Question

I have somehow hard time in understanding the following program in LOOP (you can also read the article in its original German), which is supposed to simulate IF x=0 THEN A END.

y:=1;
LOOP x DO y:=0 END;
LOOP y DO A END;

How exactly does this simulate an IF statement? Why can’t I do this:

x:=0;
LOOP x DO A END;

Answer 1

I have no idea what this language is but if one assumes that LOOP cntr DO stmt END; executes stmt as long as cntr is greater than zero (and it’s decremented after each iteration), then this works as follows:

1 is assigned to y. If x is greater than zero, then y is assigned 0. If x is zero the value of y remains 1. In the next loop, A is executed once if y is 1 which is the case only if the previous LOOP statement didn’t execute, that is if x was equal to 0. The main advantage of this construct is that A is executed only once and not as much times as is the value of x if it is greater than zero which LOOP x DO A END; would do.

Here is the logic once again, step by step:

• y is set to 1
• y is set to zero x times. This is equivalent to IF x <> 0 THEN y:=0 END
• A is executed y times (once if x is zero and zero times if x is non-zero)

Those three steps translate to: if x is zero, execute A once, otherwise execute A zero times (i.e. don’t execute).