UMBC CS 202, Spring 00

UMBC CMSC 202 Spring '00

Project 3: Fixed Point Arithmetic

Due Date:

Project 3 is due at midnight Sunday, March 19, 2000. Note that this means before 23:59:59 on Sunday evening. The standard late policy applies.

Objectives

The objectives of this project are:

To gain more experience writing classes in C++
To further practice operator overloading
To learn to implement an overloaded stream output operator

Background

Mathematicians, scientists and engineers have long required real numbers to perform the calculations required in their respective fields. With the advent of the computer to do integer operations, real number arithmetic was sure to follow. This real number arithmetic manifested itself as floating point arithmetic. The idea is that the number of places of precision is variable based on the requirements of the calculation in question. The decimal point "floats", hence the name.

However, floating point arithmetic was originally emulated, very slow, and computationally intensive. Because of this, many machines have a coprocessor (a separate, dedicated processor) for handling these floating point instructions. In many cases, they can be made to run at the same speed or even faster than integer calculations, but at the expense of heat and power.

An alternative to floating point arithmetic is fixed point arithmetic. Fixed point arithmetic sacrifices precision in order to gain back some of the performance of integer arithmetic. In the distant past, floating point coprocessors were expensive so applications often relied on fixed point arithmetic as an inexpensive solution.

Despite the fact that floating point coprocessors are now standard with consumer desktop machines, the ever shrinking computer is now plagued with what seemed like lesser concerns before: heat and power consumption. Guess what? Most personal digital assistants (PDAs -- like 3Com's Palm series) don't have coprocessors for floating point arithmetic. We're back to emulation and again we find a home for our hybrid, fixed point arithmetic.

Tasks

You have been contracted by 202Soft, Inc., a leading (pre IPO) campus-wide developer of platform independent PDA software, to write a C++ class to handle fixed point arithmetic. This library will allow their software developers to freely switch between their own floating point emulation code and your fixed point code.

The crunch is on. With a product release only weeks away, 202Soft urgently needs a beta release of your library. They've made it abundantly clear that performance is not an issue at this time. However, they are desperate to make sure that compatibility with your library is a resolved issue. If your code is well abstracted and provides an API that integrates seamlessly with builtin types, their product release can incorporate an optimized version of your library in a bug fix after the initial release.

You should write a Fixed class that handles fixed point arithmetic. The following is an example Fixed.H file.

#ifndef CMSC202_FIXED_H #define CMSC202_FIXED_H #include <iostream.h> class Fixed { public: // ------------------------------------------------ // A Fixed constructor // ------------------------------------------------ Fixed(float value=0.00, int precision=2); // ------------------------------------------------ // Change the precision of the current number. // This should not change the value stored, only // it's precision (i.e. 30.00 becomes 30.000 not // 3.000 with a change of precision from 2 to 3.) // ------------------------------------------------ void setPrecision(int); // ------------------------------------------------ // General arithmetic compatibility methods // ------------------------------------------------ Fixed operator +(const Fixed &) const; Fixed operator -(const Fixed &) const; Fixed operator *(const Fixed &) const; Fixed operator /(const Fixed &) const; Fixed operator =(const Fixed &); Fixed operator =(float); // ------------------------------------------------ // Type cast operators to convert Fixed into // primitive types // ------------------------------------------------ operator float() const; // ------------------------------------------------ // An overloaded output operator for Fixed // ------------------------------------------------ friend ostream & operator <<(ostream &, const Fixed &); // ------------------------------------------------ // The below are included for math.h compatibility // ------------------------------------------------ friend Fixed sin(const Fixed &); friend Fixed cos(const Fixed &); friend Fixed atan(const Fixed &); friend Fixed exp(const Fixed &, int); private: int value; // The value stored * 10^precision int precision; }; #endif Your program must implement all of the methods listed above without changing their signatures. This is the public interface of the class and other developers are counting on the fact that has the interface that you see above. You may add private members and methods as you see fit.

Remember: Your chance at getting in at the pre-IPO price is on the line here.

Your implementations of the above methods must satisfy the requirements listed.

Discussion About Fixed Point Arithmetic

There are a few peculiarities with fixed point arithmetic. The following section outlines a few of them.

Watch out for overflow. Overflow is what happens when you try to store a value in a variable that is too large for that variable's type. Your program should catch overflow errors and try to correct by:
1. Reducing the precision of the result
2. Printing a warning message
In the event that the overflow cannot be avoided, a second warning should be printed and the calculation should be allowed to proceed. At this point, the results of the operation are undefined. (That means you're not responsible for what your program evaluates for that expression beyond your warning.)
Similarly, if a call to setPrecision would overflow the value, print a warning. In this case however, do not change the value of the Fixed instance.
Addition and subtraction of fixed point numbers is fairly straightforward. If two numbers have the same precision, check for overflow, add the values and return the result. For example: 30 and 20, as fixed point values with two places of precision would be 3000 and 2000. Their sum is 5000, or 50.00. Subtraction works the same way. How does multiplication differ? How about division?
As a rule, when two operands are combined in an expression, the result's precision should be equal to the maximum precision of the maximum of the two operands. For example: 1.001 + 2.00 should give 3.001 as a result. Neither operand should be changed in the process.
The math.h functions listed can use the underlying math.h functions in their implementation. The important part here is that they are present as placeholders for later, optimized versions of the library routines.

Coding Standards

For this project you must follow the class coding standards.

Using Code You Didn't Write

Claiming authorship of code that you didn't write is plagiarism and will be dealt with as academic dishonesty. You should not use code that you didn't write; it defeats the learning experience of the project. If you do use outside code on your project, you must site the source. If a significant portion of the project comes from an outside source, your grade may be reduced.

Testing Your Program

Remember, your program must compile and run using the CC compiler on the IRIX machines (umbc8/9). If your program does not compile or does not generate the correct output, you will not receive full credit for this project.

You should make sure that you have a makefile named either makefile or Makefile and that when you type make your program compiles to an executable called proj3. Failure to follow these simple directions will result in a reduced grade.

The following sample main will help you test your program. A sample Makefile has also been provided.

What to Turn In

You should submit your Makefile, proj3.C and any files needed to build the Fixed class.

The contents of proj3.C will not be graded. This is a placeholder for the "real" test routines that the graders will use. The graders' proj3.C will expect the public interface defined above and will expect to find it in Fixed.H.

Submitting Your Project

When you have finished and tested your project you can submit it electronically using submit. The project name for this assignment is proj3. As an example, to submit your Makefile, you would type: submit cs202 proj3 Makefile More complete documentation for submit and related commands can be found here.

Last modified: 5 March 2000