COS 126

Rational Arithmetic
Programming Assignment

Due: Wednesday, 11:59pm

Note on collaboration: On Assignment 3, if you like, you may collaborate with one other person, someone with very nearly your level of prior programming experience (but who need not be in your precept). Here's what you should do:

Implement a rational arithmetic package and a client program that uses it to compute rational approximations to e. The purpose of this assignment is to learn about type definitions, packaging interfaces and implementations, separate compilation, and arithmetic overflow.

Rational numbers are numbers that can be represented as the ratio of two integers, i.e., any number p/q where p and q are integers is a rational number. C approximates non-integer rational numbers using floats or doubles, but these types are imprecise representations. For this assignment, you will represent rational numbers with a C structure that comprises two integers:

struct { unsigned int num; unsigned int den; }
represents the rational number with numerator num and denominator den. When a C type is used repeatedly for representing another, more abstract, type, it's conventional to specify a new type name in a   type definition as follows:
typedef struct { unsigned int num; unsigned int den; } Rational;
This directive allows us to use rational numbers in C programs in much the same way that we use other types, as in the following sample code:
Rational a, b, c;
a = RATinit(1, 3);
b = RATinit(1, 4);
c = RATadd(a, b);
In this assignment, you will learn an important general method for implementing and using new data types and their associated functions.

Step 0.  Copy the file RAT.h that contains the following code:

typedef struct { unsigned int num; unsigned int den; } Rational;
Rational RATinit(unsigned int numerator, unsigned int denominator);
    void RATshow(Rational r);
Rational RATadd(Rational r1, Rational r2);
Rational RATmul(Rational r1, Rational r2);
This file is called an interface. Its purpose is to precisely spell out what the data type is (the possible values of variables having the type and functions that manipulate such variables). In this case, we are saying that Rational numbers are pairs of integers, and that we will have functions for: initializing them; showing (printing) them; adding two of them and putting the result in a third; and multiplying two of them and putting the result in a third.

Step 1.  Make a file named rat.c and write straightforward implementations of all the functions. The first line of this file should include the interface, as follows:

#include "RAT.h"
This gives your function implementations access to the Rational type definition, and provides a check that the functions that you write have the same types of arguments and return values as promised in the interface. This file is called the implementation of the data type.

Step 2.  Write a program named e.c that uses your package to compute a rational approximation to e, using the Taylor series expansion e = 1/0! + 1/1! + 1/2! +1/3! + 1/4! + 1/5! + . . .   Print out the value that you get after each term is added to the approximation. Read in an integer n from standard input using scanf(), and print the first n approximations. The output for n = 6 is:

1/1  2/1  5/2  32/12  780/288  93888/34560
In the present context we are interested in this program as an example of a client: a program that uses the data type, but is implemented separately. To use the Rational data type in e.c, make the first line
#include "RAT.h"
Now, you can use variables of type Rational in your client program, but be sure to only access the data type through the interface functions. Compile your program jointly using "gcc126 rat.c e.c", then "a.out" as usual.

Step 3.  Develop an improved implementation ratbetter.c that staves off overflow longer. A trivial way to do this would be to use long int, but that is less helpful than you might think, so we consider algorithmic improvements.

A straightforward implementation of the rational arithmetic interface has two flaws that will jump out at you as you add terms to the list above to get more accurate approximations. You can't avoid overflow, but you can stave it off by exercising sound judgment. For example, in the fourth term in the example above, both numerator and denominator are divisible by 4, so we would prefer to see the result 8/3 instead of 32/12. You can fix this problem with Euclid's algorithm (see Sedgewick, p. 191), using it to change your package to adhere to the convention that functions only returns rational numbers whose numerator and denominator are relatively prime.

The second problem, which is more subtle, is overflow: The products formed by the rational arithmetic functions might overflow, which leads to erroneous results, even if the result can be represented. For example, a/b × c/d is equal the reduced value of ac/bd, but either ac or bd might overflow before the reduction, even though the reduced answer might represent no problem (e.g., the naive method for computing 20/33 × 77/50 = 14/15 would overflow a variable capable of storing only two digit integers). Similarly, a/b + c/d is equal to the reduced value of (ad + bc)/bd, but either ad, bc, or bd might overflow before the reduction, even though the reduced answer might represent no problem (e.g., 7/20 + 7/30 = 7/12 would create intermediate values as big as 600).

Submission.  You should submit the 4 files: readme.txt, rat.c, ratbetter.c, and e.c. Be sure to carefully explain the approach that you use to stave off overflow in your readme.txt file. Do not change or submit the interface file RAT.h. Your client program e.c should work with either implementation rat.c or ratbetter.c. Presumably, e.c will get more accurate answers with the improved implementation.

Extra Credit.  Write a client program that computes rational approximations to π, using the identities π / 4 = arctan(1) and the continued fraction expansion arctan(x) = x/(1 + (x^2)/(3 + ((2x)^2)/(5 + ((3x)^2)/(7 + ...)))) to compute rational approximations to the arctangent.

This is an extensively modified version of an assignment originally developed by A. LaPaugh and S. Arora.

Copyright © 1999 R. Sedgewick