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13.2.4 The For..to/downto..do statement

Free Pascal supports the For loop construction. A for loop is used in case one wants to calculate something a fixed number of times. The prototype syntax is as follows:

_________________________________________________________________________________________________________
For statement

-- - - - - ----- ----- for statement for control variable := initial value - to --| ----final value-do -statement------------------------downto-----------

--control variable-variable identifier-----------------------------------

--initial value expression -------------------------------------------

-- - -------------------------------------------- final value expression
___________________________________________________________________

Here, Statement can be a compound statement. When the For statement is encountered, the control variable is initialized with the initial value, and is compared with the final value. What happens next depends on whether to or downto is used:

  1. In the case To is used, if the initial value is larger than the final value then Statement will never be executed.
  2. In the case DownTo is used, if the initial value is less than the final value then Statement will never be executed.

After this check, the statement after Do is executed. After the execution of the statement, the control variable is increased or decreased with 1, depending on whether To or Downto is used. The control variable must be an ordinal type, no other types can be used as counters in a loop.

Remark: Free Pascal always calculates the upper bound before initializing the counter variable with the initial value.

Remark: It is not allowed to change (i.e. assign a value to) the value of a loop variable inside the loop.

The following are valid loops:

For Day := Monday to Friday do Work;  
For I := 100 downto 1 do  
  WriteLn (’Counting down : ’,i);  
For I := 1 to 7*dwarfs do KissDwarf(i);

The following will generate an error:

For I:=0 to 100 do  
  begin  
  DoSomething;  
  I:=I*2;  
  end;

because the loop variable I cannot be assigned to inside the loop.

If the statement is a compound statement, then the Break and Continue system routines can be used to jump to the end or just after the end of the For statement. Note that Break and Continue are not reserved words and therefor can be overloaded.

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