Great Ideas in Computer Science

(3.  Programming in Pascal, continued)

    In the program "conversation.p",
    what would happen if you entered your class
    instead of your name?
        *    Try it!

d)    Pascal so far:

*    basic instructions (or "statements", or "commands"):
        writeln()
        readln()

*    sequencing (or "compound statement"):

        begin
            <instruction>;
            .
            .
            .
            <instruction
        end

*    indentation for human readability
        (not enforced by compiler, as it was for Karel)

*    semicolons:    signal sequencing
                            therefore, go between 2 statements
                            (just as in Karel)

*    how to keep track of "begin"s & "end"s:

            label them with comments
            (as I did in "How to Read")

e)    Programming a Decision Tree

*    Here's a very simple DT:

                    ----------------
                   /| Take CSE 115 |
             yes  / ----------------
                 /
--------------- /
| Do you plan |/
| to major in |\
|     CSE?    | \
---------------  \
                  \
               no  \
                    \-------------------
                     | Take 101 or 111 |
                     |     or 113      |
                     -------------------

    TDD-1:

    var answer : varying [20] of char;
    writeln('Do you plan to major in CSE?');
    readln(answer);
    if answer = 'yes'
        then {response for "yes" branch}
        else {response for "no" branch}

    SWR-2:

    if answer = 'yes'
        then writeln('Take CSE 115')
        else begin
                    writeln('Take 101 or 111');
                    writeln('or 113')
                end

    SWR-3:

    program DecisionTree1;
    var answer : varying [20] of char;
    begin {DecisionTree1}
        writeln('Do you plan to major in CSE?');
        readln(answer);
        if answer = 'yes'
        then writeln('Take CSE 115')
        else begin {else if answer <> 'yes'}
                    writeln('Take 101 or 111');
                    writeln('or 113')
                end {else if answer <> 'yes'}
    end. {DecisionTree1}

    Comments:

    1.    In the test, "answer = 'yes' ",
           what happens if the user types in "YES" or "y"?
                Try it to see!

    2.    Note the way that I commented the
            "begin/end" pairs

    3.    Note the way you have to show an
            "unequals" sign in Pascal:  <>

f)    Coding the CSE DT:

*    Here's the general form of a DT:
 

    ----------
    |      C |
    |     /  | = A
    | yes/   |
    |   /    |
    | Q2     |
    |   \    |
    |  no\   |
    |     \  |
    |      D |
    ----------
     /
yes /
   /
  /
Q1              Etc.
  \
   \
  no\
     \
      ----------
      |      E |
      |     /  | = B
      | yes/   |
      |   /    |
      | Q3     |
      |   \    |
      |  no\   |
      |     \  |
      |      F |
      ----------

        if Q1 = yes
            then A
            else B

    where A is:

        if Q2 = yes
            then C
            else D

    & where B is:

        if Q3 = yes
            then E
            else F

*    Putting it all together:

        if Q1 = yes
            then  if Q2 = yes
                        then C
                        else D
            else if Q3 = yes
                        then E
                        else F

*    In our case:

        if Q1 = yes
            then {write something}
            else if Q3 = yes
                        then {write something}
                        else F

        Etc.

(to be continued in Lecture Notes #15...)