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TURBO-LESSON 4:  DECLARATIONS, INPUT

OBJECTIVES - In Lesson 4 you will learn about:

1.  The DECLARATIONS part of a program
2.  VAR declaration
3.  Input using the ReadLn statement
4.  Integer variables


1.  The DECLARATIONS part of a program.

You learned in the previous lesson that there are two main parts 
to any PASCAL program: DECLARATIONS, and MAIN BODY.   The various 
entries in the DECLARATIONS section define the data items used in 
the processing in the MAIN BODY.  Not all declaration entries 
will occur in every program, but only the ones needed to support 
the processing.  

The various types of declaration entries will be introduced as 
needed in the sample programs.  Only the VAR entry will be used 
in this program.  


2.  VAR declaration.

All variables, (spelled A-L-L, no exceptions), must be defined 
before they are referenced by processing statements.  The VAR 
entry is used to define variables.  The form of the entry is:

variable-name   :  type;

The variable-name may be one or several variable-names separated 
by commas.  The type may be a predefined type, such as Integer, 
or a type you have constructed useing the predefined types. The 
colon must occur between the variable-name(s) and the type.  
Extra spaces are acceptable to allow more readable format.  Below 
are some VAR entries: 

VAR
      i,j,k    : Integer;
      Inkey    : Char;
      Rate     : Real;
      Count    : Integer;

(The example above includes types not yet discussed, to 
illustrate the form of the VAR entry.)
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##### DO:

Look at PROG4.  

(You know from earlier lessons how to load the program and use 
the editor to look at the program.)  A segment of PROG4 is: 

VAR
  Number   : Integer;


The VAR entry above defines a variable called "Number" to be of 
type "Integer".  This means the computer must set up a memory 
location large enough to store an integer, which can be accessed 
by referring to the name "Number".  

Notice the variable, Number, is later referenced in the 
processing statements.

##### DO:

Add an integer variable called "Age" to Prog4.  This will be used 
later in this lesson.  You can either add the new variable to the 
declaration of Number

      Number, Age   : Integer;

or add another declaration

      Number   : Integer;
      Age      : Integer;

Compile the program to be sure you haven't made a syntax error.     


3.  Input using the ReadLn statement.

ReadLn is the statement used to input variables.  The form of the 
statement is:

ReadLn(var_1, var_2, . . . ,var_n);

When the statement is executed, the computer will wait for you to 
type values for the variables, separated by one or more spaces, 
followed by depressing the enter key.  
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Part of PROG4 follows:
                                                             
BEGIN
  Write('Enter a number (no decimals please): ');
  ReadLn(Number);
  WriteLn;                        { Display one blank line }
  WriteLn('Number: ',Number);     { Display the number entered }
END.

The Write statement provides a prompt before the ReadLn accepts 
Number.  Notice that ReadLn does not provide a "?" or any other 
prompt.  The programmer must provide any prompting require.  

The use of the Write statement for prompting, instead of the 
WriteLn, keeps the cursor on the same line so the input will 
occur right after the prompt message.

The ReadLn accepts the number you type and stores it in the 
memory location which the computer has set aside for the Integer 
variable, Number.  

##### DO:

Compile and run PROG4.  When prompted, type 12 and enter.  Run it 
again and enter -34.  

##### DO:

Using the statements of PROG4 as an example, expand the program 
to do the following:

(1)   Write a prompt to 'Enter your age'.
(2)   Read a value for age to be stored in the integer variable, 
Age, which you added to the VAR declarations in the previous 
section.
(3)   Write a message which prints the age entered, in a manner 
similar to the way Number was printed.

Run the program.


4.  Integer Variables. 

Integers are counting numbers, with no decimal points.  They 
may be positive or negative.  In TURBO the range of Integers 
permitted is:

       -32768 to +32767

Integers are used for subscripts, indexes, counting, input and 
output of such things as counts, limits, menu choices.  Decimal 
numbers (Real type, discussed later), are needed for such things 
as dollar amounts and calculations.   When decimal numbers are 
not actually needed, Integers should be used since they are 
easier to use and take less memory.  
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##### DO:

Run PROG4 again, this time entering data which will cause errors.

When prompted to enter a number, enter the letter A instead.  
What happened?

A message appears:

I/O error 10, PC=287C   
Program aborted

Searching 
  15 lines

Run-time error position found. Press <ESC>

What this means is as follows:

The program aborted because "I/O error 10" occurred at program 
code address, 287C.  (Your program may produce a different 
program code address.)  To find out what "I/O error 10" is, look 
in your reference manual in the appendix containing "I/O ERROR 
MESSAGES".  Error 10 is an "Error in numeric format".

TURBO searched 15 lines before finding where the error occurred.

When you press the ESC key, the editor is activated with the 
cursor positioned at the end of the statement which caused the 
error.  (In this case, it is not the statement which caused the 
problem, but the type of data entered.)

##### DO:

Run the program again, entering too large a number, 88888.  Note 
that the same error occurs.  Run it again with too small a 
number, -55555.  You may want to experiment with the end-points 
of the acceptable integer range: -32768 to 32767.  For example, 
try entering the values -32767, -32768, -32769.

NOTE: THERE ARE WAYS TO HANDLE ERRORS TO AVOID ABORTING THE 
PROGRAM.  MORE ABOUT ERROR-HANDLING IN LATER LESSONS.
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