Delphi Programming/Flow control

The if-structure executes a block of commands if a boolean expression returns True. A more simple to understand explanation is: It executes a block or a single command if a condition is true. Example:

begin
ifa=Falsethen
WriteLn('a is false')
elseWriteLn('a is true');
end.

Never write "if a = True" but simply write "if a". Writing "if a = False" is correct, but you can also write "if not a" or (with brackets) "if (((((a)))))" (or as many brackets you want), also "(if (not(a)))".

Structure:

begin
ifCONDITIONthen
DO_ANYTHING
elseDO_THIS;
end.

or (for more than one command to execute):

begin
ifCONDITIONthen
begin
DO_ANYTHING;
end
elsebegin
DO_THIS;
end;
end.

Or without the else:

begin
ifCONDITIONthen
begin
DO_THIS;
end;
end.

Except the last end there's always a semicolon behind the end. There is never a semicolon before an "Else"!

Example:

var
_Answer:string;
begin
WriteLn('Do you want to order a pizza?');
ReadLn(_Answer);
if_Answer='Yes'then
WriteLn('You decided for yes!')
elseWriteLn('Don''t want to have a pizza?');
end.

You can start and end a string with a quote (') or a double quote ("). How to write a quote or double quote in a string? It would end the string in the middle! If you have to write a quote in the text, you can start and end your string with a double quote or write your quote twice as it has been done at line 8. Do the same thing for a double quote.

The Case structure is quite similar to the if structure with the following difference: You can more easily ask for several cases!

Structure:

caseVARIABLE_NAMEof
VALUE_1:
DO_THIS;
VALUE_N:
DO_THIS
else
DO_THIS
end;
end;

But with a case-structure you can only ask for Integers and chars.

You can expand your condition with a few operators:

• AND (like && in C): logical 'and': if (a = 1) and (b = 2). The value of the expression "(a = 1) and (b = 2)" is TRUE if a is 1 and b is 2. Else, the value is FALSE and the ELSE-part will be executed (and not the part after THEN). Don't forget the brackets!

• OR (like || in C): 'or': if (a = 1) or (b = 1). If a is 1, b is 1 or both is 1, the value of the expression is TRUE.
• XOR: If only one of the conditions is true: if (a = 1) xor (b = 2). The expression is true if a is 1 or b is 2. If a is 1 AND b is 2, the value will be FALSE!
• NOT: The opposite of the expression.

It's also possible to interlink that operators. But then don't forget the brackets!

By the way: Every condition returns a boolean value. If it is TRUE, the then-part will be executed. If not, the processor goes to the else-part.

Operators such as '=' are:

• = equals
• > greater than
• < less than
• <= less or equals
• >= greater or equals
• <> not equal (less or greater, not the same)

and conjunction or disjunction xor exclusive disjunction

• You can use ( and ) as brackets.
• / means 'divided by', the result is a float
• div means 'divided by', the result is a rounded integer
• * means 'times'
• + means 'plus'
• - means 'minus'

+ also means linking of strings or chars:

• string + string : string
• string + char : string
• char + char : string
• char + string : string
• string + number : error
• number + string : error
• number + number : number
• number + char : error
• char + number : error

Loop means: A block will be executed many times. There are four types of loops:

for[var]:=[start]to[end]do
begin
[executethefollowingcode]
end;

The var will count from [start] to [end] and after every counting step the code will be executed. Normally the [var] is defined as i, j or k, but you can also choose counting_var_with_this_name or any name.

for[var]:=[end]downto[start]do
begin
[executethefollowingcode]
end;

The var will count down from [end] to [start] and after every counting step the code will be executed.

while[condition]do
begin
[code]
end;

While the condition is true, the code will be executed. Whether the condition is TRUE or FALSE will be checked BEFORE executing the code.

repeat
[code]
until[condition];

The code will be executed until the condition is true. Whether the condition is TRUE or FALSE will be checked AFTER executing the code.

The operator for setting values is :=

a:=b;

By executing, a will get the value of b.

EXAMPLE:

a equals 1; b equals 3

After executing:

a equals 3; b equals 3

and not:

a equals 1; b equals 1

Be careful with the order!

• Book:Delphi Programming