For statement
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.
Usually execution of a for statement is as follows:
- The initial value is stored in the control variable.
- The statement contained in the for statement is executed.
- If the value stored in the control variable is equal to the final value then
execution of the for statement is terminated.
- The value stored in the control variable is incremented by one (in loops counting upwards),
or decremented by one (in loops counting downwards).
- Execution of the for statement continues with step 2.
However the following situations can affect the execution of a for statement.
- 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 its execution is terminated.
You should not depend on the control variable having any particular value after the execution
of a for statement is terminated, unless program execution was transfered out
of the for statement by a goto statement.
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 its own control variable.
Example
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
