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                                                    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.         LOOPDEMO.PAS
This is 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
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

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starting point, and the ending point all be of the same type
or it will generate an error message during compilation.  In
addition, they 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               IFDEMO.PAS
statements.  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.

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                                 Loops and Control Structures


THE IF-THEN-ELSE BLOCK
____________________________________________________________

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.

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          LOOPIF.PAS
have studied so far 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.  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
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

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                                 Loops and Control Structures

conditional statement.  The compiler would not generate an
error and you would get no warning.  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       TEMPCONV.PAS
program 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.
Run it 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     DUMBCONV.PAS
structure 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.

Retrieve the file REPEATLP.PAS to see an     ================
example of a repeat loop.  Two more            REPEATLP.PAS
reserved 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

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following the until is found to be TRUE.  This is the only
expression in Pascal that operates on a range of 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.      WHILELP.PAS
This 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      CASEDEMO.PAS
to execute 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

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                                 Loops and Control Structures

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 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      BIGCASE.PAS
statement with a 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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