Learn Pascal in a Three Days (2nd Ed.)
(Publisher: Wordware Publishing, Inc.)
Author(s):
ISBN: 1556225679
Publication Date: 07/01/97

Using Blocks

If you want to use more than one statement as a result of one condition, you can use the BEGIN-END blocks. You can actually use any number of blocks inside the program main body, using BEGIN and END to mark the territories of each block. A block will be treated as one unit, no matter how many statements it includes. Look at the following example:

{ ------------------------------ figure 3-3 ------------------------------ }
PROGRAM UsingBlocks(INPUT,OUTPUT);
CONST
 Limit = 1000;
VAR
 Amount:REAL;
BEGIN
 WRITE('Please enter the amount:');
 READLN(Amount);
 IF Amount <= Limit THEN
    BEGIN
     WRITELN('Your charge is accepted.');
     WRITELN('Your price plus tax is $',1.05*Amount:0:2)
                         { The semicolon is optional }
    END;
 IF Amount > Limit THEN
    BEGIN
     WRITELN('The amount exceeds your credit limit.') ;
     WRITELN('The maximum limit is $',Limit)
                         { The semicolon is optional }
    END;
 WRITELN('Thank you for using Pascal credit card.');
 WRITELN('Press ENTER to continue..');
 READLN                 { The semicolon is optional }
END.

In this example more than one statement is executed in either case (TRUE or FALSE). For this reason two blocks were used.

---

TIP:  Notice that in three positions in this program, the statement is not terminated by a semicolon, as the semicolon is optional. The statement in each of these positions is the last one inside a block.

---

Here are two sample runs:

Run 1:

Please enter the amount:120
Your charge is accepted.
Your price plus tax is $126.00
Thank you for using Pascal credit card.
Press ENTER to continue..

Run 2:

Please enter the amount:2000
The amount exceeds your credit limit.
The maximum limit is $1000
Thank you for using Pascal credit card.
Press ENTER to continue..

If you try the program without the blocks, you will find that only the first statement that follows the keyword THEN belongs to the IF statement, but any other statement belongs to the main program and will be executed regardless of the condition.

3-3 The IF-ELSE-THEN Construct

The form you have used so far for the IF statement is actually a simplified version of the complete construct. The complete IF statement includes the two cases that result from testing the condition. It takes the form:

    IF condition THEN
          statement
    ELSE
          statement;

Notice here that only one semicolon is used, because the whole construct is treated as one statement. Here is an example:

    IF AGE < 18 THEN
       WRITELN('Underage.')
    ELSE
       WRITELN('Age is OK.');

This statement will display the message “Underage” if “Age” is less than 18. In the other case the message “Age is OK” is displayed.

If you add another statement to either of the two cases, you have to use the BEGIN-END blocks. The new construct will look like this:

    IF AGE < 18 THEN
       BEGIN
        WRITELN('Underage.');
        WRITELN('Wait another couple of years.')
       END   { No semicolon is used here }
    ELSE
       BEGIN
        WRITELN('Age is OK.');
        WRITELN('You don''t have to wait.')
    END;   { A semicolon is mandatory here }

---

CAUTION:  At this point the use of semicolons becomes critical and may lead to errors if not done properly. Notice here that the keyword END in the first block is not terminated by a semicolon (as it is not the end of the statement), while in the second block it is terminated by a semicolon, indicating the end of the conditional statement.

---

Now, back to the “Pascal credit card” program to enhance it with the complete IF-THEN-ELSE statement.

{ ------------------------------ figure 3-4 ------------------------------ }
PROGRAM CreditCard(INPUT,OUTPUT);
CONST
 Limit = 1000;
VAR
 Amount:REAL;
BEGIN
 WRITE('Please enter the amount:');
 READLN(Amount);
{ Beginning of the IF construct }
{ ----------------------------- }
 IF Amount <= Limit THEN
    BEGIN
     WRITELN('Your charge is accepted.');
     WRITELN('Your price plus tax is $',1.05*Amount:0:2)
    END
 ELSE
    BEGIN
     WRITELN('The amount exceeds your credit limit.');
     WRITELN('The maximum limit is $',Limit)
    END;
{ End of the IF construct }
{ ----------------------- }
 WRITELN('Thank you for using Pascal credit card.');
 WRITELN('Press ENTER to continue..');
 READLN
END.

Sample runs of the program give the following results:

Run 1:

Please enter the amount:1000
Your charge is accepted.
Your price plus tax is $1050.00
Thank you for using Pascal credit card.
Press ENTER to continue..

Run 2:

Please enter the amount:1001
The amount exceeds your credit limit.
The maximum limit is $1000
Thank you for using Pascal credit card.
Press ENTER to continue..

3-4 The ELSE-IF Ladders

Although the IF-THEN-ELSE statement is intended for binary choice, it can be extended to handle more complicated choices. Look at this new arrangement of the construct, which is sometimes referred to as the ELSE-IF ladder:

    IF condition-1 THEN
          statement-1
    ELSE IF condition-2
          statement-2
    ELSE IF condition-3
          statement-3
    ...
    ELSE
          statement-n;

The conditions in the ladder are evaluated from the top down, and whenever a condition is evaluated as TRUE, the corresponding statement is executed and the rest of the construct is skipped. If no condition has been satisfied, the last ELSE will be brought into action.

Notice that the condition ladder is considered one statement ending with a semicolon, but no semicolons are used inside. If you want to use more than one result-statement, you have to use the BEGIN-END blocks according to the rules mentioned earlier.