Monthly Archives: October 2015

RPN Calc Assignment

Use A Stack Class to create a RPN calculator

Part I : RPN Math Class

Input: String representing RPN syntax

Parse the String  (Maybe a separate method)

Calculate the return value of the RPN syntax.

Examples of Syntax

  • “3 -5 * “ -15


IF invalid syntax is entered, your method should throw an ArrithmeticException

Things to Support

  • negative numbers (vs subtraction)
  • decimal points
  • multiple spaces between characters

Part II: The GUI

  • Numbers 0-9
  • negative
  • 5 operators
  • Enter button
  • Clear button
  • System.exit(0) button
  • ** EC: support for keypresses including enter

Part III: **EC : Infix to postfix (allow users to enter numbers in infix)


Send me the jar file.

String Loop Assignments [Java]

Write the body for the methods described below.

Tester File here  (This one does not yet test the newest method)

As always, do not import any external libraries. All of the coding can be accomplished by using just  loops, arrays, strings and variables. Using anything else will lead to total loss of credit on associated methods.

Once you have completed each method, copy and paste the code in this file into your class. Then run the test() method and you will get feedback on your code. Note: This does not test all the methods. I have added new methods since writing that tester file. Also, please make sure that:

  • all methods are spelled/capitalized exactly  as shown on this page
  • variables are camelcase and have meaningful names
  • for i  loops use i as a variable
  • for-each  loops do not use i  as the variable

For each method below, you can assume that the parameters are not null

Part I

New Method 11.16 – Tester file updated 11.17 

boolean isAfterN(String str, String chrs, int index)

DescriptionThis method returns true if chrs  appears in  str  after index . 



int countF(String str)

Description: This method returns the number of times that the the lower case letter ‘f’ occurs in str .

Method Call return value/output
countF(“abcdef”) 1
countF(“abcdfef”) 2
countF(“fff”) 3
countF(“xxx”) 0

boolean hasN_Fs(String str, int n )

Description: This method returns true if str has exactly n occurrences of the letter ‘f’ in it.


String threeTimes(String str)

Description: This method returns the String str concatenated with itself three times

Method Call return value/output
threeTimes(“foog”) “foogfoogfoog”
threeTimes(“abc”) “abcabcabc”
threeTimes(“zt”) “ztztzt”
threeTimes(“”) “”

String nTimes(String str, int n)

Description: This method returns the String str concatenated with itself n times

Method Call return value/output
nTimes(“fg” , 2 ) “fgfg”
nTimes(“abc” , 0 ) “”
nTimes(“zt” , 3 ) “ztztzt”
nTimes(“uiz” , 4 ) “uizuizuizuiz”

int countMiddleChar(String str)

Precondition: str.length() ≥ 3.

Description: This method returns the number of times the middle letter of str appears in str.

Method Call return value/output
countMiddleChar(“acbcb” ) 2
countMiddleChar(“acbcx” ) 1
countMiddleChar(“bbbbb” ) 5
countMiddleChar(“xytbtzy”) 1

int indexOf(String haystack, String needle)

Description: Write your own indexOf() method.  Just to avoid any confusion – you cannot make use of the String’s built in indexOf() method. Our method returns the index of the 1st occurrence of needle in haystack . (Full credit if you can get this to work with Strings whose length is greater than 1).


int countChars(String str, String chars)

@precondtion : chars.length() <= str.lenght()  

Description: This method returns the number of times that chars  occurs in String str .

Method Call return value/output
countChars(“momdadmom” , “dad” ) 1
countChars(“foobofoo” , “foo”) 2
countChars(“foobofoofoo” , “foo”) 3
countChars(“foobofoofoo” , “xy” ) 0

Part II

String[] toArray(String str)

Description: This method returns an array comprised of the individual characters of str 

Method Call return value/output
toArray(“abc”  ) { “a”, “b”, “c”}
toArray(“xyz”  ) {“x”, “y”, “z”}

String[] toArraySansChar(String str, char ch)

DescriptionThis method returns an array of Strings comprised of  single letter strings extracted from the input String str ; however, we will always skip the char ch :




String[] reversedBy2s(String str)

DescriptionThis method returns an array of Strings comprised of pairs of characters from the input, — in reverse order as shown below:



String reverseBy3s(String str)

DescriptionThis is like the prior method, except you are going by 3’s and returning a String , not an array of Strings.




boolean isPalindrome(String str)

Description: This method returns true  if  str  is a palindrome

Method Call return value/output
isPalindrome(“abc”  ) false
isPalindrome(“aba”  ) true
 isPalindrome(“abba”  )  true



Java Stack Assignment

A Stack is a “LIFO” data structure (last in first out).

Here’ s an animation of how stacks are used in an RPN calculator.

More RPN practice


Create a stack Class that is based off of your linkedList code.


A stack has the following 3 methods




Create an RPN calculator Class




String Assignments SUPA 2015-16

Unit 3 Strings


Alice Unit 2

This Page

      1) Quiz topics overview
      2) Unit 2 assignments

Unit 2 Quiz (mid -november-ish)

  • Review sheet with answers posted in Google Drive
  • converting other bases to decimals (unit 1)
  • methods vs functions
    • definitions
    • meaning of ‘return a value’
    • screenshots from Alice with connection to these definitions
  • variables
    • definition
    • types of variables
      • number
      • boolean
      • string
    • use of strings
  • identify where the ‘local variable’ button is in Alice



Drag Racing:

      1.  ( DO NOT SUBMIT THIS VIA Google classroom.   Show me this in class)
      2. Use at least 1  comment  Link
      3. Create a world with a road (from the City collection) and two cars (from the Vehicles collection).
      4. Resize the road as necessary to match the size of the cars. The cars should be lined up side by side at one end of the road, which is a drag strip.
      5. Put an object at the end of the race to signify the ‘finish line’
      6. When the world is played, the cars should simultaneously move to the other end of the road and stop (You must use “do together”).
      7. use the “move” method for both carsmove-method
      8. Change the duration  of the move method (see pic below) so that one car arrives before the other).
      9. Position the camera  so you can see the cars as they approach the end of the drag strip.
      10.  use a function  to figure out how far they should move (You want to replace the value “5 meters” with a distance function)

Then modify it

      1. create a variable called  driveDistance  and set its value to 1
        1. set the value of driveDistance   to the returned value of a the distance in front of  function
      2.  the car should move forward   driveDistance   meters

Space Ship Repair


Create a space world with an astronaut and two spaceships(from the Web gallery’s Space collection). One of the spaceships is stranded with engine trouble, and the astronaut has arrived in the other spaceship to perform a repair.

      1. The astronaut should initially be positioned just outside one of the spaceships, about to perform a space walk to the other ship. When the world is played, the astronaut should initially be positioned just outside one of the spaceships, about to perform a space walk to the other ship.
      2. create a variable called walkingDistance (to represent how far the astronaut must walk)
      3. set the value of walkingDistance  to the returned value of  a proximity function.screenshot.44
      4. When the world is played, the astronaut should float to the stranded spaceship, appear to work for a few moments, and then float back to the rescue ship. Hint : to get the spaceship guy to move in the right way (towards the ship). Use the “turn to face” method turn-to-face
      5. The repaired ship should then fly away, off the screen.

Before submitting check out some examples of code that prior students used and that did not get full credit here .


Apollo 15


      • During the Apollo 15 mission to the moon, astronaut David Scott performed an experiment to prove that Galileo was right when he said that any two objects dropped at the same time would land on the ground at the same time in the absence of air. Scott, standing on the surface of the moon, dropped a hammer and a feather and indeed both objects hit the ground simultaneously.(Again, you must use “do together”)
        1. Create an Alice world that recreates the experiment. You will find classes for the astronaut, lunar Lander, and the moon’s surface in the Web gallery’s Space collection. You can find a class for the Hammer in the Objects collection. Substitute any object of your choice for the feather.
        2. Create a variable called  dropDistance
        3. set the value of this variable to the returned value of a distance function to make the objects fall the distance above the ground. Use screenshot.1289
        4. Make the two objects fall down the value of the dropDistance



B Before handing it in, look at some examples of work that students did and that did not receive full credit,. here.


End of Unit Open assignment

      • Tell a short story that
      • at least 3 objects
      • at least 2  variables
      • at least 2 functions
        • sets the value of a variable to a returned value of a function
      • uses that variable to move one of the objects
        • ie make sure that your variable is actually used to do something.

Below is just one example. Your story can be about any topic.


Old assignments (ignore)

Penguin on a Table: Create a world with a penguin and a table. The penguin should be facing the table, standing next to it. The penguin should simultaneously flap its wings (use the wing_flap method) and move up in the air a distance that is exactly 1 meter higher than the height of the table. You can call the table’s height function to get its height. The penguin should then move forward to approximately the center of the table, and then move down to the surface of the table


What as a String