and (ii) how to convert each string into a BigInteger.
We've looked at the problem of separating strings in the past, so
that part should be fairly straightforward. We identify the position
of the separator (the slash) and take the stuff before it and the
stuff after it.
The problem of converting strings to BigIntegers should only require
a quick check of the documentation. Amazingly enough, although
BigInteger does not provide a constructor that takes an int
as a parameter, it does provide a constructor that takes a string.
Putting it all together,
/**
* "Parse" a string of the form "numerator/denominator" to
* a Fraction.
*/
public Fraction(String description)
{
// Determine the position of the slash
int slash = description.indexOf("/");
// Grab the numerator
String num = description.substring(0,slash);
// Grab the denominator
String denom = description.substring(slash+1);
// Convert to BigIntegers
this.numerator = new BigInteger(num);
this.denominator = new BigInteger(denom);
} // Fraction(String)
We might also express this more concisely as,
public Fraction(String description);
{
// Determine the position of the slash
int slash = description.indexOf("/");
// Build the two fields
this.numerator = new BigInteger(description.substring(0,slash));
this.denominator = new BigInteger(description.substring(slash+1));
} // Fraction(String)
Write a main class that reads in two fractions and prints out their sum
and product in both fractional and decimal form.
An Answer
See
Calculator.java.
Write and test a class, Counter, that generates
objects that can count. Objects in class Counter
should provide two methods: increment, which adds 1
to the counter, and get, which gets the current value
of the counter.
Make sure to verify that if you create two separate objects in
class Counter, you can change the two objects separately.
An Answer
Counter objects will have one field, count,
which is an int. increment adds one to
the field. get returns the field. The one constructor
will take no parameters and set the counter to 0.
See the details in
Counter.java.
Here are some tests that suggest the two counters work independently
Initially ...
c1 = 0, c2 = 0
After incrementing c1 ...
c1 = 1, c2 = 0
After incrementing c2 three times ...
c1 = 1, c2 = 3
After incrementing c1 again ...
c1 = 2, c2 = 3
a. Update your Counter class to include a second constructor that
Allows the user to specify a starting value.
b. Update your Counter class to include a reset method that reset the counter to the starting value.
c. Test both updates.
An Answer
Although I expected you to update Counter.java, I've made
a second class, which I've called ExtendedCounter.java.
The key implementation details have to do with resetting.
Since we have to reset to the starting value
, we need to add a field
to keep track of that value. I'll call that field start.
Reset simply sets count to start.
See the details in
ExtendedCounter.java.
Here are some tests that show that we can create counters that start at 0,
at a positive number, and at a negative number; that those counters increment
independently; that those counters reset correctly and independently; and
that increment continues to work after the reset.
Initially ...
c1=0, c2=5, c3=-3
After incrementing c1 ...
c1=1, c2=5, c3=-3
After incrementing c2 three times ...
c1=1, c2=8, c3=-3
After incrementing c1 again ...
c1=2, c2=8, c3=-3
After incrementing c3 twice ...
c1=2, c2=8, c3=-1
After resetting c1 ...
c1=0, c2=8, c3=-1
After incrementing c1 again ...
c1=1, c2=8, c3=-1
After resetting c2 ...
c1=1, c2=5, c3=-1
After incrementing c2 again ...
c1=1, c2=6, c3=-1
After resetting c3 ...
c1=1, c2=6, c3=-3
After incrementing c2 again ...
c1=1, c2=6, c3=-2
After incrementing c2 again ...
c1=1, c2=7, c3=-2
Sunday, 25 September 2005 [Samuel A. Rebelsky]
Wednesday, 28 September 2005 [Samuel A. Rebelsky]
- Finished writing answers.
;
Check with Bobby