Great Ideas in Computer Science

(6.  Concatenation, continued)

            concatconversation.p

        which is just our old "conversation.p" program
        rewritten using the concatenation operator.

e.g.)    Here's yet another example, from Biermann,
        Ch. 2, p. 48, #2:

        I/P:    noun
        O/P:    plural of noun

        program plurals;
        var userstring, pluralstring : varying [21] of char;
        begin

            writeln('Enter a noun:');
            readln(userstring);
 
            pluralstring := userstring + 's';

            writeln('The plural of your noun is:');
            writeln(pluralstring)

        end.

        Try it!  Note that I've divided the body of the
        program into 3 parts (indicated by spacing):
        an input section, a computation section (where
        the process of creating the plural is done, using
        concatenation), and an output section.

        Although this program compiles, and
        although it does correctly form the plural
        of certain English nouns, it doesn't really
        work as advertised:
 
        What does it output as the plural of "mouse"
        or of "sheep"?  Hopefully, we'll have time later
        to return to this problem and write a slightly
        better program.
 
 

7.  A Digression from Text Processing:  Arithmetic

a)    Here's a new type, to add to our collection
        that, so far, includes these types:

            char    -    any printable character from
                            the ASCII set
 
            varying [n] of char (where n is an integer)
                        -   any string of n such characters

        Here's the new type:

            integer    -    any integer

        By the way, the integers are the following
        numbers:

        ..., -3, -2, -1, 0, 1, 2, 3, ...

        So, e.g., -796, -2, 0, 5, and 3987 are all
        integers, but 2.5, 3 1/2, pi, etc., are not.

        Well, the "integer" type does not really
        contain "any" integer:  Computers are
        finite machines, so there's a largest and a
        smallest integer that we can deal with.  To
        find out what they are, here's a small Pascal
        program that will show you what they are on
        our system.

            integertest.p

e.g.)  Here's a small Pascal program that shows you
        how we can use the "integer" type:

        program add;
        var x, y, z : integer;
        begin

            x := 2;
            y := 3;
            z := x + y;
 
            {note that this use of "+" is arithmetic}
            {addition, NOT string concatenation!  }

            writeln('x = ', x);
            writeln('y = ', y);
            writeln('x + y = ', z)

        end.

What does this program do?

1.  It sets up 3 memory locations, called "x", "y",
    and "z", each of which can contain an integer.

2.  It puts the integer 2 in location "x"
3.  It puts the integer 3 in location "y"

4.  It gets the value stored in "x" (i.e., 2)
    and the value stored in "y" (i.e., 3),
    and then it adds them; finally, it puts
    the sum in location "z"

5.  It outputs the message:

        x = <value stored in x>

6.    It outputs the message:

        y = <value stored in y>

7.    It outputs the message:

        x + y = <value stored in z>

Try it!!

What happens if we write:

    x := '1'

instead of

    x := 1

?  Try it!!