TOPS-20 PASCAL Language Manual - Bitsavers

PASCAL STATE~ENTS
5.4.3 The WHILE Statement
The WHILE statement causes one or more statements to be executed while
a specified condition is true.
Fo rrna t
where:
WHILE expression DO statement;
expression
specifies a Boolean expression.
The WHILE statement causes the statement following the word DO to be
executed while the expression is true. Unlike the REPEAT statement,
the WHILE statement controls the execution of only one statement.
Hence, to execute a group of statements repetitively, you must use a
compound statement. Otherwise, PASCAL repeats only the single
statement immediately following the word DO.
The expression is evaluated before the statement is executed. If the
expression is initially false, the statement is never executed. The
repeated statement must change the value of the expression. If the
value of the expression never changes, the result is an infinite loop.
Example 1
l"'HILE NOT EOF(Filel) DO
READL.N(Fil(·?l) y
This statement reads through to the end of the text file Filel.
Example 2
WHILE NOT EOLN(INPUT) DO
BEGIN
/:;:EAD(X) ;
IF NOT (X IN ['A' •• 'Z','a' •• 'z','O' •• '9'J) THEN
IE r T' ::::: E In' + 1
END;
This example reads an input character from the current line on the
terminal. If the character is not a digit or letter, the error count
(Err) is incremented by one.
Example 1
SUITt != 0;
Ntests ::::: 1;
AV9 := 100;
WHILE (AV9 >= 90) AND (Ntests