Problem Set 9 - Due Tue, Nov. 26, 2019 at 23:59pm


  1. New material: Slides and notebooks from Lec 18 File Operations, Lec 20 Recursion, Lab 11: File Operations, and Lab12: Recurison.
  2. Think Python, Ch. 14: Files and Think Python, Ch. 5 Section 8: Recursion

About this Problem Set

The purpose of this problem set is to give you practice with:

There are four tasks:

Use this shared Google Doc to find a pair programming partner.


Task 0: Scrambled Solutions

This is an individual problem which you must complete on your own, although you can ask for help from the CS111 staff.

This time we have two puzzles for you to solve to review the solutions to problem set 8. Go to:

CS 111 Puzzles

and select the options under Problem Set 8.

As before, please download and submit your solution files, and email if you run into trouble.

Task 1: buildHourglass and growHourglass

For this task you must write two functions: buildHourglass which creates and returns a multi-line string, and growHourglass, which reads an hourglass file and writes a larger hourglass to a different file. In this task and the next task, an "hourglass" is made of alternating rows of characters like these three examples:

         -----      ======
%%%       |||        ****
 $         -          ==
%%%       |||        ****
         -----      ======

Subtask 1a: buildHourglass

buildHourglass accepts three parameters: the size (i.e., the length of the top and bottom rows), and the two characters to use (the first argument specifies the character for the top and bottom rows, and the two characters alternate every row. So for example, buildHourglass(6, '-', '|') would produce the following multi-line string:


That string could also be written as (note the new line at the end):

"------\n ||||\n  --\n ||||\n------\n"

In addition to filling out the buildHourglass function in the file, you must also fill out its docstring, describing in your own words what the function does.

You may create the buildHourglass function using whatever programming techniques you wish; it does not have to use recursion. Using some kind of loop is a natural approach to the problem.

Subtask 1b: growHourglass

Another way to approach building hourglass patterns is to grow them row by row from the inside out. For this subtask, you will write a function growHourglass that takes two parameters: an input filename and an output filename. It must look at the hourglass inside the input file to figure out what characters are used and how large the longest row is, and then it must write a new hourglass into the output file that has two more rows than the hourglass in the input file (i.e., one more row on the top and an identical extra row on the bottom, to make the hourglass 2 characters wider).

For example, if the input file contains this hourglass:


then growHourglass should write the following hourglass into the output file:


If the input file has too few rows to determine what character to use to expand it, the hyphen character ('-') should be used. Besides such cases, you may assume that the input file will always contain a valid hourglass pattern.

As with the previous subtask, you need to fill in the docstring for the growHourglass function with an explanation of what it does.

Task 2: Recursive Hourglass

For this task, you will need to write an hourglassRecursive function which prints an hourglass shape, just like the ones from Task 1. The complication is that this hourglass function must be recursive!

In particular, you are not allowed to use any loops, and the hourglassRecursive function must include a function call to itself. Remember to think about your base case, and how the recursive case will build on the base case one step at a time.

As with the previous functions, you must fill out the docstring for this function. The hourglassRecursive function takes one more parameter than buildHourglass from Task 1 (as the first parameter): a number of spaces to use for indentation. This parameter is important for enabling recursion in hourglassRecursive.

For example, the call hourglassRecursive(4, 7, '*', '+'), would print the following output:


Task 3: Shrubs

(Use this shared Google Doc to find a pair programming partner.)

For this task, you will create a recursive shrub function that draws a branching pattern using turtle graphics. Here is an example of the output:

A branching pattern.

Subtask 3a: shrub

In the shrub, everything starts with a trunk, and the first parameter of the shrub function specifies the length of the trunk. Each branch is 80% as long as the trunk that it branches out from, and is at a 30° angle either left or right from the trunk orientation (so there is a 60° angle between the two branches).

The second parameter to shrub sets an upper limit on the total number of branches that are drawn (although in some cases fewer branches may be drawn). In order for this limit to be enforced, the recursive calls to shrub must be given smaller limit values than the original call receives.

Your job is to fill in the shrub function in so that it draws the exact shape shown above when given a trunk length of 100 and a branch limit of 63. It should also draw the following shapes for shrub(100, 7) and shrub(50, 15):

A shrub with 7 branches and base length 100. A shrub with 15 branches and base length 50.

Note one more important fact: when the shrub function is finished, the turtle cursor ends up back in the exact position it started from. This is a critical factor in making the recursion work properly (shrub must be recursive and may not use loops).

As with tasks 1 and 2, you must also fill in the docstring for the shrub function.

Subtask 3b: fancyShrub

Now that your shrub function is working, copy your original code into a second function called fancyShrub. For this function, modify your code so that the color and thickness of each shrub branch changes in different parts of the shrub (use the pensize and pencolor functions).

It's completely up to you how to set the color and thickness, and you are free to change the branching angle, number of branches, and branch size as well, as long as your function still respects the trunkLength and branchesAllowed parameters (in particular, it may not draw more than the given number of allowed branches, although it is fine if it draws fewer branches). Your docstring should explain how the fancy shrub is drawn. Here are some examples of possible fancy shrubs:

A fancy shrub with brown branches, green leaves, and pink flowers. A metalic 'shrub' with branches at 60° angles and a silvery color. A fern that branches differently to the right and to the left.

Along with the code in your file, when your fancyShrub function is done, take a screenshot of it and submit a file named myShrub.png showcasing what it looks like.

As long as your fancyShrub function varies the pen width and color at different layers of the shrub, and respects the trunkLength and branchesAllowed parameters, you will get full credit for this subtask.

Honor Code Form and Final Checks

  1. As in the previous psets, your honor code submission for this pset will involve entering values for the variables in the file.
  2. If you wrote any function invocations or print statements in your Python files to test your code, please remove them, comment them out before you submit, or wrap them in a if __name__=='__main__' block at the end of a .py file. Points will be deducted for isolated function invocations or superfluous print statements.
  3. Note that there are no Codder checks for this assignment: you have to test things on your own.

How to turn in this Problem Set

Electronic submission