CS171, Homework 4

Preparation

To do this project, you must obtain a copy of the prepared files with these commands:

mkdir ~/cs171/hw4 cp /home/cs171000/Handouts/hw4/*.java ~/cs171/hw4

List of files:

DoubleStack.java Interface of a stack that store double type DoubleArrayStack.java An implementation of DoubleStack interface StringStack.java Interface of a stack that store String type StringArrayStack.java An implementation of StringStack interface Eval1.java Work file for homework 4 testProg1.java Test program for homework 4 Eval2.java Work file for bonus project testProg2.java Test program for bonsu project

Instructions:

Do not make any changes to these files:

DoubleStack.java Interface of a stack that store double type DoubleArrayStack.java An implementation of DoubleStack interface StringStack.java Interface of a stack that store String type StringArrayStack.java An implementation of StringStack interface testProg1.java Test program for homework 4 testProg2.java Test program for bonsu project

Only edit (change) this file for homework 4:
If you want to do the bonus part, you must edit this file:

Description of the files

DoubleStack.java

This file is an interface defining a more sophistcated stack than the one discussed in class.

This stack can store double typed values

It provides the following functions:

void push(double x): store the value x in the stack
double pop(): remove and return the value on the top of the stack
double peek(): return (but does not remove) the value on the top of the stack
boolean isEmpty(): returns true if the stack is empty and false otherwise

DoubleArrayStack.java

This is an implementation of the interface DoubleStack with an array
For this project, it is sufficient to use a stack of 100 elements.
So you can create a stack using:

StringStack.java

This file is an interface defining a more sophistcated stack than the one discussed in class.

This stack can store String typed values

It provides the following functions:

void push(String x): store the value x in the stack
String pop(): remove and return the value on the top of the stack
String peek(): return (but does not remove) the value on the top of the stack
boolean isEmpty(): returns true if the stack is empty and false otherwise

StringArrayStack.java

This is an implementation of the interface StringStack with an array
For this project, it is sufficient to use a stack of 100 elements.
So you can create a stack using:

The home work assignment

Look at the Eval1.java program file: ayy

public class Eval1 { /* ===================================================== isNumber(s): returns true if s is a "numeric string" i.e., a string that can be converted to a double value using: Double.parseDouble( s ) returns false otherwise ===================================================== */ public static boolean isNumber( String s ) { return s.matches("-?\\d+(\\.\\d+)?"); // Check if s matches a decimal number // with optional '-' and decimal. } /* ===================================================== evalExpr(s): s is an array of strings that makes up an expression with + and - operations Operators must be evaluated left to right: 1 - 1 - 1 = -1 1. evaluates the expression 2. return the final result ===================================================== */ public static double evalExpr( String[] s ) { // Write this method } }

Assignment:

Write the method double evalExpr( String[] s ) that:

Receive an array of string (variable s) that contain a numerical expression in string format
The numerical expression will only contain the following:
The method double evalExpr( String[] s ) must evaluate the numerical expression in the input array s and return the result

Examples:

If the input string array s contains:
then evalExpr(s) will return 1.0 (because the input array represents the expression 3.0 - 2.0)
If the input string array s contains:
then evalExpr(s) will return -1.0 (because the input array represents the expression 3.0 - 2.0 - 2.0)

Test program

You can test your project using the test program

To compile:

javac testProg1.java

To run:

java testProg1 ... ... ... (The ... ... ... represents a numerical expression)

Sample outputs:

java testProg1 3.0 + 2.0 result = 5.0 java testProg1 3 + 2.1 - 5.1 - 1 + 2 result = 1.0 java testProg1 4.1 - 2.2 - 7.1 - 3 + 2 result = -6.199999999999999

Warning:: you must leave a space between each input item (or else, you program will crash)

The evaluation algorithm for a numerical expression with only + and − operations

You need to maintain the following information:

A stack of double (I called it numStack in my example below) to store the numbers in the input
A stack of String (I called it operStack in my example below) to store the operators in the input

I will illustrate the evalExpr() algorithm with an example

Input: s[] = {"3", "-", "2", "+", "4"}
First, create a stack of double and a stack of String:

Go through the input array and process each array element separately:

Process first item: s[0] = "3"

It is a numerical string

A numerical string is processed as follows:

Convert to a double (use the Java method Double.parseDouble() (see: click here )
Push the converted double value on to numStack:
Check the operator stack:

After processing s[0] = "3", the stacks will look like this:

(The first number is now stored in the numStack)

Process next item: s[1] = "-"
After processing s[1] = "-", the stacks will look like this:

Process next item: s[2] = "2"

It is a numerical string

A numerical string is processed as follows:

Convert to a double
Push the converted double value on to numStack:

Check the operator stack:

If the operator stack is not empty, we perform the operation given in the operator stack on the 2 operands in the number stack:

We pop the operator off the operStack (we will obtain "-" so we know we need to subtract
We then pop 2 values off the numStack and subtract them: 3 − 2 = 1
The result (the value 1) is always pushed onto the numStack

After processing s[2] = "2", the stacks will look like this:

(The operation "3 - 2" has been performed and the result 1 is stored in the numStack)

Process next item: s[3] = "+"
After processing s[3] = "+", the stacks will look like this:

Process next item: s[4] = "4"

It is a numerical string

A numerical string is processed as follows:

Convert to a double
Push the converted double value on to numStack:

Check the operator stack:

If the operator stack is not empty, we perform the operation given in the operator stack on the 2 operands in the number stack:

We pop the operator off the operStack (we will obtain "+" so we know we need to add)
We then pop 2 values off the numStack and subtract them: 1 + 4 = 5
The result (the value 5) is always pushed onto the numStack

After processing s[4] = "4", the stacks will look like this:

(The operation "1 + 4" has been performed and the result 5 is stored in the numStack)

We have gone through all array elements
At the end of the iteration, we always have the final value of the expression on the numStack
All you need to do is:

Bonus project: Extra 50 pts

Bonus project:

Use the program file Eval2.java to do the bonus project
Use the test program testProg2.java to test the function in Eval2.java
Write a method evalExpr(s) that can handle: +, −, x and / operations
You must figure out the algorithm yourself.... I will only give you this hint: when you read in a high priority operator, you can execute it right away, but when you read in a low priority operator, you must use the peek() method to look in the operator stack and then decide what to do.

Sample outputs:

java testProg2 2 x 3 result = 6.0 java testProg2 7 - 2 x 3 result = 1.0 java testProg2 2 - 3 - 2 x 4 x 3 - 3 x 3 result = -34.0 java testProg2 3 / 2 x 4 - 3 / 2 x 3 result = 1.5 java testProg2 8 / 2 / 4 + 6 / 2 / 3 result = 2.0

Turnin
- Use the following command (from inside your cs171/hw4 project directory) to turn in your program:
- If you have done the bonus project, then turn that in also.
  But use a different turn in code.
  Use the following command (from inside your cs171/hw4 project directory) to turn in your bonus prject (only if you have done it):