Contents | Prev | Next

Description

For statements perform an action in a counting loop (i.e. perform the action for a specified number of times and counts each performance of the action). For statements contain the following:

• A variable (called the control variable) that stores the current loop counter.
• The initial value of the loop counter, which is specified by an expression of ordinal type.
• The direction of the loop count, specified either by the reserved word to which indicates that the loop counts upwards, or by the reserved word downto which indicates that the loop counts downwards.
• The final value of the loop counter, which is specified by an expression of ordinal type.
• A statement that specifies the action performed in the counting loop. This statement can be a compound statement, containing a statement-sequence.

1. The initial value is stored in the control variable.
2. The statement contained in the for statement is executed.
3. If the value stored in the control variable is equal to the final value then execution of the for statement is terminated.
4. The value stored in the control variable is incremented by one (in loops counting upwards), or decremented by one (in loops counting downwards).
5. Execution of the for statement continues with step 2.

• If the statement contained in the for statement transfers program execution to a statement outside of the for statement then execution of the for statement is terminated when program execution is transfered.
• The for statement's counting loop can specify that the action be performed zero times, in which case no value is stored in the control variable and the statement contained in the for statement is not executed. This situation occurs when either the initial value is greater than the final value in loops counting upwards, or the initial value is less than the final value in loops counting downwards (in either case the initial value and the final value are evaluated and compared but no other action is performed by the for statement before it's execution is terminated.

Normally when executing a for statement it is best if the value of the control variable is changed only by the for statement. If another statement alters the value of a for statement's control variable it usually causes problems in the program. In an apparent effort to protect programmers from making that kind of mistake Standard Pascal (ISO/IEC 7185) defines a number of rules that restrict how control variables are used, and Irie Pascal implements these rules.

The first rule is that control variables must be declared at the same level as the for statement containing them (i.e. if the for statement is in a function or procedure then the control variable must be local to that function or procedure, and if the for statement is in the main program block then the control variable must be global). Keeping the control variable local makes it easier to control access.

The second rule is that neither the for statement nor any function or procedure local to the block containing the for statement shall contain a statement that threatens the control variable. A statement threatens the control variable if the execution of the statement could possibly alter the value stored in the control variable.

• An assignment statement threatens a control variable if the control variable is the variable being assigned to.
• A function or procedure call threatens a control variable if the control variable is being passed by reference.
• A read or readln statement threatens a control variable if the control variable is passed as an actual parameter to these procedures.
• A for statement threatens it's own control variable.

The following program uses a for statement to count from 1 to 10.

program CountToTen(output);

   procedure CountToTen;
   var
      i : 1..10;
   begin (* CountToTen *)
      for i := 1 to 10 do
         writeln(i);
   end; (* CountToTen *)

begin
   CountToTen;
end.

Syntax

(NOTE: for clarity some parts of the syntax are omitted, see Irie Pascal Grammar for the full syntax):

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

   control-variable = entire-variable

   initial-value = ordinal-expression

   final-value = ordinal-expression

Contents | Prev | Next