Fixing problems in Chapter 11

AllenDowney · AllenDowney · commit fb6404e543ee · 2015-11-09T11:25:55.000-05:00
diff --git a/book/book.tex b/book/book.tex
@@ -33,7 +33,7 @@
 \title{Think Python}
 \author{Allen B. Downey}
 \newcommand{\thetitle}{Think Python: How to Think Like a Computer Scientist}
-\newcommand{\theversion}{2nd Edition, Version 2.2.10}
+\newcommand{\theversion}{2nd Edition, Version 2.2.14}
 \newcommand{\thedate}{}
 
 % these styles get translated in CSS for the HTML version
@@ -330,10 +330,8 @@ \section*{The strange history of this book}
 
 
 Allen B. Downey \\
-Needham MA\\
 
-Allen Downey is a Professor of Computer Science at 
-the Franklin W. Olin College of Engineering.
+Olin College \\
 
 
 \section*{Acknowledgments}
@@ -376,7 +374,7 @@ \section*{Contributor List}
 huge help.
 
 If you have a suggestion or correction, please send email to 
-{\tt feedback@thinkpython2.com}.  If I make a change based on your
+{\tt feedback@thinkpython.com}.  If I make a change based on your
 feedback, I will add you to the contributor list
 (unless you ask to be omitted).
 
@@ -733,8 +731,14 @@ \section*{Contributor List}
 \item Andrea Zanella translated the book into Italian, and sent a
 number of corrections along the way.
 
-\item Many thanks to Melissa Lewis for excellent comments and 
-suggestions on the second edition.
+\item Many, many thanks to Melissa Lewis and Luciano Ramalho for
+  excellent comments and suggestions on the second edition.
+
+\item Thanks to Harry Percival from PythonAnywhere for his help
+getting people started running Python in a browser.
+
+\item Xavier Van Aubel made several useful corrections in the second
+edition.
 
 % ENDCONTRIB
 
@@ -1098,14 +1102,14 @@ \section{Formal and natural languages}
 Syntax rules come in two flavors, pertaining to {\bf tokens} and
 structure.  Tokens are the basic elements of the language, such as
 words, numbers, and chemical elements.  One of the problems with
-$3 + = 3 \$ 6$ is that \( \$ \) is not a legal token in mathematics
+$3 += 3 \$ 6$ is that \( \$ \) is not a legal token in mathematics
 (at least as far as I know).  Similarly, $_2Zz$ is not legal because
 there is no element with the abbreviation $Zz$.
 \index{token}
 \index{structure}
 
 The second type of syntax rule pertains to the way tokens are
-combined.  The equation $3 + = 3$ is illegal because even though $+$
+combined.  The equation $3 += 3$ is illegal because even though $+$
 and $=$ are legal tokens, you can't have one right after the other.
 Similarly, in a chemical formula the subscript comes after the element
 name, not before.
@@ -4660,8 +4664,8 @@ \section{Incremental development}
     return 0.0
 \end{verbatim}
 %
-If the function is working, it should display \verb"'dx is 3'" and {\tt
-'dy is 4'}.  If so, we know that the function is getting the right
+If the function is working, it should display \verb"dx is 3" and 
+\verb"dy is 4".  If so, we know that the function is getting the right
 arguments and performing the first computation correctly.  If not,
 there are only a few lines to check.
 
@@ -4728,7 +4732,7 @@ \section{Incremental development}
 
 As an exercise, use incremental development to write a function
 called {\tt hypotenuse} that returns the length of the hypotenuse of a
-right triangle given the lengths of the two legs as arguments.
+right triangle given the lengths of the other two legs as arguments.
 Record each stage of the development process as you go.
 \index{hypotenuse}
 
@@ -5144,8 +5148,9 @@ \section{Debugging}
 \label{factdebug}
 
 Breaking a large program into smaller functions creates natural
-checkpoints for debugging.  \index{debugging} If a function is not
+checkpoints for debugging.  If a function is not
 working, there are three possibilities to consider:
+\index{debugging} 
 
 \begin{itemize}
 
@@ -6669,7 +6674,7 @@ \section{Exercises}
 \index{method!string}
 
 Read the documentation of the string methods at
-\url{http://docs.python.org/2/library/stdtypes.html#string-methods}.
+\url{http://docs.python.org/3/library/stdtypes.html#string-methods}.
 You might want to experiment with some of them to make sure you
 understand how they work.  {\tt strip} and {\tt replace} are
 particularly useful.
@@ -6692,7 +6697,7 @@ \section{Exercises}
 There is a string method called {\tt count} that is similar
 to the function in Section~\ref{counter}.  Read the documentation
 of this method
-and write an invocation that counts the number of {\tt a}s
+and write an invocation that counts the number of {\tt a}'s
 in \verb"'banana'".
 \end{exercise}
 
@@ -6703,7 +6708,7 @@ \section{Exercises}
 \index{operator!slice}
 
 A string slice can take a third index that specifies the ``step
-size;'' that is, the number of spaces between successive characters.
+size''; that is, the number of spaces between successive characters.
 A step size of 2 means every other character; 3 means every third,
 etc.
 
@@ -6817,7 +6822,7 @@ \chapter{Case study: word play}
 properties.  For example, we'll find the longest palindromes
 in English and search for words whose letters appear in
 alphabetical order.  And I will present another program development
-plan: reduction to a previously-solved problem.
+plan: reduction to a previously solved problem.
 
 
 \section{Reading word lists}
@@ -7065,18 +7070,18 @@ \section{Search}
 
 If you were really thinking like a computer scientist, you would
 have recognized that \verb"uses_all" was an instance of a
-previously-solved problem, and you would have written:
+previously solved problem, and you would have written:
 
 \begin{verbatim}
 def uses_all(word, required):
     return uses_only(required, word)
 \end{verbatim}
 %
 This is an example of a program development plan called {\bf
-  reduction to a previously-solved problem}, which means that you
+  reduction to a previously solved problem}, which means that you
 recognize the problem you are working on as an instance of a solved
 problem and apply an existing solution.  \index{reduction to a
-  previously-solved problem} \index{development plan!reduction}
+  previously solved problem} \index{development plan!reduction}
 
 
 \section{Looping with indices}
@@ -7160,9 +7165,9 @@ \section{Looping with indices}
     return True
 \end{verbatim}
 
-Or we could reduce to a previously-solved
+Or we could reduce to a previously solved
 problem and write:
-\index{reduction to a previously-solved problem}
+\index{reduction to a previously solved problem}
 \index{development plan!reduction}
 
 \begin{verbatim}
@@ -7226,9 +7231,9 @@ \section{Glossary}
 \index{file object}
 \index{object!file}
 
-\item[reduction to a previously-solved problem:] A way of solving a
-  problem by expressing it as an instance of a previously-solved
-  problem.  \index{reduction to a previously-solved problem}
+\item[reduction to a previously solved problem:] A way of solving a
+  problem by expressing it as an instance of a previously solved
+  problem.  \index{reduction to a previously solved problem}
   \index{development plan!reduction}
 
 \item[special case:] A test case that is atypical or non-obvious
@@ -8365,7 +8370,7 @@ \section{Glossary}
 \section{Exercises}
 
 You can download solutions to these exercises from
-from \url{http://thinkpython2.com/code/list_exercises.py}.
+\url{http://thinkpython2.com/code/list_exercises.py}.
 
 \begin{exercise}
 
@@ -8629,7 +8634,7 @@ \section{A dictionary is a mapping}
 \end{verbatim}
 %
 This line creates an item that maps from the key
-{\tt 'one'} to the value \verb"'uno'".  If we print the
+\verb"'one'" to the value \verb"'uno'".  If we print the
 dictionary again, we see a key-value pair with a colon
 between the key and value:
 
@@ -8666,7 +8671,7 @@ \section{A dictionary is a mapping}
 'dos'
 \end{verbatim}
 %
-The key {\tt 'two'} always maps to the value \verb"'dos'" so the order
+The key \verb"'two'" always maps to the value \verb"'dos'" so the order
 of the items doesn't matter.
 
 If the key isn't in the dictionary, you get an exception:
@@ -8798,7 +8803,7 @@ \section{Dictionary as a collection of counters}
 {'a': 1, 'b': 1, 'o': 2, 'n': 1, 's': 2, 'r': 2, 'u': 2, 't': 1}
 \end{verbatim}
 %
-The histogram indicates that the letters {\tt 'a'} and \verb"'b'"
+The histogram indicates that the letters \verb"'a'" and \verb"'b'"
 appear once; \verb"'o'" appears twice, and so on.
 
 
@@ -8850,14 +8855,23 @@ \section{Looping and dictionaries}
 o 1
 \end{verbatim}
 %
-Again, the keys are in no particular order.
-Dictionaries have a method called {\tt keys} that returns
-the keys of the dictionary, in no particular order, as a list.
-As an exercise, modify \verb"print_hist" to print the keys and their values
-in alphabetical order.
+Again, the keys are in no particular order.  To traverse the keys
+in sorted order, you can use the built-in function {\tt sorted}:
 \index{keys method}
 \index{method!keys}
 
+\begin{verbatim}
+>>> for key in sorted(h):
+...     print(key, h[key])
+a 1
+o 1
+p 1
+r 2
+t 1
+\end{verbatim}
+
+%TODO: get this on Atlas
+
 
 \section{Reverse lookup}
 \label{raise}
@@ -8891,7 +8905,7 @@ \section{Reverse lookup}
 This function is yet another example of the search pattern, but it
 uses a feature we haven't seen before, {\tt raise}.  The 
 {\bf raise statement} causes an exception; in this case it causes a
-{\tt LookupError}, which is a built-in exception use to indicate
+{\tt LookupError}, which is a built-in exception used to indicate
 that a lookup operation failed.
 \index{search}
 \index{pattern!search} \index{raise statement} \index{statement!raise}
@@ -8905,19 +8919,19 @@ \section{Reverse lookup}
 
 \begin{verbatim}
 >>> h = histogram('parrot')
->>> k = reverse_lookup(h, 2)
->>> k
+>>> key = reverse_lookup(h, 2)
+>>> key
 'r'
 \end{verbatim}
 %
 And an unsuccessful one:
 
 \begin{verbatim}
->>> k = reverse_lookup(h, 3)
+>>> key = reverse_lookup(h, 3)
 Traceback (most recent call last):
   File "<stdin>", line 1, in <module>
   File "<stdin>", line 5, in reverse_lookup
-ValueError
+LookupError
 \end{verbatim}
 %
 The effect when you raise an exception is the same as when
@@ -8930,10 +8944,10 @@ \section{Reverse lookup}
 optional argument.  For example:
 
 \begin{verbatim}
->>> raise ValueError('value does not appear in the dictionary')
+>>> raise LookupError('value does not appear in the dictionary')
 Traceback (most recent call last):
   File "<stdin>", line 1, in ?
-ValueError: value does not appear in the dictionary
+LookupError: value does not appear in the dictionary
 \end{verbatim}
 %
 A reverse lookup is much slower than a forward lookup; if you
@@ -10453,7 +10467,7 @@ \section{Word frequency analysis}
 \begin{exercise}
 
 Modify the program from the previous exercise to print the
-20 most frequently-used words in the book.
+20 most frequently used words in the book.
 
 \end{exercise}
 
@@ -10555,7 +10569,7 @@ \section{Random numbers}
 {'a': 2, 'b': 1}
 \end{verbatim}
 %
-your function should return {\tt 'a'} with probability $2/3$ and {\tt 'b'}
+your function should return \verb"'a'" with probability $2/3$ and \verb"'b'"
 with probability $1/3$.
 \end{exercise}
 
@@ -10606,8 +10620,8 @@ \section{Word histogram}
 hyphens with spaces before using {\tt split} to break the line into a
 list of strings.  It traverses the list of words and uses {\tt strip}
 and {\tt lower} to remove punctuation and convert to lower case.  (It
-is a shorthand to say that strings are ``converted;'' remember that
-string are immutable, so methods like {\tt strip} and {\tt lower}
+is a shorthand to say that strings are ``converted''; remember that
+strings are immutable, so methods like {\tt strip} and {\tt lower}
 return new strings.)
 
 Finally, \verb"process_line" updates the histogram by creating a new
@@ -11452,7 +11466,7 @@ \section{Filenames and paths}
 \index{working directory}
 \index{directory!working}
 
-A string like {\tt '/home/dinsdale'} that identifies a file or
+A string like \verb"'/home/dinsdale'" that identifies a file or
 directory is called a {\bf path}.
 
 A simple filename, like {\tt memo.txt} is also considered a path,
@@ -12029,15 +12043,15 @@ \section{Exercises}
 If you download my solution to Exercise~\ref{anagrams} from
 \url{http://thinkpython2.com/code/anagram_sets.py}, you'll see that it creates
 a dictionary that maps from a sorted string of letters to the list of
-words that can be spelled with those letters.  For example, {\tt
-  'opst'} maps to the list {\tt ['opts', 'post', 'pots', 'spot',
-    'stop', 'tops']}.
+words that can be spelled with those letters.  For example,
+\verb"'opst'" maps to the list
+\verb"['opts', 'post', 'pots', 'spot', 'stop', 'tops']".
 
 Write a module that imports \verb"anagram_sets" and provides
 two new functions: \verb"store_anagrams" should store the
 anagram dictionary in a ``shelf''; \verb"read_anagrams" should
 look up a word and return a list of its anagrams.
-Solution: \url{http://thinkpython2.com/code/anagram_db.py}
+Solution: \url{http://thinkpython2.com/code/anagram_db.py}.
 
 \end{exercise}
 
@@ -13915,7 +13929,7 @@ \section{Exercises}
     \verb"int_to_time") to work with the new implementation.  You
     should not have to modify the test code in {\tt main}.  When you
     are done, the output should be the same as before.  Solution:
-    \url{http://thinkpython2.com/code/Time2_soln.py}
+    \url{http://thinkpython2.com/code/Time2_soln.py}.
 
 \end{exercise}
 
@@ -15615,7 +15629,7 @@ \section{Gathering keyword args}
 %
 When you are working with functions that have a large number of
 parameters, it is often useful to create and pass around dictionaries
-that specify frequently-used options.
+that specify frequently used options.
 
 
 \section{Glossary}
@@ -16069,7 +16083,7 @@ \subsection{When I run the program I get an exception.}
 \end{description}
 
 The Python debugger ({\tt pdb}) is useful for tracking down
-Exceptions because it allows you to examine the state of the
+exceptions because it allows you to examine the state of the
 program immediately before the error.  You can read
 about {\tt pdb} at \url{https://docs.python.org/3/library/pdb.html}.
 \index{debugger (pdb)}
