TURBO-LESSONS - A Pascal Tutorial        Version 1.01    Page 25


TURBO-LESSON 6:  CONDITIONAL PROCESSING

OBJECTIVES - You will learn, in this lesson, about:

1.  Selection structures used for conditional processing
2.  IF statement  (one-way selection)
3.  IF statement  (two-way selection)


1.  Selection structures used for conditional processing.

There are three types of statement sequencing used in PASCAL:

   (1)  SIMPLE SEQUENCE.  One statement follows another with no 
branching.  

   (2)  SELECTION STRUCTURES.  Based on a condition, supplied by 
the programmer, the next statement to execute is chosen from two 
alternatives (IF statement) or chosen from many alternatives 
(CASE statement).  

   (3)  REPETITION STRUCTURES.  A group of program statements may 
be repeated more than once, dependent on a condition supplied by 
the programmer.  The repetition statements are WHILE, REPEAT, and 
FOR, included in later lessons.

The Selection statement, IF .. THEN .. ELSE, is illustrated in 
this lesson,  the CASE statement appears in a later lesson.
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2.  IF statement (one-way selection).

The one-way IF is really a special case of the two-way IF, where 
the second alternative is to do nothing, just go on to the next 
statement.  The form of the IF statement is:

IF condition THEN statement;

The condition is an expression or comparison which the computer 
can evaluate as TRUE or FALSE.  Examples of a condition:

   7 < 10        TRUE
   I < 10        TRUE, if the memory location named I holds a 
                       value less than 10, otherwise FALSE.
   NOT(7 < 10)   FALSE (7 < 10 is TRUE, but NOT reverses the 
                       value to FALSE).

##### DO:

Look at PROG6.  

The first IF statement is a one-way selection.  If the condition 
is true, the WriteLn statement will be executed.  If the 
condition is false, the WriteLn will be ignored.  

Run the program using 0 for the no of computers owned.

Run it again with 1 for the no of computers owned.  

The "No Computer!" message should print for 0 computers owned, 
but not print for 1 computer owned.  

##### DO:

Examine the last IF in the program.  

This is a one-way IF with a slightly more complicated condition.  
The condition contains an integer expression, (Want - Have).  The 
computer first evaluates the integer expression to get a number 
to compare with the 2 on the right side of the ">".  

##### DO:

Run the program several times with different input to see the 
effect of this IF.

Also note the misspelled "Aren''t" in the message.  The double 
apostrophe is used in the message to represent a single 
apostrophe.  If a single apostrophe were used, it would appear to 
be the end of the message.  
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3.  IF statement (Two-way selection).

The two-way IF causes one of two alternative statements to be 
executed, based on whether the condition is TRUE or FALSE.  The 
form of the statement is:

IF condition THEN Statement_1 ELSE Statement_2;

If the condition is TRUE, the statement following the THEN is 
executed.  If the condition is FALSE, the statement following the 
ELSE is executed. 

##### DO:

Look at the second IF in PROG6.  

This is a two-way IF.  A congratulations message is printed if 
the condition is true,  condolences if false.  

The condition is (Have >= Want).  This condition is true if the 
number you enter for computers owned is greater than or equal to 
the number you enter for computers you would like to have.

##### DO:

Run the program several times, experimenting with various input 
values.  

##### DO:

Try your hand at writing an IF statement to do the following:

If the number of computers owned is more than the number of 
computers wanted, print a message 'Send extra computers to 
(put your name here?) '.
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