CMU 15-112 Spring 2017: Fundamentals of Programming and Computer Science
Homework 3 (Due Saturday 16-Sep, at 8pm)

  1. mostFrequentLetters(s) [30pts]
    Write the function mostFrequentLetters(s), that takes a string s, and ignoring case (so "A" and "a" are treated the same), returns a lowercase string containing the letters of s in most frequently used order. (In the event of a tie between two letters, follow alphabetic order.) So:
       mostFrequentLetters("We Attack at Dawn")
    returns "atwcdekn". Note that digits, punctuation, and whitespace are not letters! Also note that seeing as we have not yet covered lists, sets, maps, or efficiency, you are not expected to write the most efficient solution. (And you should not use lists, sets, or maps in your solution.) Finally, if s does not contain any alphabetic characters, the result should be the empty string ("").

  2. patternedMessage(message, pattern) [30 pts]
    Write the function patternedMessage(message, pattern) that takes two strings, a message and a pattern, and returns a string produced by replacing the non-whitespace characters in the pattern with the non-whitespace characters in the message (where any leading or trailing newlines in the pattern are first removed). As a first example:

    patternedMessage("Go Pirates!!!", """
    ******   ******
    irates   !!!GoP

    Here, the message is "Go Pirates!!!" and the pattern is a block of asterisks with a few missing in the middle. Notice how the whitespace in the pattern is preserved, but the whitespace in the message is removed. Again, note that any leading or trailing newlines in the pattern are removed.

    Here is another example:

    patternedMessage("Three Diamonds!","""
        *     *     *
       ***   ***   ***
      ***** ***** *****
       ***   ***   ***
        *     *     *
        T     h     r
       eeD   iam   ond
      s!Thr eeDia monds
       !Th   ree   Dia
        m     o     n

    Hint: While you may solve this how you wish, our sample solution did not use replace in any way. Instead, we started with the empty string, and built up the result character by character. How did we determine the next character? Using both the message and the pattern in some way...

    And here is one last example, just for fun:

    patternedMessage("Go Steelers!",
                       oo$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$o         o$   $$ o$
       o $ oo        o$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$o       $$ $$ $$o$
    oo $ $ '$      o$$$$$$$$$    $$$$$$$$$$$$$    $$$$$$$$$o       $$$o$$o$
    '$$$$$$o$     o$$$$$$$$$      $$$$$$$$$$$      $$$$$$$$$$o    $$$$$$$$
      $$$$$$$    $$$$$$$$$$$      $$$$$$$$$$$      $$$$$$$$$$$$$$$$$$$$$$$
      $$$$$$$$$$$$$$$$$$$$$$$    $$$$$$$$$$$$$    $$$$$$$$$$$$$$  '$$$
       '$$$'$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$     '$$$
        $$$   o$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$     '$$$o
       o$$'   $$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$       $$$o
       $$$    $$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$' '$$$$$$ooooo$$$$o
      o$$$oooo$$$$$  $$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$   o$$$$$$$$$$$$$$$$$
      $$$$$$$$'$$$$   $$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$     $$$$'
     ''''       $$$$    '$$$$$$$$$$$$$$$$$$$$$$$$$$$$'      o$$$
                '$$$o     '$$$$$$$$$$$$$$$$$$'$$'         $$$
                  $$$o          '$$'$$$$$$'           o$$$
                   $$$$o                                o$$$'
                    '$$$$o      o$$$$$$o'$$$$o        o$$$$
                      '$$$$$oo     '$$$$o$$$$$o   o$$$$'
                         '$$$$$oooo  '$$$o$$$$$$$$$'
                            '$$$$$$$oo $$$$$$$$$$

    Returns this:

                       teelers!GoSteelers!GoSteelers!GoS         te   el er
       s ! Go        Steelers!GoSteelers!GoSteelers!GoSteel       er s! GoSt
    ee l e rs      !GoSteeler    s!GoSteelers!    GoSteelers       !GoSteel
    ers!GoSte     elers!GoSt      eelers!GoSt      eelers!GoSt    eelers!G
      oSteele    rs!GoSteele      rs!GoSteele      rs!GoSteelers!GoSteeler
      s!GoSteelers!GoSteelers    !GoSteelers!G    oSteelers!GoSt  eele
       rs!GoSteelers!GoSteelers!GoSteelers!GoSteelers!GoSteel     ers!
        GoS   teelers!GoSteelers!GoSteelers!GoSteelers!GoSteelers     !GoSt
       eele   rs!GoSteelers!GoSteelers!GoSteelers!GoSteelers!GoSt       eele
       rs!    GoSteelers!GoSteelers!GoSteelers!GoSteelers!Go Steelers!GoSteele
      rs!GoSteelers  !GoSteelers!GoSteelers!GoSteelers!GoS   teelers!GoSteelers
      !GoSteelers!G   oSteelers!GoSteelers!GoSteelers!Go     Steel
     ers!       GoSt    eelers!GoSteelers!GoSteelers!G      oSte
                elers     !GoSteelers!GoSteelers!         GoS
                  teel          ers!GoSteel           ers!
                   GoSte                                elers
                    !GoSte      elers!GoSteele        rs!Go
                      Steelers     !GoSteelers!   GoStee
                         lers!GoSte  elers!GoSteeler
                            s!GoSteele rs!GoSteel

  3. decodeColumnShuffleCipher(encodedMessage) [30 pts]
    Write the function decodeColumnShuffleCipher, which takes an encoding from encodeColumnShuffleCipher in lab3 and runs it in reverse, returning the plaintext that generated the encoding. For example, decodeColumnShuffleCipher("0213WTAWACD-EATNTKA-") returns "WEATTACKATDAWN".

    Reminder: Even though you wrote encodeColumnShuffleCipher() in a collaborative group, this problem is not collaborative. You must solve it solo.

  4. decodeColumnShuffleCipherNoDashes(encodedMessage) [10 pts]
    Imagine an alternative version of encodeColumnShuffleCipher that does not add dashes to the encoded message.
    Consider the following example:
    Function call: encodeColumnShuffleCipherNoDashes("WEATTACKATDAWN", "0213")
    Key: 0213
    Initial Grid:		Rearranged Grid:
    W E A T			W A E T
    T A C K			T C A K
    A T D A			A D T A
    W N 			W   N  
    Encrypted Message: WTAWACDEATNTKA
    Message to Return: 0213WTAWACDEATNTKA
    Write the function decodeColumnShuffleCipherNoDashes, which takes an encoding as described above, and returns the plaintext that generated the encoding. For example, decodeColumnShuffleCipherNoDashes("0213WTAWACDEATNTKA") returns "WEATTACKATDAWN".

    Note: We have not provided any testcases for you in the homework file. Your are responsible for creating your own testcases.

  5. Bonus/Optional: topLevelFunctionNames(code) [3 pts]
    Write the function topLevelFunctionNames(code) that takes a possibly-multi-line string of Python code, and returns a string with the names of the top-level functions in that code, separated by dots ("."), in the order they appear in the code.

    You may assume that the code is syntactically correct, with no non-ascii or non-printable characters in it. You may further assume that every top-level function is defined with a "def" statement, where the "def" is left-flush (so no spaces before the "def"), following by exactly one space (" "), followed by the function name, followed by no spaces, followed by the open parenthesis for the parameters.

    This task is complicated by the fact that there can be multi-line strings in the code, and a "def" inside a multi-line string is not a function definition, and should be ignored.

    This is further complicated by the fact that there can be comments (#) in the code, and everything after the comment on that line should be ignored, including any potential string delimiters.

    Also, note that comment characters (#) can appear inside strings, in which case they are not the start of comments, and should be ignored.

    Here is a sample test case for you:
        # f is redefined
        code = """\
    def f(x): return x+42
    def g(x): return x+f(x)
    def f(x): return x-42
        assert(topLevelFunctionNames(code) == "f.g")
    And here is another one:
        # g() is in triple-quotes (""")
        code = '''\
    def f(): return """
    def g(): pass"""
        assert(topLevelFunctionNames(code) == "f")

  6. Bonus/Optional: getEvalSteps(expr) [3 pts]
    Write the function getEvalSteps(expr), that takes a string containing a simple arithmetic expression, and returns a multi-line string containing the step-by-step (one operator at a time) evaluation of that expression. For example, this call:
    produces this result (which is a single multi-line string):
    2+3*4-8**3%3 = 2+3*4-512%3
                 = 2+12-512%3
                 = 2+12-2
                 = 14-2
                 = 12
    Here are some considerations and hints:
    • You are only responsible for legal input as described below. Numbers are limited to non-negative integers.
    • Operators are limited to +, -, *, /, //, %, and **.
    • All operators are binary, so they take two operands. So there are no unary operators, meaning "-5" is not a legal input. For that, you'd need "0-5".
    • In fact, the previous restriction is even stronger: no intermediate value of expr can be negative! So "1-2+3" is not legal, since it would be converted first into "-1+3" which is not legal. So you can safely ignore all such cases.
    • There are no parentheses.
    • Operators must be evaluated by precedence (** first, then *,/,//,%, then +,-).
    • Equal-precedence operators are evaluated left-to-right.
    • Evaluation proceeds until a single integer with no operators remains after the equals sign.
    • The equal signs must all be stacked directly over each other.
    • You may write this however you wish, though you may want to write a helper function that finds the next operator to be applied, and another helper function that just applies a single operator. Something like that. In any case, top-down design is crucial here. And don't forget to thoroughly test your helper functions before using them!
    • In our sample solution, we used very few string methods, just "find" and "isdigit". You may use others, but you should not spin your wheels trying to find that awesome string method that will make this problem remarkably easier, as that method does not exist.
    • For this function, as any other function, you may not use the eval function, so you will have to write your own equivalent just for the kinds of simple expressions you will deal with here. Eval is dangerous and should be avoided, as it can lead to serious bugs or security holes.

  7. Bonus/Optional: bonusDecode [3 pts; 1 pt each]
    1. Bonus/Optional: bonusDecode1(msg) [1 pt]
      For this problem, you are given the function bonusEncode1(msg). This function takes a string, msg, and returns an encoded version of the string. The encoding is fairly straightforward. Your job is to write the corresponding decoder. Specifically, write the function bonusDecode1(msg) so that, for any string s:
      assert(bonusDecode1(bonusEncode1(s)) == s)
      To make things more interesting, you are not given the function bonusEncode1(msg), but rather you are given the result of calling that function on itself, so you have the encoded version of the encoder! You still write the plaintext decoder! Here is the encoded encoder:
      efg cpovtEodpef1(nth):
          sftvmu = ""
          gps d jo nth:
              jg (d.jtmpxfs()):
                  d = dis(pse('b') + (pse(d) - pse('b') + 1)%26)
              sftvmu += d
          sfuvso sftvmu
      Have fun!

    2. Bonus/Optional: bonusDecode2(msg) [1 pt]
      Same basic problem: write bonusDecode2(msg) given this self-encoded version of bonusEncode2(msg):
      ddd 7jhnkvd1c00N(5aX):
          0MZ0QX = ""
          I = HHEuyq.izinm_nftscoo + kkh7b3.Y2Z0a8
          PXZ O MQ SAMEB(GyG(DIv)):
              e = kpc[c]
              1X (R VZ Z): I = R[(O.CEIx(u) - v) % tlt(t)]
              j5ij9g += U
          3P33ZU WIVWMT

    3. Bonus/Optional: bonusDecode3(msg) [1 pt]
      Once more: write bonusDecode3(msg) given this self-encoded version of bonusEncode3(msg):