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CHAPTER 4 - The Pascal loops and 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 used to repeat a single
Pascal statement any number of times we desire. Load
LOOPDEMO and we will discuss the loops presented there.
The first example is the simplest and is simply a
repeat of 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 and it must be defined in a var
statement. Following the "do" reserved word 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 second example 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 17 and
extending through and including the end statement in line 21
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
variable, the starting point, and the ending point all be of
the same type or it will generate an error message. In
addition, it must be a variable of type integer, byte, or
char. The starting point and ending point can be constants
or expressions of arbitrary complexity.
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CHAPTER 4 - The Pascal loops and control structures
The fourth example is a decrementing loop as mentioned
earlier. It uses the reserved word "downto".
THE IF STATEMENT
Pascal has two conditional branching capabilities, the
"if" and the "case". We will look at one of them now, the
if statement. Load IFDEMO for an onscreen look at the "if
then" pair of reserved words. 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 following single Pascal statement is
executed, and if it resolves to FALSE, then the following
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 file.
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 easy 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 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 the 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 do so.
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
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CHAPTER 4 - The Pascal loops and control structures
end separators. This is shown in the fourth example of the
IFDEMO 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.
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 and observe it for a few minutes. It
contains most of what you have studied so far and should be
understandable to you at this point. It contains a loop
(lines 7 & 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.
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. Add a
semicolon at the end of line 8 to see the error.
FINALLY, A MEANINGFUL PROGRAM
Load TEMPCONV and study its structure. Notice the
header block that defines the 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.
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CHAPTER 4 - The Pascal loops and control structures
Load, examine, and run DUMBCONV for a good example of
poor variable naming. The 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. 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 to see an example of a
repeat loop. Two more 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 following the "until" is found
to be TRUE. This is the only expression I know of 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 run REPEATLP to observe the output.
THE WHILE LOOP
The file WHILELP contains an example of another new
construct, the "while" loop. 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
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CHAPTER 4 - The Pascal loops and control structures
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.
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 I chose to save 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 based on the
value of a simple variable. Load the file CASEDEMO 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. 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 a variable and 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 for another
example of a case 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
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CHAPTER 4 - The Pascal loops and control structures
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 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 in
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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