Delphi Programming/Flow control

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The if-structure[edit]

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:

1  begin
2    if a = False then
3      WriteLn('a is false')
4    else WriteLn('a is true');
5  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)))".


1  begin
2    if CONDITION then
4    else DO_THIS;
5  end.

or (for more than one command to execute):

1  begin
2    if CONDITION then
3    begin
5    end
6    else begin
7      DO_THIS;
8    end;
9  end.

Or without the else:

1  begin
2    if CONDITION then
3    begin
4      DO_THIS;
5    end;
6  end.

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


1  var
2    _Answer: string;
3  begin
4    WriteLn('Do you want to order a pizza?');
5    ReadLn(_Answer);
6    if _Answer = 'Yes' then
7      WriteLn('You decided for yes!')
8    else WriteLn('Don''t want to have a pizza?');
9  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[edit]

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


1  case VARIABLE_NAME of
2    VALUE_1:
3      DO_THIS;
4    VALUE_N:
5      DO_THIS
6    else
7      DO_THIS
8    end;
9  end;

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


Expanding the condition[edit]

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'[edit]

Operators such as '=' are:

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

Operators for calculating[edit]

  • 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:


1  for [var] := [start] to [end] do
2  begin
3    [execute the following code]
4  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.


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

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


1  while [condition] do
2  begin
3    [code]
4  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.


1  repeat
2    [code]
3  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[edit]

The operator for setting values is :=

1  a := b;

By executing, a will get the value of b.


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!