TURBO-LESSONS - A Pascal Tutorial        Version 1.01    Page 28


TURBO-LESSON 7:   REPEAT STATEMENT                             

OBJECTIVES - In lesson 7 you should learn about:

1.  CHARacter variables
2.  BOOLEAN variables
3.  REPEAT statement

1.  CHARacter variables.

The reserved word, CHAR, is used to declare a variable of 
character type.  A variable of type CHAR can be used to refer to 
or store a single character.  

VAR
   Alpha : CHAR;

Alpha is declared to be a variable which can be used to store any 
of the characters in the character set.  This includes the upper 
and lower case alphabet, the digits, 0 to 9, special characters 
such as #, $, %, *, and the rest of the 256 characters in the 
PC's character set.

##### DO:

Examine PROG7.  

A variable named Response, of type CHAR, is used to store the 
character entered in response to a multiple choice question.  

##### DO:

Run the program several times, entering wrong responses, and the 
correct response, D.  Also try lower case d.  

##### DO:

Study the first IF statement in PROG7.  

IF (Response = 'D') OR (Response = 'd')
    THEN . . .

The character 'D' must be enclosed in single quotes in the 
program.  Note that you enter the character as input data without 
quotes when running the program.  

Notice the compound condition using OR to combine two simple 
conditions.  This condition will be true if either  or both of 
the simple conditions are true.  The correct response, D, is 
checked in both upper and lower case to make responding easier.

##### DO:

Modify the IF statement to accept A, B, or C as the right response.  
Assume that the correct answer is A or B or C.  (Ignore lower case 
responses to keep the statement short.) 
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2.  BOOLEAN variables.

BOOLEAN variables can have only two values, TRUE or FALSE.   The 
value of a condition, TRUE or FALSE, may be stored in a BOOLEAN 
variable for later use.

VAR
  Correct_Response : Boolean;

In PROG7, the Boolean variable, Correct_Response, is used to 
store the truth value (TRUE or FALSE) of the condition in the 
first IF statement.  If the correct character, 'D' or 'd', is 
entered, TRUE is stored in Correct_Response.  If anything else is 
entered, FALSE is stored.  

Actually, TRUE and FALSE are stored as 1 and 0 to take up less 
space, but you can always view a BOOLEAN variable as having a 
value of TRUE or FALSE.

##### DO:

Identify the condition in the second IF statement in PROG7.  

Since BOOLEAN variables can only have two values, TRUE or FALSE, 
and conditions always evaluate to the same two values, a BOOLEAN 
variable may be substituted for a condition.  

If would be permissible, but unnecessary to write the IF 
statement: 

IF Correct_Response = TRUE
   THEN . . .

##### DO:

Modify the IF statement as indicated above and run the program to 
verify that the IF still works exactly as before.

There is another way to assign the correct value to the variable, 
Correct_Response.  

##### DO:

Replace the FIRST IF statement in PROG7 with the following 
statement:

Correct_Response := (Response = 'D') OR (Response = 'd');

Run the program.

How does the program change when you run it?  (If it doesn't do 
exactly as before, maybe you typed it wrong, or replaced the 
second IF instead of the first?)  

Since the condition on the right of the := must be evaluated by 
the computer and assigned a value of TRUE or FALSE, this value 
can be stored directly in a BOOLEAN variable without using the IF 
statement.
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3.  REPEAT Statement.

In a previous lesson, you learned that there are three ways to 
sequence the execution of statements in PASCAL: SIMPLE SEQUENCE, 
SELECTION STRUCTURES, and REPETITION STRUCTURES.

One of the statements used for REPETITION is REPEAT . . UNTIL.  
The form of the REPEAT statement is:

REPEAT
  Statement 1;
  Statement 2;
    .
    .
    .
  Statement n
UNTIL condition;

Statements 1, 2, . . . , n will be executed repeatedly until the 
condition becomes true.  This implies that the condition is 
checking something that can be changed by the statements 1 to n.  
If this is not so, the statements will be repeated forever!

In PROG7, you are prompted to respond to the multiple choice 
question.  A REPEAT statement controls the block of statements 
which prompt for a response, and then check the response.  The 
block of statements will be repeated until the UNTIL condition is 
true.

##### DO:

Change the condition in the UNTIL in PROG7 to:

UNTIL 'A' = 'B';

Run the program.  

Does the program correctly identify a correct response?  What 
happens then.  

(Use ctrl-c or ctrl-Scroll-Lock to stop the program.) 

##### DO:

Change the condition again to:

UNTIL 'A' = 'A';

How many times are you prompted for a response?  

Notice that the statements in a REPEAT structure are ALWAYS 
executed at least once.  Even if the UNTIL condition is TRUE 
before entering the REPEAT, the condition is not checked until 
the end of the statements in the REPEAT block.  
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##### DO:

Try to find another way to terminate the REPEAT without using the 
BOOLEAN variable, Correct_Response, in the UNTIL condition.

Hint: Check the statement which assigns a value of TRUE or FALSE 
to Correct_Response.
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