CS170, Homework 8

Warning:

You can do scoping part of homework very easily by cutting and pasting the Java code into a file, compile it and run it
The program execution will give you the answers... BUT....
If you do this, you will miss out on the opportunity to learn about scoping rules
I can guarantee you that you will be tested on scoping rules in Midterm 2 and there may even be some questions on scoping on the Final.
Therefore, use this homework to reinforce your knowlegde on scoping.

Preparation

Make a hw8 directory and copy over the prepared files:

mkdir ~/cs170/hw8 cp ~cs170002/share/hw8/* ~/cs170/hw8 cd ~/cs170/hw8

You should have 3 files:

hw8.ans
EvalExpr.java
TestEvalExpr.java (do not edit this file, it is used to test the program EvalExpr.java)
TestEvalExpr2.java (do not edit this file, it is used to run the program EvalExpr.java)

Use the file hw8.ans to answer the questions 1 through 5 (each question is worth 10 pts).

Use the file EvalExpr.java to do question 6 (50 pts).

(You can use gedit to edit the file - add your answer to the file. You will turn in the updated hw8.ans as part of this homework)

Question 1 (10 pts)
- What is printed by the following Java program: (you may need to scroll the window to see the whole program)
Question 2 (10 pts)
- What is printed by the following Java program: (you may need to scroll the window to see the whole program)
Question 3 (10 pts)
- What is printed by the following Java program: (you may need to scroll the window to see the whole program)
Question 4 (10 pts)
- Consider the following program:
  What is the scope of the variable i ? (Describe it with your own words)
  Hint: compile the program, look at the error messages and draw your conclussion on the scope of the variable i)
Question 5 (10 pts)
- Indicate which lines in the following program will have compilation errors any what is the cause of the error:

Question 6: write a method (50 pts)

Problem Description:

Write a method called public static int eval(char[] s) that receives a array of characters (which is nothing more than a different representation of a string) containing an expression, and returns the result of that expression.
The input array contains an expression that consists of a number of integers, and the operations + and −.

For example:

int r; char[] a = {'1', '1', '+', '2'}; // a[0] = '1', a[1] = '1', // a[2] = '+', a[3] = '2' r = EvalExpr.eval( a ); will return: 13 (integer) (and assign 13 to r)
char[] a = {'1', '1', '+', '1', '2'}; // a[0] = '1', a[1] = '1', // a[2] = '+', a[3] = '1', a[4] = '2' r = EvalExpr.eval( a ); will return: 23 (integer) (and assign 23 to r)

The character array is a sequence of characters, which is nothing more than a string (in fact, the C/C++ programming language use character arrays to represent strings)

The algorithm (I will describe the algorithm with a concrete input parameter, but will use this: "23-3+12-4+56" to represent the input array where: s[0] = '2', s[1] = '3', s[2] = '-', and so on....)

A general observation:
This means that you must break down the problem into a task that can be repeated

How to break this task into a repeatable task:

Input: "23-3+12-4+56" We must identify a "repeatable task" in the input.
How: divide input "23-3+12-4+56" into 2 portions: "23-3+12-4+56" The blue portion (23) must be "handled separated". The red portion of the input can be processed as a repeatable task that consists of 2 steps: 1. extract the operation 2. extract a number In other words: processing the portion "-3+12-4+56" can be done repeatedly: Iteration 1: 1. extract the operation ---> - 2. extract a number ---> 3 Iteration 2: 1. extract the operation ---> + 2. extract a number ---> 12 Iteration 3: 1. extract the operation ---> - 2. extract a number ---> 4 Iteration 4: 1. extract the operation ---> + 2. extract a number ---> 56 The repeatable task (consisting of 2 steps) forms the body of a while loop of the algorithm !

Preliminary algorithm design:

Extract the first number string in the input; // It's a string ! result = convert number string to integer; // Use: Integer.parseInt(..) while ( we have not processed the whole string ) { operation = next element of the character array !!! Extract the next number string; x = convert the number string to integer; if ( operation == '+' ) result = result + x; // do the + operation else if ( operation == '-' ) result = result - x; // do the - operation else print "illegal operation", and return 0; Prepare variables for the next loop; }

How to extract a number:

Array s: 12+.... (a sequence of characters !!!) ^ | pos = 0 Define: String w; // Used to collect the extracted number w = ""; // Start w as empty Check: s[pos(=0)] = '1' it is a digit ==> add to w ==> w = "1", goto next pos (pos=1) Check: s[pos(=1)] = '2' it is a digit ==> add to w ==> w = "12", goto next pos (pos=2) Check: s[pos(=2)] = '+' it is NOT a digit ==> you have extracted a number ("12") as a string in w Notice that: The next character is always an operation !

NOTE: you must take care when extracting a number, it can reach the end of the array

Array s: ....+45 (suppose we are extracting the last number) ^ | pos = 20 Define: w = ""; // Start w as empty Check: s[pos(=20)] = '4' it is a digit ==> add to w ==> w = "4", goto next pos (pos=21) Check: s[pos(=21)] = '5' it is a digit ==> add to w ==> w = "45", goto next pos (pos=22) Check: pos=22 will be equal to the LENGTH of the array You MUST STOP ==> you have extracted a number ("45") as a string in w Notice that: Your (while) must make sure to check if pos access a valid array element !!!

How the algorithm works on the input "23-3+12-4+56":

input = "23-3+12-4+56"
Execute the algorithm: Get the first number string in the input; ----> "23" result = convert number string to integer; ----> result = 23
(Processed so far: "23-3+12-4+56") Make sure your variables RECORD this fact !!! The scan position must be positioned here: "23-3+12-4+56" ^ From the scan position, we can conclude that we have not processed the whole string, so exec while-body: find the next operation; ----> next operation = '-' find the next number string; ----> next number string = "3" x = convert the number string to integer; --> x = 3 if ( operation == '+' ) result = result + x; // Skip... elese result = result - x; // --> result = 23-3 = 20 (Processed so far: "23-3+12-4+56") Prepare variables for the next loop execution // Make sure you update your variables correctly !!! End of loop, go back to the loop condition....
(Processed so far: "23-3+12-4+56") Make sure your variables RECORD this fact !!! The scan position must be positioned here: "23-3+12-4+56" ^ From the scan position, we can conclude that we have not processed the whole string, so exec while-body: find the next operation; ----> next operation = '+' find the next number string; ----> next number string = "12" x = convert the number string to integer; --> x = 12 if ( operation == '+' ) result = result + x; // --> result = 20+12 = 32 elese result = result - x; // Skip... (Processed so far: "23-3+12-4+56") Prepare variables for the next loop execution // Make sure you update your variables correctly !!! End of loop, go back to the loop condition.... And so on !

Assignment:

Write the eval() method in the program file EvalExpr.java

I have included a test program called TestEvalExpr.java.

The TestEvalExpr.java program contains a main() method that invoke the EvalExpr.eval() method.

Test your eval() method as follows:

UNIX>> javac TestEvalExpr.java UNIX>> java TestEvalExpr 1+2=3 2-3-4=-5 3+1-3-1+4-1=3 4+1-1+1-1+1-1=4 Illegal operator: / 4/1-1+1-1+1-1=0 Illegal operator: / 4+1-1/1-1+1-1=0

Note:

I have also included another program called TestEvalExpr2.java.
This program reads in an expression from the terminal and invokes EvalExpr.eval() to compute the result.

Try it out after you have tested your EvalExpr.java program.

Use these command to run it:

UNIX>> javac TestEvalExpr2.java UNIX>> java TestEvalExpr2 Please input a simple expression containing: 1. integer numbers 2. arithemtic operators + and - Enter an expression (no spaces): 67+76 67+76=143 Enter an expression (no spaces):

Turn in

Turn in your hw8 answers using these commands:

  /home/cs170002/turnin-hw   hw8.ans         hw8 

  /home/cs170002/turnin-hw   EvalExpr.java   hw8a