writingscripts.html

Shall高级编程

></SPAN
>,
	      so you don't end up complaining about yourself.</P
><P
>As necessary, use the appropriate <A
HREF="communications.html#COMMUNINFO1"
>network analysis commands</A
>.</P
><P
>For some ideas, see <A
HREF="communications.html#ISSPAMMER"
>Example 15-40</A
> and <A
HREF="contributed-scripts.html#ISSPAMMER2"
>Example A-30</A
>.</P
><P
>Optional: Write a script that searches through a list of
	      e-mail messages and deletes the spam according to specified
	      filters.</P
></DD
><DT
><B
CLASS="COMMAND"
>Creating man pages</B
></DT
><DD
><P
>Write a script that automates the process of creating
	      <A
HREF="external.html#MANREF"
>man pages</A
>.</P
><P
>Given a text file which contains information to be
	      formatted into a <I
CLASS="FIRSTTERM"
>man page</I
>, the
	      script will read the file, then invoke the appropriate
	      <A
HREF="textproc.html#GROFFREF"
>groff</A
> commands to
	      output the corresponding <I
CLASS="FIRSTTERM"
>man page</I
>
	      to <TT
CLASS="FILENAME"
>stdout</TT
>. The text file contains
	      blocks of information under the standard <I
CLASS="FIRSTTERM"
>man
	      page</I
> headings, i.e., <SPAN
CLASS="QUOTE"
>"NAME,"</SPAN
>
	      <SPAN
CLASS="QUOTE"
>"SYNOPSIS,"</SPAN
> <SPAN
CLASS="QUOTE"
>"DESCRIPTION,"</SPAN
>
	      etc.</P
><P
>See <A
HREF="textproc.html#MANVIEW"
>Example 15-28</A
>.</P
></DD
><DT
><B
CLASS="COMMAND"
>Morse Code</B
></DT
><DD
><P
>Convert a text file to Morse code. Each character of the
	      text file will be represented as a corresponding Morse
	      code group of dots and dashes (underscores), separated by
	      whitespace from the next. For example:
		<TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;Invoke the "morse.sh" script with "script"
   2&nbsp;as an argument to convert to Morse.
   3&nbsp;
   4&nbsp;
   5&nbsp;$ sh morse.sh script
   6&nbsp;
   7&nbsp;... _._. ._. .. .__. _
   8&nbsp;s   c    r   i   p   t</PRE
></TD
></TR
></TABLE
></P
></DD
><DT
><B
CLASS="COMMAND"
>Hex Dump</B
></DT
><DD
><P
>Do a hex(adecimal) dump on a binary file
	      specified as an argument. The output should be in neat
	      tabular fields, with the first field showing the address,
	      each of the next 8 fields a 4-byte hex number, and the final
	      field the ASCII equivalent of the previous 8 fields.</P
><P
>The obvious followup to this is to extend the hex dump
	      script into a disassembler. Using a lookup table, or some other
	      clever gimmick, convert the hex values into 80x86 op
	      codes.</P
></DD
><DT
><B
CLASS="COMMAND"
>Emulating a Shift Register</B
></DT
><DD
><P
>Using <A
HREF="arrays.html#STACKEX"
>Example 26-15</A
> as an inspiration,
	      write a script that emulates a 64-bit shift register as
	      an <A
HREF="arrays.html#ARRAYREF"
>array</A
>. Implement
	      functions to <I
CLASS="FIRSTTERM"
>load</I
> the register,
	      <I
CLASS="FIRSTTERM"
>shift left</I
>, <I
CLASS="FIRSTTERM"
>shift
	      right</I
>, and <I
CLASS="FIRSTTERM"
>rotate</I
>
	      it. Finally, write a function that interprets the register
	      contents as eight 8-bit ASCII characters.</P
></DD
><DT
><B
CLASS="COMMAND"
>Determinant</B
></DT
><DD
><P
>Solve a 4 x 4 determinant.</P
></DD
><DT
><B
CLASS="COMMAND"
>Hidden Words</B
></DT
><DD
><P
>Write a <SPAN
CLASS="QUOTE"
>"word-find"</SPAN
> puzzle generator,
	      a script that hides 10 input words in a 10 x 10 matrix
	      of random letters. The words may be hidden across, down,
	      or diagonally.</P
><P
>Optional: Write a script that <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>solves</I
></SPAN
>
	      word-find puzzles. To keep this from becoming too difficult,
	      the solution script will find only horizontal and vertical
	      words. (Hint: Treat each row and column as a string, and
	      search for substrings.)</P
></DD
><DT
><B
CLASS="COMMAND"
>Anagramming</B
></DT
><DD
><P
> Anagram 4-letter input. For example, the
	      anagrams of <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>word</I
></SPAN
> are:
	      <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>do or rod row word</I
></SPAN
>. You may use
	      <TT
CLASS="FILENAME"
>/usr/share/dict/linux.words</TT
> as the
	      reference list.</P
></DD
><DT
><B
CLASS="COMMAND"
>Word Ladders</B
></DT
><DD
><P
>A <SPAN
CLASS="QUOTE"
>"word ladder"</SPAN
> is a sequence of words,
              with each successive word in the sequence differing from
              the previous one by a single letter.</P
><P
>For example, to <SPAN
CLASS="QUOTE"
>"ladder"</SPAN
> from
              <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>mark</I
></SPAN
> to
              <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>vase</I
></SPAN
>:</P
><P
>	    <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;mark --&#62; park --&#62; part --&#62; past --&#62; vast --&#62; vase
   2&nbsp;         ^           ^       ^      ^           ^</PRE
></TD
></TR
></TABLE
>
            </P
><P
>Write a script that solves word ladder puzzles. Given
	      a starting and an ending word, the script will list all
	      intermediate steps in the <SPAN
CLASS="QUOTE"
>"ladder."</SPAN
> Note
	      that <SPAN
CLASS="emphasis"
><I
CLASS="EMPHASIS"
>all</I
></SPAN
> words in the sequence must
	      be legitimate dictionary words.</P
></DD
><DT
><B
CLASS="COMMAND"
>Fog Index</B
></DT
><DD
><P
>The <SPAN
CLASS="QUOTE"
>"fog index"</SPAN
> of a passage of text
	      estimates its reading difficulty, as a number corresponding
	      roughly to a school grade level. For example, a passage
	      with a fog index of 12 should be comprehensible to anyone
	      with 12 years of schooling.</P
><P
>The Gunning version of the fog index uses the following
	      algorithm.</P
><OL
TYPE="1"
><LI
><P
>Choose a section of the text at least
                 100 words in length.</P
></LI
><LI
><P
>Count the number of sentences (a portion of
                 a sentence truncated by the boundary of the text section
                 counts as one).</P
></LI
><LI
><P
>Find the average number of words per
                   sentence.</P
><P
>AVE_WDS_SEN = TOTAL_WORDS / SENTENCES</P
></LI
><LI
><P
>Count the number of <SPAN
CLASS="QUOTE"
>"difficult"</SPAN
>
		   words in the segment -- those containing at least
		   3 syllables. Divide this quantity by total words to
		   get the proportion of difficult words.</P
><P
>PRO_DIFF_WORDS = LONG_WORDS / TOTAL_WORDS</P
></LI
><LI
><P
>The Gunning fog index is the sum of the above two
                   quantities, multiplied by 0.4, then rounded to the
                   nearest integer.</P
><P
>G_FOG_INDEX = int ( 0.4 * ( AVE_WDS_SEN  + PRO_DIFF_WORDS ) )</P
></LI
></OL
><P
>Step 4 is by far the most difficult portion of the
	      exercise. There exist various algorithms for estimating
	      the syllable count of a word. A rule-of-thumb formula
	      might consider the number of letters in a word and the
	      vowel-consonant mix.</P
><P
>A strict interpretation of the Gunning fog index does
	      not count compound words and proper nouns as
	      <SPAN
CLASS="QUOTE"
>"difficult"</SPAN
> words, but this would enormously
	      complicate the script.</P
></DD
><DT
><B
CLASS="COMMAND"
>Calculating PI using Buffon's Needle</B
></DT
><DD
><P
>The Eighteenth Century French mathematician de Buffon
	      came up with a novel experiment. Repeatedly drop a
	      needle of length <SPAN
CLASS="QUOTE"
>"n"</SPAN
> onto a wooden floor
	      composed of long and narrow parallel boards. The cracks
	      separating the equal-width floorboards are a fixed distance
	      <SPAN
CLASS="QUOTE"
>"d"</SPAN
> apart. Keep track of the total drops and
	      the number of times the needle intersects a crack on the
	      floor. The ratio of these two quantities turns out to be
	      a fractional multiple of PI.</P
><P
>In the spirit of <A
HREF="mathc.html#CANNON"
>Example 15-48</A
>, write a
	      script that runs a Monte Carlo simulation of Buffon's
	      Needle. To simplify matters, set the needle length
	      equal to the distance between the cracks, <TT
CLASS="PARAMETER"
><I
>n =
	      d</I
></TT
>.</P
><P
>Hint: there are actually two critical variables:
              the distance from the center of the needle to the crack
              nearest to it, and the angle of the needle to that
              crack. You may use <A
HREF="mathc.html#BCREF"
>bc</A
> to
              handle the calculations.</P
></DD
><DT
><B
CLASS="COMMAND"
>Playfair Cipher</B
></DT
><DD
><P
>Implement the Playfair (Wheatstone) Cipher in a
	      script.</P
><P
>The Playfair Cipher encrypts text by substitution
	      of <SPAN
CLASS="QUOTE"
>"digrams"</SPAN
> (2-letter groupings).  It is
	      traditional to use a 5 x 5 letter scrambled-alphabet
	      <I
CLASS="FIRSTTERM"
>key square</I
> for the encryption and
	      decryption.</P
><P
>	     <TABLE
BORDER="0"
BGCOLOR="#E0E0E0"
WIDTH="90%"
><TR
><TD
><PRE
CLASS="PROGRAMLISTING"
>   1&nbsp;   C O D E S
   2&nbsp;   A B F G H
   3&nbsp;   I K L M N
   4&nbsp;   P Q R T U
   5&nbsp;   V W X Y Z
   6&nbsp;
   7&nbsp;Each letter of the alphabet appears once, except "I" also represents
   8&nbsp;"J". The arbitrarily chosen key word, "CODES" comes first, then all
   9&nbsp;the rest of the alphabet, in order from left to right, skipping letters
  10&nbsp;already used.
  11&nbsp;
  12&nbsp;To encrypt, separate the plaintext message into digrams (2-letter
  13&nbsp;groups). If a group has two identical letters, delete the second, and
  14&nbsp;form a new group. If there is a single letter left over at the end,
  15&nbsp;insert a "null" character, typically an "X."
  16&nbsp;
  17&nbsp;THIS IS A TOP SECRET MESSAGE
  18&nbsp;
  19&nbsp;TH IS IS AT OP SE CR ET ME SA GE
  20&nbsp;
  21&nbsp;For each digram, there are three possibilities.
  22&nbsp;----------------------------------------------
  23&nbsp;1) Both letters will be on the same row of the key square
  24&nbsp;   For each letter, substitute the one immediately to the right, in that
  25&nbsp;   row. If necessary, wrap around left to the beginning of the row.
  26&nbsp;
  27&nbsp;or
  28&nbsp;
  29&nbsp;2) Both letters will be in the same column of the key square
  30&nbsp;   For each letter, substitute the one immediately below it, in that
  31&nbsp;   row. If necessary, wrap around to the top of the column.
  32&nbsp;
  33&nbsp;or
  34&nbsp;
  35&nbsp;3) Both letters will form the corners of a rectangle within the key
  36&nbsp;   square. For each letter, substitute the one on the other corner the
  37&nbsp;   rectangle which lies on the same row.
  38&nbsp;
  39&nbsp;
  40&nbsp;The "TH" digram falls under case #3.
  41&nbsp;G H
  42&nbsp;M N
  43&nbsp;T U           (Rectangle with "T" and "H" at corners)
  44&nbsp;
  45&nbsp;T --&#62; U
  46&nbsp;H --&#62; G
  47&nbsp;
  48&nbsp;
  49&nbsp;The "SE" digram falls under case #1.
  50&nbsp;C O D E S     (Row containing "S" and "E")
  51&nbsp;
  52&nbsp;S --&#62; C  (wraps around left to beginning of row)
  53&nbsp;E --&#62; S
  54&nbsp;
  55&nbsp;=========================================================================
  56&nbsp;
  57&nbsp;To decrypt encrypted text, reverse the above procedure under cases #1
  58&nbsp;and #2 (move in opposite direction for substitution). Under case #3,
  59&nbsp;just take the remaining two corners of the rectangle.
  60&nbsp;
  61&nbsp;
  62&nbsp;Helen Fouche Gaines' classic work, ELEMENTARY CRYPTANALYSIS (1939), gives a
  63&nbsp;fairly detailed rundown on the Playfair Cipher and its solution methods.</PRE
></TD
></TR
></TABLE
>
             </P
><P
>This script will have three main sections</P
><OL
TYPE="I"
><LI
><P
>Generating the <SPAN
CLASS="QUOTE"
>"key square"</SPAN
>,
	         based on a user-input keyword.</P
></LI
><LI
><P
>Encrypting a <SPAN
CLASS="QUOTE"
>"plaintext"</SPAN
>
	         message.</P
></LI
><LI
><P
>Decrypting encrypted
	         text.</P
></LI
></OL
><P
>The script will make extensive use of <A
HREF="arrays.html#ARRAYREF"
>arrays</A
> and <A
HREF="functions.html#FUNCTIONREF"
>functions</A
>.</P
></DD
></DL
></DIV
><P
>--</P
><P
>Please do not send the author your solutions to these
            exercises. There are better ways to impress him with your
            cleverness, such as submitting bugfixes and suggestions for
            improving this book.</P
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