


                                                        Chapter 4
                                       LOOPS & CONTROL STRUCTURES


Every program we have examined to this point has been a simple one
pass through with no statements being repeated.  As in all other
languages, Pascal has extensive capabilities to do looping and
conditional branching.  We will look at these now.


THE FOR LOOP
_________________________________________________________________

We will start with what may be the easiest       ================
structure to understand, the for loop.  This is    LOOPDEMO.PAS
used to repeat a single Pascal statement any     ================
number of times we desire.  Load LOOPDEMO.PAS
and we will discuss the loops presented there. 

The example illustrated in lines 13 and 14, is the simplest loop
and does nothing more than execute a Writeln 7 times.  We have
three new reserved words, for, to, and do which are used as shown. 
Any simple variable of type integer, byte, or char can be used for
the loop index but due to the requirement that everything must be
defined prior to use in Pascal, the loop index must be defined in
a var statement.  Following the reserved word do is any single
Pascal statement that will be repeated the specified number of
times.  Note that the loop is an incrementing loop but substitution
of downto for to will make it a decrementing loop as is illustrated
in the last example in this program.  It should be pointed out that
the loop control variable can only be incremented or decremented
by 1 each time through the loop in Pascal.


A COMPOUND PASCAL STATEMENT
_________________________________________________________________

The example given in lines 18 through 22 contains our first
compound Pascal statement.  It was mentioned in Chapter 1 that the
begin end pair of reserved words could be used to mark the limits
of a compound statement.  In this case, the single compound
statement starting with the begin at the end of line 18 and
extending through and including the end statement in line 22 is the
single Pascal statement that will be executed 10 times.  A second
variable Total has been introduced to simply add another operation
to the loop.  Any valid Pascal operation can be performed within
the begin end pair, including another for loop, resulting in nested
loops to whatever depth you desire. 

The third example shows how the char type variable could be used
in a for loop.  Pascal requires that the loop index, the starting
point, and the ending point all be of the same type or it will
generate an error message during compilation.  In addition, they

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                         Chapter 4 - Loops and Control Structures

must be variables of type integer, byte, or char.  The starting
point and ending point can be constants or expressions of arbitrary
complexity.

The fourth example is a decrementing loop as mentioned earlier. 
It uses the reserved word downto, and should be self explanatory.



THE IF STATEMENT
_________________________________________________________________

Pascal has two conditional branching               ==============
capabilities, the if and the case statements.        IFDEMO.PAS
We will look at the simplest of the two now, the   ==============
if statement.  Load IFDEMO.PAS for an onscreen
look at the if then pair of reserved words first
illustrated in lines 11 and 12.  Any condition that can be reduced
to a boolean answer is put between the if then pair of words.  If
the resulting expression resolves to TRUE, then the single Pascal
statement following the reserved word then is executed, and if it
resolves to FALSE, then the single statement is skipped over.  Of
course, you can probably guess that the single statement can be
replaced with a compound statement bracketed with a begin end pair
and you are correct.  Study example 1 and you will see that the
line will always be printed in this particular fragment because
Three is equal to One + Two.  It is very difficult to come up with
a good example without combining some of the other control
structures but we will do so in the next example program.

The second example in lines 14 through 19, is similar to the first
but has the single statement replaced with a compound statement and
should be simple for you to understand.

The third example in lines 21 through 24, contains a new reserved
word, else.  When the if condition is FALSE, the single statement
following the word then is skipped and if a semicolon is
encountered, the if clause is totally complete.  If instead of a
semicolon, the reserved word else is encountered, then the single
Pascal statement following else is executed.  One and only one of
the two statements will be executed every time this if statement
is encountered in the program.  Examination of the third example
should clear this up in your mind.

Notice that the Pascal compiler is looking for either a semicolon
to end the if, or the reserved word else to continue the logic. 
It is therefore not legal to use a semicolon immediately preceding
the reserved word else.  You will get a compiler error if you
include the semicolon.  This is a common error, but easy to fix,
so you will get used to writing it correctly.





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THE IF-THEN-ELSE BLOCK
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Put on your thinking cap because the next principle is difficult
to grasp at first but will suddenly clear up and be one of the most
useful facts of Pascal programming.  Since the entire if then else
block of code is itself a single Pascal statement by definition,
it can be used anywhere that an executable statement is legal
without begin end separators.  This is shown in the fourth example
of the IFDEMO.PAS Pascal example program.  Lines 27 through 30
comprise a single Pascal statement, and lines 32 through 35
comprise another.  The if statement begun in line 26 therefore has
a single statement in each of its branches, and it is a single
Pascal statement itself beginning in line 26 and ending in line 35. 
Reread this paragraph until you understand it completely, because
it is a very important concept.

The if then else construct is one of the most used, most useful,
and therefore most important aspects of Pascal.  For this reason
you should become very familiar with it.

Try changing some of the conditions in the example program to see
if you can get it to print when you expect it to for your own
practice.  When you are ready, we will go on to a program with
loops and conditional statements combined and working together.


LOOPS AND IFS TOGETHER
_________________________________________________________________

Load LOOPIF.PAS and study it for a few minutes.    ==============
It contains most of what you have studied so far     LOOPIF.PAS
and should be understandable to you at this        ==============
point.  It contains a loop (lines 7 through 17)
with two if statements within it (lines 8 & 9
and lines 10 through 16), and another loop (lines 11 through 15)
within one of the if statements.

You should make careful note of the formatting used here.  The
begin is at the end of the line which starts the control and the
end is lined up under the control word such that it is very clear
which control word it is associated with.  All statements within
each control structure are indented three spaces which greatly adds
to the readability.  You will develop your own clear method of
formatting your code in time but until then it is suggested that
you follow this example which is written in a manner which is
acceptable within the general Pascal programming community.

An easily made error should be pointed out at this time.  If an
extraneous semicolon were put at the end of the if statement in
line 8, the code following the statement would always be executed
because the null statement (the nothing statement between the then
and the semicolon) would be the conditional statement.  The
compiler would not generate an error and you would get no warning. 

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                         Chapter 4 - Loops and Control Structures

Add a semicolon at the end of line 8 to see the error.  Of course,
you will need to compile and execute the program to see line 9
print for all 10 values of Count.


FINALLY, A MEANINGFUL PROGRAM
_________________________________________________________________

Load TEMPCONV.PAS and study its structure.       ================
Notice the header block that defines the program   TEMPCONV.PAS
and gives a very brief explanation of what the   ================
program does.  This program should pose no
problem to you in understanding what it does
since it is so clearly documented.  If you study the style of
indenting used here, you will learn to appreciate the clarity
afforded by the indentation.  Compile and run this program and you
will have a list of Centigrade to Fahrenheit temperature
conversions with a few added notes.  

Load, examine, and run DUMBCONV.PAS for a good   ================
example of poor variable naming.  The structure    DUMBCONV.PAS
of the program is identical to the last program  ================
and when you run it, you will see that it is
identical in output, but compared to the last
program, it is difficult to understand what it does by studying the
listing.  We studied UGLYFORM.PAS in chapter 2 of this tutorial and
it illustrated really stupid formatting that nobody would ever use. 
The poor choice of variable names and lack of comments in the
present program is nearly as unreadable, but many programmers are
content to write code similar to this example.  You should be
conscious of good formatting style and naming conventions from the
start and your programs will be clear, easy to understand, and will
run efficiently.  This program, like the last should be easily
understood by you, so we will go on to our next Pascal control
structure.


THE REPEAT UNTIL LOOP
_________________________________________________________________

The next two Pascal constructs are very similar because they are
both indefinite loops (indefinite because they are not executed a
fixed number of times).  One of the loops is evaluated at the top
and the other at the bottom.  It will probably be easier to start
with the repeat until construct which is the loop that is evaluated
at the bottom.

Examine the file named REPEATLP.PAS to see  an   ================
example of a repeat loop.  Two more reserved       REPEATLP.PAS
words are defined here, namely repeat and until. ================
This rather simple construct simply repeats all
statements between the two reserved words until
the boolean expression following the until is found to be TRUE. 
This is the only expression in Pascal that operates on a range of

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statements rather than a single statement and begin end delimiters
are not required.  

A word of caution is in order here.  Since the loop is executed
until some condition becomes TRUE, it is possible that the
condition will never be TRUE and the loop will never terminate. 
It is up to you, the programmer, to insure that the loop will
eventually terminate.  Compile and execute REPEATLP.PAS to observe
the output.


THE WHILE LOOP
_________________________________________________________________

The file WHILELP.PAS contains an example of       ===============
another new construct, the while loop.  This        WHILELP.PAS
uses the while do reserved words and will         ===============
execute one Pascal statement (or one compound
statement bounded with begin and end)
continuously until the boolean expression between the two words
becomes FALSE. 
This loop is also indeterminate and could, like the repeat until
loop, never terminate.  You should therefore exercise care in using
it. 

There are two basic differences in the last two loops.  The repeat
until loop is evaluated at the bottom of the loop and must
therefore always go through the loop at least one time.  The while
loop is evaluated at the top and may not go through even once. 
This gives you flexibility when choosing the loop to do the job at
hand.  Compile, run, and examine the output from the example
program WHILELP.PAS.


THE CASE STATEMENT
_________________________________________________________________

The final control structure introduces one more reserved word,
case.  The case construct actually should be included with the if
statement since it is a conditional execution statement, but we
saved it for last because it is rather unusual and will probably
be used less than the others we have discussed in this chapter.

The case statement is used to select one of many ================
possible simple Pascal statements to execute       CASEDEMO.PAS
based on the value of a simple variable.  Load   ================
the file CASEDEMO.PAS and observe the program
for an example of a case statement.  The
variable between the case and of reserved words in line 9 is the
variable used to make the selection and is called the case
selector.  Following that, the various selections are listed as a
possible value or range, followed by a colon, a single Pascal
statement, and a semicolon for each selector.  Following the list
of selections, an else can be added to cover the possibility that

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none of the selections were executed.  Finally, an end statement
is used to terminate the case construct.  Note that this is one of
the few places in Pascal that an end is used without a
corresponding begin.

The example file uses Count for the case selector, prints the
numbers one through five in text form, and declares that numbers
outside this range are not in the allowable list.  The program
should be self explanatory beyond that point.  Be sure to compile
and run this example program.

Load and display the sample program BIGCASE.PAS   ===============
for another example of a case statement with a      BIGCASE.PAS
few more added features.  This program uses the   ===============
identical structure as the previous program but
in line 11 a range is used as the selector so
that if the value of Count is 7, 8, or 9 this selection will be
made.  In line 12, three different listed values will cause
selection of this part of the code.  Of greater importance are the
compound statements used in some of the selections.  If the
variable Count has the value of 2, 4, or 6, a compound statement
will be executed and if the value is 3, a for loop is executed. 
If the value is 1, an if statement is executed which will cause a
compound statement to be executed.  In this case the if statement
will always be executed because TRUE will always be true, but any
Boolean expression could be used in the expression.  Be sure to
compile and run this program, then study the output until you
understand the result thoroughly.

This brings us to the end of chapter 4 and you now have enough
information to write essentially any program desired in Pascal. 
You would find however, that you would have a few difficulties if
you attempted to try to write a very big program without the topics
coming up in the next few chapters.  The additional topics will
greatly add to the flexibility of Pascal and will greatly ease
programming with it.


PROGRAMMING EXERCISES
_________________________________________________________________

1.   Write a program that lists the numbers from 1 to 12 and writes
     a special message beside the number representing your month
     of birth.

2.   Write a program that lists all of the numbers from 1 to 12
     except for the numbers 2 and 9.








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