# Programming Assignment 1

## Computer Science 102

Spring 2007

Due date: WED, FEB 7, 11:59 pm (midnight). Draft #2 is due one week after your etutor returns the first draft but start working on it as soon as possible. .

### Draft #1

Please send your programs to the etutor. The URL for this is:

http://cs.nyu.edu/courses/spring07/V22.0102-002/submitting.html

and is found on the homepage under the heading Where to submit your programs .

Introduction
Your project is to design a program to implement a calculator. The calculator will take an infix expression convert it to a postfix expression and then evaluate it. In draft #1, the infix expression will consist of operands that are single digits and the operators {+, -, *, /}. An example of such an expression is 2+3*4/6. This will translate to 234*6/+ and will then be evaluated as 4.0. Full credit for doing this will be a grade of 3.75. To get a grade of 4.0 in draft #1, your program must evaluate infix expressions containing parentheses such as (2+4)*(8-2)/(9-5). The course homepage contains explanations of how to do all of this under the category Stack programs http://cs.nyu.edu/courses/spring04/V22.0102-002/infix or if you have the Goodruch text see page 218 C-5.18 for an explanation of a postfix expression or in the Weiss text, see pp 78-86. I feel the web page explanations are sufficient.

Your project for draft #1 divides into two parts:

• A Converter class that will convert the input string to postfix.
• A Calculator class that will evaluate the postfix expression.

Normally we would use a stack of char to do the conversion and a stack of double to do the evaluation. Java, however, allows us to use a stack of Object for both (please see StackADT done on Jan 24, but alter it so that there is a constructor without any parameter that intializes n to 100). Unfortunately the primitive types, in our case double and char are not classes. However, we can use wrapper classes Character and Double to accomplish our goal. Since both of these are subclasses of Object, instances of both these types can be pushed on the stack. It's like assigning an instance of a smaller class to an instance of a wider class. For instance, when pushing ans make it a field of a Double instance variable and push that instance variable on the stack by using:

prim = new Double(ans);
stack.push(prim );
Here the Double instance variable is prim. Or, you can use autoboxing and simply write
stack.push(ans)
.

Special consideration must be given to removing items from the stack. Since the Object class is wider than the Double class, writing

prim = stack.pop();
will cause an error since you cannnot assign an object of a wider class to an object of a narrower class with out type casting. If you use autoboxing, you would write
ans = (Double)stack.pop();

In both the Converter and Calculator classes, use a StackADT that checks for an empty stack when popping and a full stack when pushing. You should write your own MTstackException and FullStackException classes to do this. As a consequence, you should do no stack checking in the driver program except when popping the stack converting from infix to postfix.

### The Converter class

This uses the following methods

1. The boolean method isOperator(char ch)
2. The boolean method isOperand(char ch)
3. The booleanmethod precedence(String op1, char op2) which determines if the incoming token op2 has precedence over the top of the stack op1.

The Calculator class

This uses the following methods

1. The boolean method isOperator(char ch)
2. The boolean method isOperand(char ch)
3. The double method result(double op1, double op2, char op) which determines the value of the binary operator op operating on the operands op1 and op2.

### Strategy for writing the program

Here is a recomended way of writing the program. If you feel comfortable skipping some steps to get to the final result, then by all means do.
1. Write the Calculator class using a stack of Object.
2. Write the Converter class using a stack of Object and instead of it printing the final postfix string, it should return a String.
3. Have the Calculator class call the Converter class in order to get a postfix expression as input.

For draft #1, write your program so that there are three files. That way it is easier to compile (one file at a time). Here are the steps:

1. A Converter class written so that the main method is rewritten as a method that returns a string. Let's call it toPostfix() with the signature: public String toPostfix()
2. A Calculator class that instantiates the Converter class so that it can access method toPostfix().
3. A StackADT class and the exception classes in one file.

### Sample input

• 3+4*5/6
• (3+2)*(4-2)/(8+7)
• (4+8)*(6-5)/((3-2)*(2+2))

Your output should show the converted postfix string and the result of the calculation. For instance the last sample input will produce the following in the output window:

(4+8)*(6-5)/((3-2)*(2+2))

converted string is 48+65-*32-22+*/