Great Ideas in Computer Science

(19.  Some sample loops, continued.)

            i := 1; {initializes the test}

            while i <= 10 do {an arithmetic test}
                begin {while loop}

                    writeln('hello');
                    i := i+1

                end    {while loop}

        end.

    Click here for a downloadable/runnable version.

b)    You can change the behavior of this program
        by changing various parts of it:

        *    to change the number of times it outputs
              'hello', change the test from "i <= 10" to
              "i <= 3" or "i <= 20"

        *    to change what it outputs, change the
              "writeln" statement; e.g., you could have
              it print out the contents of the memory
              location "i" by changing the "writeln"
              statement to:    writeln(i)

        *    to change other behaviors, you could
             change the increment:  Change "i := i+1"
             to "i := i+2"

c)    The next loop program we'll look at it is called
        a "stub" (actually, being a stub has nothing to
        do with being a loop program).

*    A program stub is a program that:

        -    runs

        -    does something to tell you that it ran
              successfully

        -    doesn't (yet) do the details of what you
              really want it to do

        -    has the structure of how you want it done

    i.e., it's a top-down design of a program that
            needs to be refined, but it runs!

*    here it is:

    program loop2;
    {stub that repeatedly executes a user-provided
     command until a "quit" command is issued}
 
    var command : varying [20] of char;

    begin {main}

        writeln('COMMAND:');
        readln(command);

        {the previous 2 instructions "prime the pump"
         for the while-loop that follows, getting the
         test ready, so that the loop will execute at
         least once}

        while command <> 'quit' do

            begin {while}

                writeln('Command ', command,
                            ' has been executed');

                {the previous line is the line that "does
                 something to tell you that the program
                 runs successfully" but "doesn't yet do
                 the details of what you really want it to do}

                {the next two lines "prime" the while-loop
                 again}

                writeln('COMMAND:');
                readln(command)

            end;   {while}

        {the next instruction is the first one after the
          loop exits, so when this gets printed out, it
            tells you that the loop has finished.}

        writeln('The ', command,
                   ' command was executed; good-bye.')

    end.   {main}

    Click here for a downloadable/runnable version.
 

20.  Putting it all together.

program CSE111Editor;
{outline; needs refinement}

<type declarations go here>
<var declarations go here>
<function declarations go here>

begin {main}

    {initialize text and pointer:  text := ''; ptr := 0}

    writeln('COMMAND:');
    readln(command);

    while command <> 'quit' do

        begin {while}

            if command = 'i' then {insert new text};
            if command = 'd' then {delete text};
            if command = 's' then {insert space};
            {etc.}

            writeln('COMMAND:');
            readln(command)

        end;   {while}

    writeln(text)
 
end.   {main}