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    24     

    25 \begin{document}

    26 \fnote{\copyright{} Christian Urban, King's College London, 2022}

    27 

    28 \section*{Scala Worksheet 2}

    29 

    30 

    31 

    32 \subsection*{Task 1 (Options)}

    33 

    34 Get familiar with constructing strings and printing strings

    35 (i.e.~\texttt{println}, \texttt{mkString}, \texttt{toString}, \ldots)

    36 

    37 \begin{lstlisting}[numbers=none]

    38 scala> List(7,2,3,4,5,6).find(_ < 4)

    39 scala> List(5,6,7,8,9).find(_ < 4)

    40 scala> 

    41 \end{lstlisting}

    42 

    43 \subsection*{Task 2 (URLs / Files)}

    44 

    45 ALso Try

    46 

    47 \begin{lstlisting}[numbers=none]

    48 scala> 

    49 scala> 

    50 scala> 

    51 \end{lstlisting}

    52 

    53 \subsection*{Task 3 (Higher-Order Functions)}

    54 

    55 Make sure you understand if-conditions in Scala: They are

    56 \emph{expressions}, meaning they need to calculate a result. For

    57 example an \texttt{if} without an else-branch is nearly always

    58 \emph{not} what you intend to write (because you are not meant to

    59 change any ``state'' outside the expression). Also, remember the quirks

    60 in Scala with if-conditions needing parentheses and there is no

    61 \texttt{then}-keyword.

    62 

    63 \begin{lstlisting}[numbers=none]

    64 scala> val s1 = "foo"

    65 scala> val s2 = "bar"

    66 scala  val s3 = "foobar"

    67 scala> if (s1 == s2) print("equal") else print("unequal")

    68 scala> if (s1 ++ s2 == s3) print("equal") else print("unequal")

    69 \end{lstlisting}

    70 

    71 \subsection*{Task 4 (Maps)}

    72 

    73 Write \texttt{for}-comprehensions that enumerate all triples containing the numbers 1 - 5. That is,

    74 print the list

    75 

    76 \[

    77 \texttt{(1,1,1)}, \texttt{(2,1,1)}, \texttt{(3,1,1)} \;\ldots\; \texttt{(5,5,5)}

    78 \]

    79 

    80 \noindent

    81 Modify the \texttt{for}-comprehensions such that only triples are

    82 printed where the sum is divisible by 3.  Then sort the list according

    83 to the middle element (for this use \texttt{sortBy}).

    84 

    85 \subsection*{Task 5 (Pattern-Matching)}

    86 

    87 \subsection*{Task 6 (Web-Crawler)}

    88 

    89 Write a pretty print function for lists of integers which

    90 ``condenses'' elements in the list, meaning if there is a number

    91 several times in a row, then print out \mbox{\texttt{n x e}}, where

    92 \texttt{n} is the number of repetitions and \texttt{e} is the

    93 number. For example

    94 

    95 \begin{lstlisting}[numbers=none]

    96   List(1,1,1,2,3,3)    =>   List(3 x 1, 2, 2 x 3)

    97   List(1,2,3,4,4,4)    =>   List(1, 2, 3, 3 x 4)

    98   List(1,1,1,1,1,1)    =>   List(6 x 1)

    99   List(1,1,1,2,1,1)    =>   List(3 x 1, 2, 2 x 1)

   100 \end{lstlisting}

   101 

   102 You might like to define a separate function that first counts the occurences

   103 for each element and returns a list of (Int, Int)-pairs .

   104 

   105 \end{document}

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