CS170 Homework 4: Operators and If-statements

Preparation
- This is your first programming assignment and you must learn to project your computer files from illegal access by other students.
  To disallow other students from reading your homework programs, you must save your file in a directory inside your cs170 directory.
- So will will first create a directory called hw4 directory inside your cs170 project directory to save your hw4 files.
  To do this, execute this command in a terminal window:
- You must use ~/cs170/hw4 directory as your current directory when editing any program files for hw4.
  To change your current directory to ~/cs170/hw4, use this command:
- After making ~/cs170/hw4 the current directory, you can run gedit to edit your programs:
  The name yourProgramName is the name of the Java program (and also the name of the class.
  Remember:

Question 1 (30 pts)

Write a Java program called Operators.java that reads in 2 integer values and then prints:

their sum
the difference of the first and the second number
their product
the quotient of the first and the second number
the remainder of the first and the second number

This is what you should see when you run the program and enters 7 as the first number and 5 as the second number:

UNIX>> java Operators Enter the first number: 7 Enter the second number: 5 7 + 5 = 12 7 - 5 = 2 7 * 5 = 35 7 / 5 = 1 7 % 5 = 2

Notes:

The filename of the first program must be Operator.java and the name of the class must be public class Operators.
Make sure your current directory is /home/yourID/cs170/hw4 when you gedit your program.
To get you started, here is a Java program discussed in class that reads in a number of seconds and prints the seconds in hours, minutes and seconds: click here
You can cut and paste the relevant statements into your Operators.java program (so you don't have to type everything in from scratch).

You can see how the correct solution should work by running this command:

Question 2 (30 pts)

Write a Java program called FirstEqLast.java that reads in a word (String) and then prints:

"Yes", if the first character in the word is equal to the last character in the word, and
"No", otherwise.

This is what you should see when you run the program and enters atlanta

UNIX>> java FirstEqLast Enter a word: atlanta Yes

This is what you should see when you run the program and enters boston

UNIX>> java FirstEqLast Enter a word: boston No

Relevant materials:

Lecture note on Character
Lecture note on String (how to read in a word).
stringVar.charAt(i) returns the character at position i in the string variable stringVar
stringVar.length()−1 is the position of the last character of the string stored in the variable stringVar

You can see how the correct solution should work by running this command:

Question 3 (40 pts)

In class, I showed you the following ShowCurrentTime.java Java program that prints the current time:

public class ShowCurrentTime { public static void main(String[] args) { long nMillis; // We need long for accuracy !!! long n, hours, minutes, seconds; nMillis = System.currentTimeMillis(); n = nMillis/1000; // Total # Second; seconds = n % 60; // Seconds n = n / 60; // Total # minutes; minutes = n % 60; // Minutes n = n / 60; // Total # hours; hours = n % 24; // Hours System.out.print(hours); System.out.print(":"); System.out.print(minutes); System.out.print(":"); System.out.print(seconds); System.out.println(" GMT"); } }

The usual output of the time is as follows:

hh:mm:ss

In other words, we always use 2 digits for to display the hours, minutes and seconds,

When the hour, minute or second is less than 10, the output of the program deviates from the usual format, e.g.:

7:8:9 (Standard: 07:08:09)

Edit your own ShowCurrentTime.java program and cut and paste the above ShowCurrentTime.java into your ShowCurrentTime.java program.

Then add some statement into the program to make the program print the time in the standard format.

Hint:

When the hours is less than 10, print a "0" before printing the variable hours.
Do the same with the variables minutes and seconds.

You can see how the correct solution should work by running this command:

Turn in

When your are done with this homework, you should have these files inside your /home/yourID/cs170/hw4 directory:

You must turn these program in using these commands from inside your hw4 directory:

cd ~/cs170/hw4 (omit this if your current directory is hw4) /home/cs170002/turnin-hw Operators.java hw4 /home/cs170002/turnin-hw FirstEqLast.java hw4a /home/cs170002/turnin-hw ShowCurrentTime.java hw4b

Your homework is not turned unless the above commands are successful (you will get a "success" message when turn in was successful).

(It works just like turn in command of labs).

Note:

If you work on a project on your laptop, you must do the following to turn in your project:

Upload your programs into your cs170 project directory on Emory
Login to a machine in the MathCS lab (e.g. lab0z)
Change directory to the directory where you have uploaded your files.
Then run the turnin commands given above.

Unlike many other professors, I do not go to students' directories to find stuff.
I rather have them put the files securely into the grader account.
That way I find out who has turned in stuff and who has not easily (look in one place rather than sillion of places).