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Delphi Programming/Flow control

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Structures

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The if-structure

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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
   if a = False then
     WriteLn('a is false')
   else WriteLn('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
   if CONDITION then
     DO_ANYTHING
   else DO_THIS;
 end.

or (for more than one command to execute):

 begin
   if CONDITION then
   begin
     DO_ANYTHING;
   end
   else begin
     DO_THIS;
   end;
 end.

Or without the else:

 begin
   if CONDITION then
   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!')
   else WriteLn('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

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

Structure:

 case VARIABLE_NAME of
   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.

Operators

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Expanding the condition

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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 'equals'

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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

Operators for calculating

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  • 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

Loops

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

For-to

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 for [var] := [start] to [end] do
 begin
   [execute the following code]
 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-downto

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 for [var] := [end] downto [start] do
 begin
   [execute the following code]
 end;

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

While-do

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 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-until

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 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.

Setting values

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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!