Repeat
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This reserved words repeat
in conjunction with until
are used to create tail-controlled loops.
syntax
A tail-controlled loops start with repeat
, followed by a possibly empty list of statements, and concluded by until
and a Boolean
expression.
The following (infinite) loops demonstrate the syntax:
// empty loop body is legal:
repeat
until false;
Note, repeat
and until
already form a frame in their own right.
You do not need to surround your statements by an extra begin
… end
-frame.
In fact, you can put as many sequences (also called compound statements) between repeat
and until
as you want:
repeat
begin
write('x');
end;
begin
write('o');
end;
until false;
Note, it is not necessary, but allowed to put a semicolon prior until
:
repeat
write('zZ')
until false;
semantics
Since the loop “head” appears at the tail, the loop body is executed at least once and the loop condition evaluated at the end of every iteration.
If the condition evaluates to false
, another iteration occurs.
Therefore, repeat
… until
loops are particularly useful to ensure a certain sequence of statements is run at least once.
repeat
write('Enter a positive number: ');
readLn(i);
// readLn loads a default value if the source is EOF.
// For integer values the default is zero.
// Since our loop condition requires _positive_ values,
// this loop would be stuck _indefinitely_ if EOF(input).
// Ergo, we check for that:
if eof(input) then
begin
writeLn;
writeLn(stdErr, 'error: input has reached EOF');
halt(1);
end;
until i > 0;
The user will be prompted again and again, but at least once, until he finally enters a positive number.
Another standard usage example is the reverse Horner scheme as demonstrated in Base converting.
see also
while
for head-controlled loopsbreak
continue
- Object Pascal Tutorial on
repeat…until
Keywords: begin — do — else — end — for — if — repeat — then — until — while