Lesson 4 - Program Control
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This lesson will cover:
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Now, it is time to learn the most important rules of programming: the if statements - decision making, for loops and the repeat-until loop. Almost, these 3 general programming constructs are common in every programming language and you have to make sure that when you have finished reading this lesson, make sure that you have practiced them enough before continuing with learning pascal because they are of outmost importance. If you fall in doubt about these programming constructs, then try to use the forums, describing your problem. |
The If Statement |
The 'if statement' executes a the proceeding statement(s) conditionally. This means that if an action comes to be true, then the statement(s) proceeding the if statement are executed, else these statements are skipped. It works like this:
If this happens(action), then do this(reaction, if action is true).
OR:
If this happens(action), then do this(reaction, if action is true), else do this(reaction, if action is false).
In Pascal, the 'if statement' should be written as follows:
If conditional expression then code ... ;{if one action}
OR:
If conditional expression then Begin instructions ... End; {if more than one action is required}
Note that you should not use an assignment statement in the 'if' construct, otherwise the compiler will raise a syntax error. I.e.:
Wrong:
If x := 20 then x := x + 1; {the underlined character must be excluded}
Correct:
If x = 20 then x := x + 1; {only an equal sign is used for comparison}
A program is shown below as an example of how the 'if statement' works:
Program lesson4_Program1;
Uses Crt;
Label 1; {this is used with a goto statement}
Var Sel: String;
N1,N2, Total : Real;
YN : Char; {this is a character variable type,
which holds single characters ONLY}
Begin
1:Clrscr;
Total := 0; {always initialise integer/real variables}
GotoXy(4,3);
Writeln('1.Addition');
GotoXy(4,4);
Writeln('2.Subtraction');
GotoXy(4,5);
Writeln('3.Exit');
GotoXy(6,8);
Write('Select: ');
Sel := Readkey;
If Sel = '1' {action} then
Begin {more than one statement}
ClrScr;
Write('Input No.1:');
Readln(N1);
Write('Input No.2:');
Readln(N2);
Total := N1 + N2;
Writeln('Addition: ',N1:2:3,' + ',N2:2:3,' = ',Total:2:3);
Write('Press any key to continue...');
Readkey;
Goto 1;{this leads back to the beginning of the program,
otherwise the program terminates}
End; {Closing the if statement(begin)}
If Sel = '2' then
{note that the assignment statement
is not used within an if statement}
Begin
ClrScr;
Write('Input No.1:');
Readln(N1);
Write('Input No.2:');
Readln(N2);
Total := N1 - N2;
Write('Subtraction: ');
Write(N1:2:3,' - ',N2:2:3,' = ',Total:2:3);
Write('Press any key to continue...');
Readkey;
Goto 1;
End; {Closing the if statement}
If Sel = '3' then
Begin
ClrScr;
Write('Are you sure?(Y/N)');
YN := Readkey;
If YN = 'y' then Halt; {1 action, so no need of Begin..End}
If YN = 'n' then Goto 1; {the goto statement is not
recommended for excessive use}
End;
End. |
In the above program, the 'goto' statement is used. So far, it has been a real damage to programs and it has produced unwanted confusions. I strongly suggest you not to use it repeatedly. |
-> If..Then..Else |
In
a normal if statement, the 'reaction' cannot be performed if the condition
is not true. But in an if..then..else statement, there is at
least one set of statements to be performed. Let's take a look at the example
below:
writeln('Who has discovered the land of America?');
Readln(ans);
If (ans = 'Christopher Colombus') then
score := score + 1 {if this is false,}
ELSE
writeln('sorry, you''ve got it wrong!'); {then this is true}
Note
that if the 'else' term is included with an if statement, then there
should be no semi-colon before the 'else' term; just as seen
in the above example. |
-> Nested If statements |
The
previous program have already shown an example of nested if statements.
I.e.:
If Sel = '3' then
Begin
ClrScr;
Write('Are you sure?(Y/N)');
YN := Readkey;
If YN = 'y' then HALT; {Nested if statement}
If YN = 'n' then Goto 1;{Another Nested if statement}
End;
A
nested if statement, is in the form:
If (this happens) then {if 1}
If (this happens) then {if 2}
(do this) etc...
Else (do this)
{if 2}
Else (do this) etc...
{if 1}
A
nested if statement is an if statement within another if statement,
as shown above. |
| The
Repeat-Until Loop |
This
loop is used to repeat the execution of a set of instructions for at
least one time. It is repeated until the conditional expression is obeyed.
The following example, shows the model of the 'repeat-until' loop:
Repeat
..(code)
..(code)
..(code)
Until conditional statement;
Here's
an example:
Uses Crt;
Var YN : String;
Begin
Writeln('Y(YES) or N(NO)?');
Repeat {repeat the code for at least one time}
YN := Readkey ;
If YN = 'y' then Halt; {Halt - exit}
If YN = 'n' then Writeln('Why not? Exiting...');
Delay(1800); { wait a second plus 800 milliseconds }
Until (YN = 'y') OR (YN = 'n');
End. |
See?
It's very simple! In the above program, there is a Boolean expression in the 10th line (or). This will be described later on. |
The For Loop |
The for loop is a sort of repeat-until loop. The for loop, repeats
a set of instructions for a number of times. The for loop is in the
form:
-
If used for only one action:
for {variable}* := {original value} to/downto {final value} do
{code...(for one action)}
-
If used for more than one action:
for {variable}* := {original value} to/downto {final value} do Begin
{code...}
{code...}
End;
*Generally,
this variable is called the 'loop counter'.
Now,
an example of the for loop is shown below, but firstly, you should have
an idea of the usefulness of the for loop. Consider the following
example:
Without for loop:
Program lesson4_Program2a;
Uses Crt;
Begin
Writeln('for loop'); {somewhat boring writing all this!!!}
Writeln('for loop');
Writeln('for loop');
Writeln('for loop');
Writeln('for loop');
Writeln('for loop');
Writeln('for loop');
Readln;
End. |
With for loop:
Program lesson4_Program2b;
Uses Crt;
Var Counter : Integer; {loop counter declared as integer}
Begin
For Counter := 1 to 7 do {it's easy and fast!}
writeln('for loop');
Readln;
End. |
Note
that the two programs above perform the same function, but which programming
style is more useful?
Suppose
we have to make a program which designs a small box with some of the
characters of the ASCII, obviously the characters which are most likely
to make up a simple box.
Without
the for loop:
Program Program3a_lesson4;
Uses Crt;
Begin
Gotoxy(25,5);Writeln('+');
Gotoxy(25,6);Writeln('I');
GotoXy(25,7);Writeln('I');
GotoXy(25,8);Writeln('I');
GotoXy(25,9);Writeln('I');
GotoXy(25,10);Writeln('I');
GotoXy(25,11);Writeln('+');
GotoXy(26,11);Writeln('-');
GotoXy(27,11);Writeln('-');
GotoXy(28,11);Writeln('-');
GotoXy(29,11);Writeln('-');
GotoXy(30,11);Writeln('-');
GotoXy(31,11);Writeln('-');
GotoXy(32,5);Writeln('+');
Gotoxy(32,6);writeln('I');
GotoXy(32,7);Writeln('I');
GotoXy(32,8);Writeln('I');
GotoXy(32,9);Writeln('I');
GotoXy(32,10);Writeln('I');
GotoXy(32,5);Writeln('+');
GotoXy(26,5);Writeln('-');
GotoXy(27,5);Writeln('-');
GotoXy(28,5);Writeln('-');
GotoXy(29,5);Writeln('-');
GotoXy(30,5);Writeln('-');
GotoXy(31,5);Writeln('-'); {oh my God!!! Phew!}
Readln; { wait for user to read }
End. |
With for loop:
Program Program3b_lesson4;
Uses Crt;
Var Counter : Integer; {loop counter}
Begin
For Counter := 1 to 5 do
Begin
gotoxy(25, 5 + Counter);
Writeln('I');
End;
For Counter := 5 Downto 1 do
Begin {an example of 'downto' instead of
'to', note the 'gotoxy(_,_)'}
gotoxy(32, 11 - Counter);
Writeln('I');
End;
For Counter := 1 to 6 do
Begin
gotoxy(25 + Counter, 11);
Writeln('-');
End;
For Counter := 6 Downto 1 do
Begin
gotoxy(32 - Counter, 5);
Writeln('-');
End;
{--------------The Corners(+)---------------}
Gotoxy(25,5);
Writeln('+');
GotoXy(25,11);
Writeln('+');
GotoXy(32,5);
Writeln('+');
GotoXy(32,11);
Writeln('+');
GotoXy(45,7);
Writeln('Just as simple as the for loop!!!');
Readln;
End. |
Again,
the two programs above perform the same function. |
| ->
Nested for loops |
A
nested for loop is similar to that of the nested if statements. A nested
for loop is in the form:
for {loop counter} := {original value} to {final
value} do {Begin-if required}
{code if any..begin should be included
(i.e more than one action)}
for {loop counter} := {original value} to {final
value} do {Begin-if required}
{code..if more than one action, include
begin in the second for loop}
{End; - if begin is included in the second for loop)}
{code if any..begin should be included in
the first for loop}
{End; - if begin is included in the first for loop)}
The
nested for loop is rarely used and it may cause problems. |
| While-Do
Loop |
This
type of loop is executed while the
condition is true. It is different from the 'Repeat-Until'
loop since the loop might not be executed for at least one time. The
code works like this:
While
<condition is true> do the following:
instruction 1;
instruction
2;
instruction
3;
etc...
End;
{If while-do loop starts with a begin statement}
Example
Program on the While-Do loop:
Program Lesson4_Program4;
Uses Crt;
Var Ch : Char;
Begin
Writeln('Press ''q'' to exit...');
Ch := Readkey;
While Ch <> 'q' do
Begin
Writeln('I told you press ''q'' to exit!!');
Ch := Readkey;
End;
End. |
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modified last: September 2005 (Article Version: 2)
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