Lab 8: Scoping Rules and Methods

For the TA

In this lab, students must:

The lab assignment is self-explanatory and your job is to

pace the first part of the lab (make sure it finish in about 20 mins) and
assist students (show them the answers if necessary but explain the reason) when they have questions.

Lab goals
- In this lab you will write statements and method invocations that help you understand the scoping concept better.
- The scoping rule assignments are very easy to do; so we will use the latter part of the lab to practice nested loops
  
  Please, make sure that you have added the honor code statement at the top of your Java file:
  The deadline for completing this lab assignment is midnight, today.
Preparation: Create your ~/cs170/lab8/, and copy files
- Copy-and-paste these terminal commands:
  In this lab, we will use gedit and javac (not BlueJ !).
  (You could use BlueJ, but you must figure out how to compile and run the program on your own.)

Overview of the provided program Scoping.java

Take a look at the program Scoping.java with gedit:

gedit Scoping.java &

The variable in the program have been high lighted in the following diagram:

public class Scoping { static double a = 123; /* ....................... line (1) */ public static void F( double a ) /* ....................... line (2) */ { System.out.println("------ inside F( double a ) !!!!!!!!!!!!!!!!"); System.out.print("a at (2) = "); // TODO: Write a System.out.println() statement that prints a on line (2) System.out.print("a at (1) = "); // TODO: Write a System.out.println() statement that prints a on line (1) } public static void F( String a ) /* ....................... line (3) */ { System.out.println("------ inside F( String a ) "); System.out.print("a at (3) = "); // TODO: Write a System.out.println() statement that prints a on line (3) System.out.print("a at (1) = "); // TODO: Write a System.out.println() statement that prints a on line (1) } public static void main( String[] args ) { double x = -444444444; /* ....................... line (4) */ // TODO: Write a method call F() that passes x at line (4) F(x); // I have given you the answer for the first problem. { double a = 9999; /* ....................... line (5) */ // TODO: Write a method call F() that passes a on line (5) // TODO: Write a method call F() that passes a on line (1) } // TODO: Write a method call F() that passes x at line (4) // TODO: Write a method call F() that passes a on line (5) { String a = "9999"; /* ....................... line (6) */ // TODO: Write a method call F() that passes x at line (4) // TODO: Write a method call F() that passes a on line (6) // TODO: Write a method call F() that passes a on line (1) } } }

Notice that:

variable a on line (1) is a class variable
variable a on line (2) is a parameter variable
variable a on line (3) is a parameter variable
variable x on line (4) is a local variable
variable a on line (5) is a local variable
variable a on line (6) is a local variable

You can use the following commands to compile and execute the program Scoping.java:

Lab Assignment - Part 1: write the F(double a) method

Edit the Scoping.java program that is provided for this lab:

Complete the F(double) method:

public static void F( double a ) /* ....................... line (2) */ { System.out.println("------ inside F( double a )"); System.out.print("a at (2) = "); // TODO: Write a System.out.println() statement that prints a on line (2) System.out.print("a at (1) = "); // TODO: Write a System.out.println() statement that prints a on line (1) }

After completing the F(double a) method, you can compile and run the program with these commands:

javac Scoping.java java Scoping

and if correct, you should see this output:

------ inside F( double a ) a at (2) = -4.44444444E8 a at (1) = 123.0

TA: make sure all students are done before moving on to the next part of the lab.
Explain the answers after 5 mins.

Lab Assignment - Part 2: write the method calls to the F() method inside the first pair of { }

Write the following method calls inside the first pair of { }:

{ double a = 9999; /* ....................... line (5) */ // TODO: Write a method call F() that passes a on line (5) // TODO: Write a method call F() that passes a on line (1) }

If it is not possible to write the requested method call, then write the following statement in its place:

System.out.println("Impossible: Write a method call F() that ... ");

After writing the method calls, you can compile and run the program with these commands:

TA: make sure all students are done before moving on to the next part of the lab.
Explain the answers after 5 mins.

Lab Assignment - Part 3: write more method calls F()
- Write the following method calls between the first and second pair of { }:
  If it is not possible to write the requested method call, then write the following statement in its place:
- After writing the method calls, you can compile and run the program with these commands:

Lab Assignment - Part 4: complete F(String a) and write more method calls F()

Complete the F(String a) method:

public static void F( String a ) /* ....................... line (3) */ { System.out.println("------ inside F( String a ) !!!!!!!!!!!!!!!!"); System.out.print("a at (3) = "); // TODO: Write a System.out.println() statement that prints a on line (3) System.out.print("a at (1) = "); // TODO: Write a System.out.println() statement that prints a on line (1) }

Also: write the following method calls inside the second pair of { }:

{ String a = "9999"; /* ....................... line (6) */ // TODO: Write a method call F() that passes x at line (4) // TODO: Write a method call F() that passes a on line (6) // TODO: Write a method call F() that passes a on line (1) }

If it is not possible to write the requested method call, then write the following statement in its place:

System.out.println("Impossible: Write a method call F() that ... ");

After writing the method calls, you can compile and run the program with these commands:
TA: make sure all students are done before moving on to the next part of the lab.
Explain the answers after 5 mins.

Part 5 - Methods (Fold and unfold a string) --- (50 %)

Let s be a string consisting of the characters "c₀c₁c₂...c_n-3c_n-2c_n-1"

We define the fold operation on a string as follows:

Original string: "c₀c₁c₂.....c_n-3c_n-2c_n-1" Folded string: "c₀c_n-1c₁c_n-2c₂c_n-3....."

In other words:

We first put the first character in the folded string
Then we put the last character in the folded string
Then we put the second character in the folded string
Then we put the one before last character in the folded string
And so on.

Examples:

Original string ------> Folded string "ab" "ba" "abc" "acb" "abcd" "adbc" "abcde" "aebde" "abcdef" "afbecd" and so on...

Algorithm Description:

Use 2 pointer (indices): i points at the start and j points at the end Use a variable named "output" to collect the folded string
Initial state: output = ""; "abcd.....xyz" ^ ^ | | i j
After 1 iteration: output = "az"; "abcd.....xyz" ^ ^ | | i j
After 2 iterations: output = "azby"; "abcd.....xyz" ^ ^ | | i j
Repeat until i catches up to j (i.e.: i >= j) 2 cases: A) If input string has an odd number of characters: then i == j "abcd .. ? .. xyz" ^ | i==j Action: add the middle character to the output Example: input = "abcde" After 2 iterations, i and j will point to the character 'c' We must add 'c' to the output. B) If input string has an even number of characters: then i > j "abcd .. ?? .. xyz" ^^ || ji Action: Don't do anything (the output is correct) Example: input = "abcd" After 2 iterations, i points to 'c' and j points to 'b' Don't need to add any characters to the output !

Assignment:

I have provided a test program named TestFoldString1.java that test the foldString() method.

Use these command to compile and test:

javac TestFoldString1.java java TestFoldString1

Correct output:

s = a, folded = a correct s = ab, folded = ab correct s = abc, folded = acb correct s = abcd, folded = adbc correct s = abcdegh, folded = ahbgced correct s = abcdefghi, folded = aibhcgdfe correct

Turn in
- You must turn in your work by enter these terminal commands:
  before midnight today.
For the Lead TA:

Extra exercise (to improve your programming skills)

Extra exercise:

Algorithm Description:

Use 1 pointer i Use 2 variables to collect to unfolded string: out1 contains the prefix of the unfolded string out2 contains the postfix of the unfolded string
How to unfold a string: input = "abcdefghijk...." Start here: "abcdefghijk...." ^ | i the character at position i goes in the prefix (out1="a") the character at position i+1 goes in the postfix (out2="b") (Make sure they go in the right order; use the concatenation operation WISELY !) Advance i by 2 !!!: "abcdefghijk...." ^ | i the character at position i goes in the prefix (out1="ac") the character at position i+1 goes in the postfix (out2="db") (Make sure they go in the right order; use the concatenation operation WISELY !) Advance i by 2 !!!: "abcdefghijk...." ^ | i the character at position i goes in the prefix (out1="ace") the character at position i+1 goes in the postfix (out2="fdb") (Make sure they go in the right order; use the concatenation operation WISELY !) And so on.
You need to handle 2 different cases: 1. input has an odd number of characters 2. input has an even number of characters

I have provided a test program named TestFoldString1.java that test the foldString() method.

Use these command to compile and test:

javac TestFoldString2.java java TestFoldString2

Correct output:

s = a, unfolded(fold(s)) = a correct s = ab, unfolded(fold(s)) = ab correct s = abcdefgh, unfolded(fold(s)) = abcdefgh correct s = abcdefghi, unfolded(fold(s)) = abcdefghi correct